Java « CodeThatAint

When to use Lambda Expressions
If you have a functional Interface which has only one Abstract Method then we can use Lambda Expression at the Class which implements the functional interface
and provides the method definition for the abstract method in Functional Interface.

We have a Function Interface Printable with abstract method print
Two classes implements functional Interface – DailyReport and MothlyReport. They provide the method definition for Print() method in Printable Interface
GenerateReport class has a downloadReport method which takes the Printable Type as argument
DownloadReport Class we use the Printable reference and call the exact implementation by passing as argument

Printable.java

@FunctionalInterface
public interface Printable {
    public void print();
}

DailyReport.java

public class DailyReport implements Printable {
    @Override
    public void print() {
        System.out.println("Printing Report in Daily Format");
    }
}

MonthlyReport.java

public class MonthlyReport implements Printable {
    @Override
    public void print() {
        System.out.println("Printing Report in Monthly Format");
    }
}

GenerateReport.java

public class GenerateReport {
    public static void downloadReport(Printable printable){
        printable.print();
    }
}

Without Lambda Expression
DownloadReport.java

public class DownloadReport {
    public static void main(String[] args) {
        GenerateReport objGenReport = new GenerateReport();
        Printable objDailyReport = new DailyReport();
        Printable objMonthlyReport = new MonthlyReport();

        objGenReport.downloadReport(objDailyReport);
        objGenReport.downloadReport(objMonthlyReport);
    }
}

Output

Printing Report in Daily Format
Printing Report in Monthly Format

With Lambda Expression
Using Lambda expression we can pass method definition directly as parameter instead of using a class to extend interface and writing a method definition for the interface method
DownloadReport.java

public class DownloadReport {
    public static void main(String[] args) {
        GenerateReport objGenReport = new GenerateReport();
        objGenReport.downloadReport(() -> System.out.println("Printing Report in Daily Format using Lambda Expr"));

        Printable mnthlyReport = () -> System.out.println("Printing Report in Monthly Format using Lambda Expr");
        objGenReport.downloadReport(mnthlyReport);     
    }
}

Output

Printing Report in Daily Format using Lambda Expr
Printing Report in Monthly Format using Lambda Expr

With Lambda Expression	Without Lambda Expression
We pass the method definition as Argument	Method Definition in MonthlyReport.java Method Definition in DailyReport.java Object creation for DailyReport and MonthlyReport

Let’s take the below Example where we find the square of even numbers

ReduceEgs.java

public class ReduceEgs {
    public static void main(String[] args) {
        List<Integer> arrNumbers = Arrays.asList(1,2,4,5,7,6);

        arrNumbers.stream()
                  .filter(x-> x%2==0)
                  .map(x->x*x)
                  .forEach(System.out::println);
    }
}

In the above code we have

Filter which takes Predicate as Param
Map which takes Function as Param
Sysout which takes Consumer as Param

The filter, Consumer and sysout have been expanded from the above code as below.

public class ReduceEgs {
        public static void main(String[] args) {
            List<Integer> arrNumbers = Arrays.asList(1,2,4,5,7,6);
    
            arrNumbers.stream()
                      .filter(getIntegerPredicate())
                      .map(getFunction())
                      .forEach(getPrintln());
        }
    
        private static Consumer<Integer> getPrintln() {
            return System.out::println;
        }
    
        private static Function<Integer, Integer> getFunction() {
            return x -> x * x;
        }
    
        private static Predicate<Integer> getIntegerPredicate() {
            return x -> x % 2 == 0;
        }
}

The Above code is refactored as below

public class ReduceEgs {
        public static void main(String[] args) {
            List<Integer> arrNumbers = Arrays.asList(1,2,4,5,7,6);

            Function<Integer, Integer> getSquare = x -> x * x;
            Predicate<Integer>         getEvenNo = x -> x % 2 == 0;
            Consumer<Integer>          showNos   = System.out::println;

            arrNumbers.stream()
                      .filter(getEvenNo)
                      .map(getSquare)
                      .forEach(showNos);
        }
}

What the java compiler does internally changes the above code as below. It created an anonymous inner class for implementing the methods in the interface.

Function has apply method which should be implemented
Consumer has accept method which should be implemented
Predicate has test method which should be implemented

public class ReduceEgs {
        public static void main(String[] args) {
            List<Integer> arrNumbers = Arrays.asList(1,2,4,5,7,6);

            Function<Integer, Integer> getSquare = new Function<Integer, Integer>() {
                @Override
                public Integer apply(Integer integer) {
                    return integer * integer;
                }
            };

            Predicate<Integer> getEvenNo =  new Predicate<Integer>() {
                @Override
                public boolean test(Integer integer) {
                    return  integer % 2 == 0;
                }
            };

            Consumer<Integer> showNos = new Consumer<Integer>() {
                @Override
                public void accept(Integer integer) {
                    System.out.println(integer);
                }
            };

            arrNumbers.stream()
                      .filter(getEvenNo)
                      .map(getSquare)
                      .forEach(showNos);
        }
}

The output of all the above code is one and same as below

4
16
36

We have a Employee Object with following fields – empId,salary,empAge,totalExp,empName,location,pincode. Now we are going to do following operations in the List containing Employee Object.

