Java – Mutable and Immutable Objects

This article shows you the difference between Mutable and Immutable objects in Java

1. Mutable object – You can change the states and fields after the object is created. For examples: StringBuilder, java.util.Date and etc.

2. Immutable object – You cannot change anything after the object is created. For examples: String, boxed primitive objects like Integer, Long and etc.

1. Java Mutable Example

Normally, it provides a method to modify the field value, and the object can be extended.

package com.mkyong;

public class MutableExample {

	private String name;

	MutableClass(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	// this setter can modify the name
	public void setName(String name) {
		this.name = name;
	}

	public static void main(String[] args) {

		MutableExample obj = new MutableExample("mkyong");
		System.out.println(obj.getName());

		// update the name, this object is mutable
		obj.setName("new mkyong");
		System.out.println(obj.getName());

	}
}

Output

mkyong
new mkyong

2. Java Immutable Example

To create an Immutable object, make the class final, and don’t provide any methods to modify the fields.

package com.mkyong;

// make this class final, no one can extend this class
public final class ImmutableExample {

	private String name;

	ImmutableExample (String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	//no setter
	
	public static void main(String[] args) {

		ImmutableExample obj = new ImmutableExample("mkyong");
		System.out.println(obj.getName());

		// there is no way to update the name after the object is created.
		// obj.setName("new mkyong");
		// System.out.println(obj.getName());

	}
}

Output

mkyong

P.S Please refer to the Effective Java Book – Item 15: Minimize mutability.

References

1. Mutable vs immutable objects
2. Immutable objects as per java docs