Simple Program to print numbers using threads
NumberPrinter.java

public class NumberPrinter implements Runnable{
    int number;
    public NumberPrinter(int number){
        this.number = number;
    }

    public void run(){
        System.out.println("Printing Number from Thread "+ this.number);
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        for (int idx=1;idx<=5;idx++){
            Thread objthread = new Thread(new NumberPrinter(idx));
            objthread.start();
        }
    }
}

Output

Printing Number from Thread 5
Printing Number from Thread 1
Printing Number from Thread 4
Printing Number from Thread 3
Printing Number from Thread 2

Simple Program using Executor Service taking Runnable as Argument

ExecutorService is a framework which allows to create thread. Threads can be created from FixedThreadPool, CachedThreadPool and ScheduledThreadPool. submit() method takes runnable or callable object (Functional Interface Type) as argument. The Same code above can be rewritten as below
Main.java

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService objExecService = Executors.newFixedThreadPool(2);

        //Lambda Expresssion passed as Argument as Runnable is FI
        objExecService.submit(() -> {
            System.out.println(Thread.currentThread().getName());
        });

        objExecService.shutdown();
    }
}

Output

pool-1-thread-1

Same code with Runnable instance passed as argument to submit

  .
  . 
  //Instance of Runnable passed as argument
  HelloThread1 objHT1 = new HelloThread1();
  objExecService.submit(objHT1);
  .
  .

Output

Hello World from Thread Name (pool-1-thread-1) using Runnable 

Same code with Runnable as Anonymous Class passed as argument

ExecutorService exec = Executors.newFixedThreadPool(2);

//Instance of Runnable passed as Anonymous class
exec.execute(new Runnable() {
  public void run() {
    System.out.println("Hello world");
  }
});

exec.shutdown();

Simple Program using Executor Service taking Callable as Argument

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService objExecService = Executors.newFixedThreadPool(2);
        Future<String> objFuture = objExecService.submit(new HelloThread2());
        System.out.println(objFuture.get());
        objExecService.shutdown();
    }
}

Output

Hello World from Thread Name (pool-1-thread-1) using Callable

Using Lambda Expression as Submi

.
.
ExecutorService objExecService = Executors.newFixedThreadPool(2);

Future<String> objFuture = objExecService.submit(() -> {
  Thread.sleep(3000);
  return Thread.currentThread().getName();
});

System.out.println(objFuture.get());
.
.

The above could be rewritten in anonymous class as below

ExecutorService objExecService = Executors.newFixedThreadPool(2);

Future<String> objFuture = objExecService.submit(new Callable<String>() {
 @Override
 public String call() throws Exception {
   Thread.sleep(3000);
   return Thread.currentThread().getName();
 }
});

System.out.println(objFuture.get());
objExecService.shutdown();

Program for Creating Thread Pool and executing Task

ThreadPoolExample.java

public class ThreadPoolExample {
    public static void main(String args[]) {
       ExecutorService service = Executors.newFixedThreadPool(10); //create 10 worker threads in Thread Pool
       for (int i =0; i<100; i++){
           service.submit(new Task(i)); //submit that to be done 
       }
       service.shutdown();
    }  
}

Task.java

final class Task implements Runnable {
    private int taskId;  
    public Task(int id){
        this.taskId = id;
    }
  
    @Override
    public void run() {
        System.out.println("Task ID : " + this.taskId +" performed by " 
                           + Thread.currentThread().getName());
    }  
}

Task ID : 0 performed by pool-1-thread-1
Task ID : 3 performed by pool-1-thread-4
Task ID : 2 performed by pool-1-thread-3
Task ID : 1 performed by pool-1-thread-2
Task ID : 5 performed by pool-1-thread-6
Task ID : 4 performed by pool-1-thread-5