import java.util.*;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class MatchEgs {
    public static void main(String[] args) {
        Employee objEmp1 = new Employee(101, "Mugil", 30, "Chennai", "600018", 5, 5000);
        Employee objEmp2 = new Employee(102, "Mani", 33, "Chennai", "600028", 4, 6000);
        Employee objEmp3 = new Employee(103, "Madhu", 32, "Chennai", "600054", 6, 10000);
        Employee objEmp4 = new Employee(104, "Vinu", 29, "Bangalore", "500234", 5, 15000);
        Employee objEmp5 = new Employee(105, "Srini", 40, "Delhi", "622142", 10, 7000);
        Employee objEmp6 = new Employee(106, "Dimple", 35, "Delhi", "622142", 5, 8000);

        List<Employee> arrEmployee = Arrays.asList(objEmp1, objEmp2, objEmp3, objEmp4, objEmp5, objEmp6);
 }
}

Check if people match the criteria by passing predicate to anyMatch, noneMatch and allMatch

        Predicate<Employee> empAgeGreaterThan35 = employee -> employee.getEmpAge() > 35;
        Predicate<Employee> empAgeGreaterThan30 = employee -> employee.getEmpAge() > 30;
        Predicate<Employee> empStayingInDelhi   = employee -> employee.getLocation().equals("Delhi");

        //Check if Some Employee Age is Greater 30
        System.out.println(arrEmployee.stream().anyMatch(empAgeGreaterThan35));

        //Check if all Employee Age is above 30
        System.out.println(arrEmployee.stream().allMatch(empAgeGreaterThan30));

        //Check if any Employee Location in Delhi
        System.out.println(arrEmployee.stream().noneMatch(empStayingInDelhi));

Sort Employee by Name passing Comparator using Sorted and reversed. Sort based on more than
one field using thenComparing

        Comparator<Employee> sortByName = Comparator.comparing(Employee::getEmpName);
        List<Employee> arrSortedByName = arrEmployee.stream()
                                                    .sorted(sortByName)
                                                    .collect(Collectors.toList());
        System.out.println("----------------Sorted By Name----------------");
        arrSortedByName.forEach(employee -> System.out.println(employee));

        Comparator<Employee> sortByNameReversed = Comparator.comparing(Employee::getEmpName).reversed();
        List<Employee> arrSortedByNameRev = arrEmployee.stream()
                                                       .sorted(sortByNameReversed)
                                                       .collect(Collectors.toList());
        System.out.println("----------------Sorted By Name Reversed----------------");
        arrSortedByNameRev.forEach(employee -> System.out.println(employee));

        Comparator<Employee> sortByNameanAge = Comparator.comparing(Employee::getEmpName).thenComparing(Employee::getEmpAge);
        List<Employee> arrSortedByNameAge = arrEmployee.stream()
                                                       .sorted(sortByNameanAge)
                                                       .collect(Collectors.toList());
        System.out.println("----------------Sorted By Name and Age----------------");
        arrSortedByNameAge.forEach(employee -> System.out.println(employee));

Limit and Skip records using limit and skip

        System.out.println("----------------Limit Records to 3 ----------------");
        List<Employee> arrEmp3 =  arrEmployee.stream()
                                             .limit(3)
                                             .collect(Collectors.toList());
        arrEmp3.forEach(employee -> System.out.println(employee));

        System.out.println("----------------Skip First 3 Records ----------------");
        List<Employee> arrEmp4 =  arrEmployee.stream()
                                             .skip(3)
                                             .collect(Collectors.toList());
        arrEmp4.forEach(employee -> System.out.println(employee));

doWhile when condition is true using takeWhile and vice-versa(Until condition is met) using dropWhile

        System.out.println("----------------Print Records until Chennai - Keeps printing until condition is True----------------");
        List<Employee> arrEmp5 =  arrEmployee.stream()
                                             .takeWhile(employee -> employee.getLocation().equals("Chennai"))
                                             .collect(Collectors.toList());
        arrEmp5.forEach(employee -> System.out.println(employee));

        System.out.println("----------------Print Records until Chennai - Stops printing when condition is Met ----------------");
        List<Employee> arrEmp6 =  arrEmployee.stream()
                                             .dropWhile(employee -> employee.getLocation().equals("Bangalore"))
                                             .collect(Collectors.toList());
        arrEmp5.forEach(employee -> System.out.println(employee));

Get Minimum and Maximum age of employee using min and max. The List should be sorted first using comparator. Similarly get Employee with max experience in a particular location using predicate, comparator and max.

        System.out.println("----------------Record with Min and Max Value ----------------");
        Comparator<Employee> sortByAge = Comparator.comparing(Employee::getEmpAge);
        List<Employee> arrEmp7 =  arrEmployee.stream()
                                             .min(sortByAge)
                                             .stream()
                                             .collect(Collectors.toList());
        List<Employee> arrEmp8 =  arrEmployee.stream()
                                             .max(sortByAge)
                                             .stream()
                                             .collect(Collectors.toList());
        arrEmp7.forEach(employee -> System.out.println(employee));
        arrEmp8.forEach(employee -> System.out.println(employee));

        System.out.println("----------------Get Employee with Max Exp in Chennai ----------------");
        Predicate<Employee> filterEmpInChennai = employee -> employee.getLocation().equals("Chennai");
        Comparator<Employee> sortByExp = Comparator.comparing(Employee::getTotalExp);
        List<Employee> arrEmp11 =  arrEmployee.stream()
                                              .filter(filterEmpInChennai)
                                              .max(sortByExp)
                                              .stream()
                                              .collect(Collectors.toList());
        arrEmp11.forEach(employee -> System.out.println(employee));

FindFirst Employee who matches criteria and FindAny who matches criteria. Both takes predicate as input.

