Mutable and immutable objects in java

Question

Can anyone explain me on a simple example the difference between mutable and immutable objects in java?

Answer 1

Mutable objects are objects whose state can be changed.

A state in Java is implemented with data fields.

An example of mutable object:

class Counter {
    private int i = 0;

    public void increase() {
         i++;
    }
}

So i represents an internal state of the class Counter here. And it can be changed as time goes by:

 Counter counter = new Counter();
 counter.increase(); // somewhere in the code

On the other hand, Immutable objects are objects whose state can't be changed once the object is created/initialized.

These objects should not have 'mutators' - setters, or in general methods that change an internal state.

Here is an example of an immutable object:

public final class Person {
    private final String name;
    private final int age;

    public Person(String name, int age) {
        this.name = name;
        this.age  = age;
    } 

    public String getName() {
        return this.name;
    }

    public int getAge() {
        return this.age;
    }
}

As you see - although this class maintains the state (in fields name and age), it's impossible to change this state after the object is created (the constructor is called).