There are many immutable classes like String, Boolean, Byte, Short, Integer, Long, Float, Double etc. In short, all the wrapper classes and String class is immutable

Immutable objects have a number of properties that make working with them easier, including relaxed synchronization requirements and the freedom to share and cache object references without concern for data corruption.

An immutable object is one whose externally visible state cannot change after it is instantiated. The String, Integer, and BigDecimal classes in the Java class library are examples of immutable objects — they represent a single value that cannot change over the lifetime of the object.

Immutable classes generally make the best map keys.

Potential problem with a mutable Date object

  Date d = new Date();
  Scheduler.scheduleTask(task1, d);
  d.setTime(d.getTime() + ONE_DAY);
  scheduler.scheduleTask(task2, d);

Since Date is mutable, the scheduleTask method must be careful to defensively copy the date parameter into its internal data structure. Otherwise, task1 and task2 might both execute tomorrow, which is not what was desired.

Classic Value Objects
Strings and integers and are often thought of as values. Therefore its not surprising to find that String class and the Integer wrapper class (as well as the other wrapper classes) are immutable in Java. A color is usually thought of as a value, thus the immutable Color class.

Playing Cards

Ever write a playing card program? I did. I could have represented a playing card as a mutable object with a mutable suit and rank. A draw-poker hand could be 5 fixed instances where replacing the 5th card in my hand would mean mutating the 5th playing card instance into a new card by changing its suit.

However, I tend to think of a playing card as an immutable object that has a fixed unchanging suit and rank once created. My draw poker hand would be 5 instances and replacing a card in my hand would involve discarding one of those instance and adding a new random instance to my hand.

Simple Immutable Class Code

public final class Employee
   {  
    final String pancardNumber;  
      
    public Employee(String pancardNumber){  
    this.pancardNumber=pancardNumber;  
    }  
      
    public String getPancardNumber(){  
    return pancardNumber;  
  }  
 }  

1. The instance variable of the class is final i.e. we cannot change the value of it after creating an object.
2. The class is final so we cannot create the subclass.
3. There is no setter methods i.e. we have no option to change the value of the instance variable.