findAny – Returns an Optional describing some element of the stream, or an empty Optional if the stream is empty.it is free to select any element in the stream. This is to allow for maximal performance in parallel operations;

        System.out.println("----------------Find First  ----------------");
        Predicate<Employee> empStayingInChennai = employee -> employee.getLocation().equals("Chennai");
        List<Employee> arrEmp9 =  arrEmployee.stream()
                                             .filter(empStayingInChennai)
                                             .findFirst()
                                             .stream()
                                             .collect(Collectors.toList());

        arrEmp9.forEach(employee -> System.out.println(employee));

        System.out.println("----------------Find Any  ----------------");
        List<Employee> arrEmp10 =  arrEmployee.stream()
                                              .filter(empAgeGreaterThan30)
                                              .findAny()
                                              .stream()
                                              .collect(Collectors.toList());
        arrEmp10.forEach(employee -> System.out.println(employee));

Get the Sum of salary of Employees in Location(Chennai) using sum

         System.out.println("----------------Sum - Get Sum of Salary in Chennai  ----------------");
        //Method 1
        Integer empTotalSalaryInChennai1 = arrEmployee.stream()
                                                      .filter(empStayingInChennai)
                                                      .map(employee -> employee.getSalary())
                                                      .collect(Collectors.toList())
                                                      .stream()
                                                      .reduce(0, Integer::sum);
        System.out.println("Sum of Empl Salary - Chennai " + empTotalSalaryInChennai1);

        //Method 2
        Integer empTotalSalaryInChennai2 = arrEmployee.stream()
                                                      .filter(empStayingInChennai)
                                                      .mapToInt(Employee::getSalary).sum();
        System.out.println("Sum of Empl Salary - Chennai " + empTotalSalaryInChennai2);

        //Method 3
        Integer empTotalSalaryInChennai3 = arrEmployee.stream() 
                                                      .filter(empStayingInChennai)
                                                      .map(employee -> employee.getSalary())
                                                      .collect(Collectors.summingInt(Integer::intValue));
        System.out.println("Sum of Empl Salary - Chennai " + empTotalSalaryInChennai3);

Get Max and Min salary of employee

  Comparator<Employee> cmpSalComp = (Employee objEmpp1, Employee objEmpp2) -> Double.compare(objEmpp1.Salary,objEmpp2.Salary);

  Employee empMaxObj = arrEmpl.stream()
                                   .max(cmpSalComp)
                                   .get();
                                   
  Employee empMinObj =   arrEmpl.stream()
                                 .min(cmpSalComp)
                                 .get();

Get Average salary of employee grouped by Location

        System.out.println("----------------Average - Grouped by Location  ----------------");
        Map<String, Double> arrAvgSalByLoc =  arrEmpl.stream()
                                                     .collect(Collectors.groupingBy(Employee::getLocation, Collectors.averagingDouble(Employee::getSalary)));
                                         
        System.out.print(arrAvgSalByLoc);

Get Average salary of employee in location using average

        System.out.println("----------------Average - Get Average of Salary in Chennai  ----------------");
        OptionalDouble empAvgSalaryInChennai = arrEmployee.stream()
                                                          .filter(empStayingInChennai)
                                                          .mapToInt(Employee::getSalary)
                                                          .average();
        System.out.println("Average Salary of Employee - Chennai " + empAvgSalaryInChennai);

Get List of Employees in a location using groupingBy. The Return type is hashmap with location as key and List of employees in value

        System.out.println("----------------Group By -  Get Employees grouped by Location  ----------------");
        Map<String, List<Employee>> hmEmp13 = arrEmployee.stream()
                                                         .collect(Collectors.groupingBy(Employee::getLocation));
        hmEmp13.forEach((s, employees) -> {
            System.out.println("Employees from "+ s);
            employees.forEach(employee -> System.out.println(employee));
        });

Get Employee grouped by Location and getting Maximum Salary groupingBy and maxBy

        System.out.println("----------------Group By -  Get Employees with Max Salary in Location  ----------------");
        Comparator<Employee> cmprSalary = Comparator.comparing(Employee::getSalary);
        Map<String, Optional<Employee>> hmEmp14 = arrEmployee.stream()
                                                             .collect(Collectors.groupingBy(Employee::getLocation, Collectors.maxBy(cmprSalary)));

        hmEmp14.forEach((s, employee) -> {
            System.out.print("Employee Location is "+ s +  " and salary is ");
            employee.ifPresent(emp -> System.out.println(emp.getSalary()));
        });

Get Employee Name using mapping grouped by Location using groupingBy

        System.out.println("---------------- Employee at Location -  Using Collectors.mapping  ----------------");
        Map<String, List<String>> hmEmp16 = arrEmployee.stream()
                                                       .collect(Collectors.groupingBy(Employee::getLocation, Collectors.mapping(Employee::getEmpName, Collectors.toList())));

        hmEmp16.forEach((s, employee) -> {
            System.out.println("Employee Name is " + employee + " and Location is "+ s );

        });

Output

true
false
false
----------------Sorted By Name----------------
Employee{empId=106, empName='Dimple', empAge=35, totalExp=5, location='Delhi', pincode='622142', salary=8000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
----------------Sorted By Name Reversed----------------
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
Employee{empId=106, empName='Dimple', empAge=35, totalExp=5, location='Delhi', pincode='622142', salary=8000}
----------------Sorted By Name and Age----------------
Employee{empId=106, empName='Dimple', empAge=35, totalExp=5, location='Delhi', pincode='622142', salary=8000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
----------------Limit Records to 3 ----------------
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
----------------Skip First 3 Records ----------------
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
Employee{empId=106, empName='Dimple', empAge=35, totalExp=5, location='Delhi', pincode='622142', salary=8000}
----------------Print Records until Chennai - Keeps printing until condition is True----------------
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
----------------Print Records until Chennai - Stops printing when condition is Met ----------------
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
----------------Record with Min and Max Value ----------------
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
----------------Get Employee with Max Exp in Chennai ----------------
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
----------------Find First  ----------------
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
----------------Find Any  ----------------
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
----------------Sum - Get Sum of Salary in Chennai  ----------------
Sum of Empl Salary - Chennai 21000
Sum of Empl Salary - Chennai 21000
Sum of Empl Salary - Chennai 21000
----------------Average - Get Average of Salary in Chennai  ----------------
Average Salary of Employee - Chennai OptionalDouble[7000.0]
----------------Group By -  Get Employees grouped by Location  ----------------
Employees from Delhi
Employee{empId=105, empName='Srini', empAge=40, totalExp=10, location='Delhi', pincode='622142', salary=7000}
Employee{empId=106, empName='Dimple', empAge=35, totalExp=5, location='Delhi', pincode='622142', salary=8000}
Employees from Chennai
Employee{empId=101, empName='Mugil', empAge=30, totalExp=5, location='Chennai', pincode='600018', salary=5000}
Employee{empId=102, empName='Mani', empAge=33, totalExp=4, location='Chennai', pincode='600028', salary=6000}
Employee{empId=103, empName='Madhu', empAge=32, totalExp=6, location='Chennai', pincode='600054', salary=10000}
Employees from Bangalore
Employee{empId=104, empName='Vinu', empAge=29, totalExp=5, location='Bangalore', pincode='500234', salary=15000}
----------------Group By -  Get Employees with Max Salary in Location  ----------------
Employee Location is Delhi and salary is 8000
Employee Location is Chennai and salary is 10000
Employee Location is Bangalore and salary is 15000
---------------- Employee at Location -  Get Employee Object grouped by Location  ----------------
Employee Location is Delhi
Srini
Dimple
Employee Location is Chennai
Mugil
Mani
Madhu
Employee Location is Bangalore
Vinu
---------------- Employee at Location -  Using Collectors.mapping  ----------------
Employee Name is [Srini, Dimple] and Location is Delhi
Employee Name is [Mugil, Mani, Madhu] and Location is Chennai
Employee Name is [Vinu] and Location is Bangalore

How to find the Stream is Primitive or Wrapper?

public class PrimStreams {
    public static void main(String[] args) {
        List<Integer> arrReviews1 = List.of(8, 9, 8 , 6, 2, 7, 6);
        System.out.println(arrReviews1.stream());

        int[] arrReviews2 = {8, 9, 8 , 6, 2, 7, 6};
        System.out.println(Arrays.stream(arrReviews2));

        System.out.println("----------Wrapper Streams Starts----------");
        System.out.println(arrReviews1.stream().reduce(Integer::sum));
        System.out.println(arrReviews1.stream().reduce(Integer::min));
        System.out.println(arrReviews1.stream().reduce(Integer::max));
        System.out.println(arrReviews1.stream().mapToInt(Integer::intValue).average().orElse(Double.NaN));

        System.out.println("----------Primitive Streams Starts----------");
        System.out.println(Arrays.stream(arrReviews2).sum());
        System.out.println(Arrays.stream(arrReviews2).min());
        System.out.println(Arrays.stream(arrReviews2).max());
        System.out.println(Arrays.stream(arrReviews2).average());
    }
}

In the below output you may see the wrapper class stream would be printed as ReferencePipeline whereas primitive stream would be displayed as IntPipeline. The above code contains doing some basic operation in both primitive and wrapper stream. If the stream is primitive type it would be easy to carry out operation over stream and more memory efficient since there would be no boxing and unboxing involved
Output

java.util.stream.ReferencePipeline$Head@1d251891
java.util.stream.IntPipeline$Head@2133c8f8
----------Wrapper Streams Starts----------
Optional[46]
Optional[2]
Optional[9]
6.571428571428571
----------Primitive Streams Starts----------
46
OptionalInt[2]
OptionalInt[9]
OptionalDouble[6.571428571428571]

How to create a primitive stream of Int?

  IntStream arrReviews3 = IntStream.of(8, 9, 8 , 6, 2, 7, 6);

Basic Operations in primitive stream of Int?

range – Considers nos in specific range.range doesnot takes upper limit no.To address that rangeClosed is used
iterate– Iterate helps in iterating till a particular range by taking Initial and urnaryOperator as params. The default behaviour of Iterator is endless loop.To stop after particular limit like one we did in range we should use limit function.
peek– peek helps going through the values in stream without making any changes. This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline
boxed– boxed will help in boxing primitive to wrapper while collecting the values in to a list.

IntStream arrReviews = IntStream.of(8, 9, 8 , 6, 2, 7, 6);

        System.out.println("----------Stream printing Range of no ----------");

        System.out.println("range doesnot include last no 10 so sum is from 1 to 9 - " + IntStream.range(0,10).sum());
        System.out.println("rangeClosed would include last no 1 to 10 - " + IntStream.rangeClosed(0,10).sum());

        System.out.println("Using Peek we can look at nos in stream");
        System.out.println(IntStream.rangeClosed(0,10)
                                    .peek(System.out::println)
                                    .sum());

        System.out.println("iterate helps in iterating till a specific value");
        System.out.println(IntStream.iterate(1,e->e+1)
                                    .peek(System.out::println)
                                    .limit(10)
                                    .sum());

        System.out.println("iterate helps in iterating till a specific value");
        System.out.println(IntStream.iterate(1,e->e+2)
                                    .peek(System.out::println)
                                    .limit(10)
                                    .sum());

        System.out.println("Iterating over range and collecting in a list");
        List<Integer> arrIntegStream = IntStream.iterate(1,e->e+1)
                                                .peek(System.out::println)
                                                .limit(10)
                                                .boxed()
                                                .collect(Collectors.toList());
        System.out.println(arrIntegStream);

Finding Factorial of No of range 5

public class Factorial {
    public static void main(String[] args) {
        System.out.println(IntStream.rangeClosed(1,5).reduce(1, (x,y)->x*y));

        int i,fact=1;
        int number=5;//It is the number to calculate factorial
        for(i=1;i<=number;i++){
            fact=fact*i;
        }
        System.out.println(fact);
    }
}

Find Maximum of Number in Stream

class HelloWorld {
    public static void main(String[] args) {
       List<Integer> arrNum  = List.of(3,4,5,2,1);
       
       Integer max = arrNum.stream()
                           .max(Integer::compare)
                           .get();
                            
       System.out.print(max);
    }
}

Output

The same when used in Objects comparator should be used as one below.

Find Maximum of Number in Stream

.
.       
      Comparator<Employee> sortByExp = Comparator.comparing(Employee::getTotalExp);
      List<Employee> arrEmp11 =  arrEmployee.stream()      
                                            .max(sortByExp);
.
.

Basic String Operation in Streams

Join strings in a list using Collectors.joining
Make Array of String using split. Split would return Array Stream
Apply transformation in elements in list using replaceAll
To flatten the Array Stream in the List use flatMap
Get rid of particular element in List of String using removeIf
Use replaceAll to carry Out Transformation similar to map()
Use peek to support debugging, where you want to see the elements as they flow past a certain point in a pipeline

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class StringOps {
    public static void main(String[] args) {
        List<String> arrSkills = List.of("Core Java", "Core Spring", "Spring MVC", "Spring Security", "Spring Boot", "RestAPI");

        System.out.println("-----Join Elements in List using Seperator - [,]---------");
        String skills = arrSkills.stream().collect(Collectors.joining(","));
        System.out.println("Skills are "+ skills);

        System.out.println("-----Split String and Store in String Array---------");
        String[] arrSkillList = skills.split(",");
        Arrays.stream(arrSkillList).forEach(System.out::println);

        System.out.println("-----Convert to UpperCase---------");
        List<String> arrModSkills = new ArrayList(arrSkills);
        arrModSkills.replaceAll(str -> str.toUpperCase());
        arrModSkills.forEach(System.out::println);

        System.out.println("-----Using flatMap to flatten Arrays of Stream and Getting Distinct Location---------");
        List<String> arrLocation = List.of("Madras,Mumbai,Bangalore", "Madras,Kolkatta,Delhi", "Delhi,Madras");
        List allLocations = arrLocation.stream()
                                       .map(location->location.split(","))
                                       .peek(loc-> System.out.println(loc))
                                       .flatMap(Arrays::stream)
                                       .distinct()
                                       .collect(Collectors.toList());

        allLocations.stream()
                    .map(loc->replaceMadras(loc.toString()))
                    .forEach(System.out::println);

        System.out.println("------------Remove Particular Location using removeIf------------");
        allLocations.removeIf(loc->loc.equals("Delhi"));
        allLocations.forEach(loc-> System.out.println(loc));
    }

    private static String replaceMadras(String location){
        if(location.equals("Madras"))
            return "Chennai";
        else
            return location;
    }
}

Converting Multiple List into Single List

public class StreamMisc {
    public static void main(String[] args) {
        List<List<String>> arrSkills = new ArrayList<>();
        arrSkills.add(Arrays.asList("Java", "C++", "C#"));
        arrSkills.add(Arrays.asList("HTML", "CSS"));
        arrSkills.add(Arrays.asList("SQL", "MySQL"));
        arrSkills.add(Arrays.asList("Azure"));

        List<String> arrAllSkills = arrSkills.stream().reduce(new ArrayList<>(), (a, b) -> {
            a.addAll(b);
            return a;
        });

        arrAllSkills.forEach((skill) -> System.out.print(skill + ","));
    }
}

Output

Java,C++,C#,HTML,CSS,SQL,MySQL,Azure

Higher-order functions are functions that take other functions as arguments or return functions as their results.

In the below example we Search employee by Location using Predicate.
If we are not using HOF we need to assign new predicate to variable each and everytime like empFromDelhi
Instead of doing that we have a HOF getEmpByLocation which takes location as string in arguments and returns function in its return type

public class HigherOrdFns {
    public static void main(String[] args) {
        Employee objEmp1 = new Employee(101, "Mugil", 30, "Chennai", "600018", 5, 5000);
        Employee objEmp2 = new Employee(102, "Mani", 33, "Chennai", "600028", 4, 6000);
        Employee objEmp3 = new Employee(103, "Madhu", 32, "Chennai", "600054", 6, 10000);
        Employee objEmp4 = new Employee(104, "Vinu", 29, "Bangalore", "500234", 5, 15000);
        Employee objEmp5 = new Employee(105, "Srini", 40, "Delhi", "622142", 10, 7000);
        Employee objEmp6 = new Employee(106, "Dimple", 35, "Delhi", "622142", 5, 8000);

        List<Employee> arrEmployee = Arrays.asList(objEmp1, objEmp2, objEmp3, objEmp4, objEmp5, objEmp6);

        Predicate<Employee> empFromDelhi = employee -> employee.getLocation().equals("Delhi");

        arrEmployee.stream().filter(empFromDelhi).forEach(System.out::println);
        arrEmployee.stream().filter(getEmpByLocation("Chennai")).forEach(System.out::println);
        arrEmployee.stream().filter(getEmpByLocation("Bangalore")).forEach(System.out::println);
    }

    //Higher Order function which return function as return type
    private static Predicate<Employee> getEmpByLocation(String location) {
        return employee -> employee.getLocation().equals(location);
    }
}

## Summary

Below are the List of Features available in Code

1. Reading payload from JSON File and using it as mocked Response
2. Object creation from JSON File
---

## Things Newly Implemented

1. JSON Payload files are found in src/test/resources/json-data
2. In the Code Converstion to Employee Object after reading Employee JSON and Stringify of JSON Object are both done  
3. MockMvc Testing is implemented for Controllers	
	- For AddEmployee Endpoint - Request from JSON file, Response from mocked response  
	- For GetEmployee Endpoint - Request by URL, Response from mocked Employee object constructed from JSON File
	- For GetAllEmployees Endpoint - Request by URL, Response from method returning Employee Object
	- For DeleteEmployee Endpoint - Request by URL, Response from mocked method	 	  
---

## Changes in pom.xml 
```xml
<build>
    <testResources>
        <testResource>
            <directory>${project.basedir}/src/test/resources</directory>
        </testResource>			
    </testResources>
    .
    .
    .
</build>
```
---

# API endpoints
## GET
`Get All Employees Detail` [/empmgmt/employees](#getallemployee) <br/>
`Get Employee Details` [/empmgmt/employees/{empId}](#getemployeesempid) <br/>

## POST
`Add Employee Details` [/empmgmt/employees](#addemployee) <br/>

## DELETE
`Delete EmpDetails` [/empmgmt/employees/{empId}](#deleteemployee) <br/>
___

<a name="getemployeesempid"></a>
### GET /empmgmt/employees/{empId}
Get Employee Detail

**Parameters**

|          Name | Required |  Type   | Description                                                                                                                                                           |
| -------------:|:--------:|:-------:| --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
|     `empId` | required | string  | Employee Id

**Response**

```
// Employee Details added Succesfully
[
    {
        "empID": 1,
        "empName": "Mugil",
        "empAge": "36"
    }
]
```
___

<a name="getallemployee"></a>
### GET /empmgmt/employees

**Response**
```
// Employee Details added Succesfully
[
    {
        "empID": 1,
        "empName": "Mugil",
        "empAge": "36"
    }
]
```
___

<a name="addemployee"></a>
### POST /empmgmt/employees
Add Employee Details

**Parameters**

|          Name | Required |  Type   | Description                                                                                                                                                           |
| -------------:|:--------:|:-------:| --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
|     `empName` | required | string  | Name of Employee
|     `empAge` | required | string  | Age of Employee

**Request**
```
// Employee Details 
{
    "empName" : "Mugil",
    "empAge" : "36"
}
```


**Response**

```
// Employee Details added Succesfully
[
    {
        "empID": 1,
        "empName": "Mugil",
        "empAge": "36"
    }
]
```
___

<a name="deleteemployee"></a>
### DELETE /empmgmt/employees/{empId}

**Parameters**

|          Name | Required |  Type   | Description                                                                                                                                                           |
| -------------:|:--------:|:-------:| --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
|     `empId` | required | string  | Employee Id


**Response**

```
Employee Deleted Successfully
```
___



## Note

* While reading file from resources folder I continously got error telling the file doesnot exists. 
  The Same problem is faced while working in office project. After adding above lines in pom.xml it started 
  working. It kept working, even after removing above lines
* The URL returned in _Link in add employee wont have a port number. The test keeps failing if you copy and paste
  URL from postman and try to asset the _links in mockMVC

___

Copy paste the code in URL to see in action : https://markdownlivepreview.com/

Readme file screen shot for above code

forEach method in java.lang.Iterable interface
Java 8 has introduced forEach method in java.lang.Iterable interface so that while writing code we focus on business logic. The forEach method takes java.util.function.Consumer object as an argument, so it helps in having our business logic at a separate location that we can reuse.

myList.forEach(new Consumer < Integer > () {
    public void accept(Integer t) {
        System.out.println("forEach anonymous class Value::" + t);
    }

});

//traversing with Consumer interface implementation
MyConsumer action = new MyConsumer();
myList.forEach(action);

Consumer implementation that can be reused

class MyConsumer implements Consumer<Integer>{

	public void accept(Integer t) {
		System.out.println("Consumer impl Value::"+t);
	}
}

Interfaces with default and static methods
From Java 8, interfaces are enhanced to have a method with implementation. We can use default and static keyword to create interfaces with method implementation

We know that Java doesn’t provide multiple inheritance in Classes because it leads to Diamond Problem. So how it will be handled with interfaces now since interfaces are now similar to abstract classes. Compiler will throw an exception in this scenario and we will have to provide implementation logic in the class implementing the interfaces.

@FunctionalInterface
public interface Interface1 {

    void method1(String str);

    default void log(String str) {
        System.out.println("I1 logging::" + str);
    }

    static void print(String str) {
        System.out.println("Printing " + str);
    }
}

interfaces are not allowed to have Object default methods.

@FunctionalInterface annotation. Functional interfaces are a new concept introduced in Java 8. An interface with exactly one abstract method becomes a Functional Interface. We don’t need to use @FunctionalInterface annotation to mark an interface as a Functional Interface.
@FunctionalInterface annotation is a facility to avoid the accidental addition of abstract methods in the functional interfaces. You can think of it like @Override annotation and it’s best practice to use it. java.lang.Runnable with a single abstract method run() is a great example of a functional interface.

Anonymous Class Implementation
```
Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("My Runnable");
    }
};
```
Lambda Expression Implementation
```
Runnable r1 = () - > {
    System.out.println("My Runnable");
};
```
If you have single statement in method implementation anonymous class can be instantiated using lambda expression as below
lambda expressions Implementation
```
Interface1 i1 = (s) -> System.out.println(s);		
i1.method1("abc");
```

New java.util.stream has been added in Java 8 to perform filter/map/reduce like operations with the collection. Stream API will allow sequential as well as parallel execution.

public static void main(String[] args) {

    List < Integer > myList = new ArrayList < > ();
    for (int i = 0; i < 100; i++) myList.add(i);

    //sequential stream
    Stream < Integer > sequentialStream = myList.stream();

    //parallel stream
    Stream < Integer > parallelStream = myList.parallelStream();

    //using lambda with Stream API, filter example
    Stream < Integer > highNums = parallelStream.filter(p - > p > 90);
    //using lambda in forEach
    highNums.forEach(p - > System.out.println("High Nums parallel=" + p));

    Stream < Integer > highNumsSeq = sequentialStream.filter(p - > p > 90);
    highNumsSeq.forEach(p - > System.out.println("High Nums sequential=" + p));

}

parallel processing values are not in order, so parallel processing will be very helpful while working with huge collections.

IO improvements known to me are:
Files.list(Path dir) that returns a lazily populated Stream, the elements of which are the entries in the directory.
Files.lines(Path path) that reads all lines from a file as a Stream.
Files.find() that returns a Stream that is lazily populated with Path by searching for files in a file tree rooted at a given starting file.
BufferedReader.lines() that return a Stream, the elements of which are lines read from this BufferedReader.
Java Time API packages, I can sense that they will be very easy to use. It has some sub-packages java.time.format that provides classes to print and parse dates and times and java.time.zone provides support for time zones and their rules.

Minimal Requirment

Java 17 or higher
Jakarta EE 10
Spring Framework 6
Works on Maven 3.5+
Tomcat 10.0
Improved observability with Micrometer and Micrometer Tracing

Improvements
Performance enhancements and optimizations to boost application responsiveness and efficiency.These improvements focus on reducing startup times, minimizing memory footprint, and optimizing resource utilization.

Changes in Code

When we wanted to configure the Security settings, we had to extend the WebSecurityConfigurerAdapter class.This class has been deprecated and removed in Spring Security 6.
Instead, we should now take a more component-based approach and create a bean of type SecurityFilterChain.

@Configuration
@EnableWebSecurity
public class WebSecurityConfig {
 @Bean
  public SecurityFilterChain configure(HttpSecurity http) throws Exception {
    http
        .authorizeHttpRequests(requests -> requests
            .requestMatchers(new AntPathRequestMatcher("/openapi/openapi.yml")).permitAll()
            .anyRequest().authenticated())
        .httpBasic();
    return http.build();
  }
}

Instead of using authorizeRequests, which has been deprecated, we should now use authorizeHttpRequests.This method is part of the HttpSecurity configuration and allows you to configure fine-grained request matching for access control.
Spring Security 6, AntMatcher, MvcMatcher, and RegexMatcher have been depreciated and replaced by requestMatchers or securityMatchers for path-based access control. This allows us to match requests based on patterns or other criteria without relying on specific matchers.

Why we need Custom Exception
Exceptions are technical causes which should be wrapped in a presentable way with a specific business logic and workflow. adding a higher abstraction layer for the exception handling, which results in more meaningful and readable API

How to write Custom Exception
All you need to do is create a new class and have it extend Exception. If you want an Exception that is unchecked, you need to extend RuntimeException.
Note: A checked Exception is one that requires you to either surround the Exception in a try/catch block or have a ‘throws’ clause on the method declaration. (like IOException) Unchecked Exceptions may be thrown just like checked Exceptions, but you aren’t required to explicitly handle them in any way (IndexOutOfBoundsException).

Exception, if you want your exception to be checked (i.e: required in a throws clause).RuntimeException, if you want your exception to be unchecked.

Creating Custom Checked Exception
Note:constructor takes a Throwable’s subclass which is the origin (cause) of the current exception

public class StudentStoreException extends Exception {
     public StudentStoreException(String message, Throwable cause) {
        super(message, cause);
    }
}

Creating Custom Unchecked Exception

public class IncorrectFileExtensionException 
  extends RuntimeException {
    public IncorrectFileExtensionException(String errorMessage, Throwable err) {
        super(errorMessage, err);
    }
}

Methods in Custom Exception

public class MyOwnException extends Exception {
    public MyOwnException() {

    }

    public MyOwnException(String message) {
        super(message);
    }

    public MyOwnException(Throwable cause) {
        super(cause);
    }

    public MyOwnException(String message, Throwable cause) {
        super(message, cause);
    }
}

Thread – direction or path that is taken while a program is executed

Below code would explain how dirty read happens when multiple thread(2 threads) tries to access Instance variable at once from two different class objects
Below we have 3 classes, One for Printing Report and Other for removing the report printed. Third for tracking the report status
The Output of the code would be Consistently 0 every time which is expected when the number of reports to be printed is in range of less than 100. However the output changes with more the no of reports to be printed. I.E. totalReportsToBePrinted = 10000000
This happens because for lower value of reports to be printed the thread executes fast with out context switching however for higher values other thread(RemovePrintedReports thread takes control before PrintExcelReports get completed) takes control which leads to inconsistency
The Same code would return 0 every time if one thread(PrintExcelReports) completes before other(RemovePrintedReports) as below when we use join and didn’t start both the threads at once.
```
.
.
        t1.start();
        t1.join();
        
        t2.start();
        t2.join();

.
.
```

PrintExcelReports.java

public class PrintExcelReports implements Runnable {
    TotalReportCount totalReportCount;

    public PrintExcelReports(TotalReportCount totalReportCount) {
        this.totalReportCount = totalReportCount;
    }

    int totalReportsToBePrinted = 1000000;

    @Override
    public void run() {
        for(int i=0;i<totalReportsToBePrinted;i++){
            totalReportCount.totalReportsCntVal -= i;
        }
    }
}

RemovePrintedReports.java

public class RemovePrintedReports implements Runnable {
    TotalReportCount totalReportCount;

    public RemovePrintedReports(TotalReportCount totalReportCount) {
        this.totalReportCount = totalReportCount;
    }

    int totalReportsToBePrinted = 1000000;

    @Override
    public void run() {
        for(int i=0;i<totalReportsToBePrinted;i++){
            totalReportCount.totalReportsCntVal -= i;
        }
    }
}

ReportCurrentStatus.java

public class ReportCurrentStatus {
    public static void main(String[] args) throws InterruptedException {
        TotalReportCount objTotalReportCount = new TotalReportCount();
        objTotalReportCount.totalReportsCntVal = 0;

        PrintExcelReports objPrinter1 = new PrintExcelReports(objTotalReportCount);
        RemovePrintedReports objPrinter2 = new RemovePrintedReports(objTotalReportCount);

        Thread t1 = new Thread(objPrinter1);
        Thread t2 = new Thread(objPrinter2);

        t1.start();
        t2.start();
        t1.join();
        t2.join();

        System.out.println(objTotalReportCount.totalReportsCntVal);
    }
}

Output when totalReportsToBePrinted is greater than 1000

RANDOM NUMBER

Output when totalReportsToBePrinted is less than 100

What is Class Loading?
Loading is the process of finding the binary representation of a class or interface type with a particular name and creating a class or interface from that binary representation.

When Class is Loaded?

Class loading done by Class Loaders may eagerly load a class or does lazy load
Eager loading happens as soon as another class references it and lazy load happens when a need of class initialization occurs.
If a Class is loaded before it’s actually being used, it can sit inside before being initialized which may vary from JVM to JVM.
However, it’s guaranteed by JLS that a class will be loaded when there is a need of static initialization.

Whenever the JVM determines it needs a class (to use its static variables, to create a new object, to use its static methods etc) it will load the class and static initialization blocks will run, static variables are initialized etc. This is (at least under normal circumstances) done only once

SomeClass.someStaticMethod(); //SomeClass is loaded (if its not already)
SomeClass.someStaticVariable; //SomeClass is loaded (if its not already)
SomeClass var=new SomeClass(); //SomeClass is BOTH loaded (if its not already) AND instantiated

Class loader actually loads byte code into JVM , runs static initializers when you want to use static fields within class without creating an instance of it, class must be loaded by class loader first.Default classloader in java is java.lang.ClassLoader

Class Loading is mostly unpredictable

When Class is Initialized?
Class Initialization happens after class loading. A class is initialized in Java when

An Instance of class is created using either new() keyword or using reflection using class.forName(), which may throw ClassNotFoundException in Java.
an static method of Class is invoked.
an static field of Class is assigned.
an static field of class is used which is not a constant variable.
if Class is a top level class and an assert statement lexically nested within class is

Which classes are Eagerly Loaded and Lazily Loaded

Eagerly Loaded Classes are those the JVM must be able to load JVM class files. The JVM class loader loads referenced JVM classes that have not already been linked to the runtime system. Classes are loaded implicitly because: The initial class file – the class file containing the public static void main(String args[]) method – must be loaded at startup. Depending on the class policy adopted by the JVM, classes referenced by this initial class can be loaded in either a lazy or eager manner.

An eager class loader loads all the classes comprising the application code at startup. Lazy class loaders wait until the first active use of a class before loading and linking its class file.

The first active use of a class occurs when one of the following occurs: • An instance of that class is created • An instance of one of its subclasses is initialized • One of its static fields is initialized. Certain classes, such as ClassNotFoundException, are loaded implicitly by the JVM to support execution. You may also load classes explicitly using the java.lang.Class.forName() method in the Java API, or through the creation of a user class loader.

Based on the above rules, lazy class loading can be forced on the system.

Use java -XX:+TraceClassLoading to trace the loading of classes.

Use java -XX:+PrintCompilation to trace when methods are JITed.

CodeThatAint

Category Archives: Java

How Lambda Expression Works Internally

Real Time Example Using Lambda Functions

Streams in Action

Higher Order Functions

Readme file example

Java 8 New Features

New Features in Spring Boot 3

Writing Custom Exception in Java

Basics of Threads

All about Class Loading and initialization