The singleton pattern restricts the instantiation of a class to one object. INSTANCE is a public static final field that represents the enum instance. We can get the instance of the class with the EnumSingleton.INSTANCE but it is better to encapsulate it in a getter in case if we want to change the implementation.
There are a few reasons why we can use an enum as a singleton in Java
- Guaranteed one instance (Cannot instantiate more than one enum even through reflection).
- Thread-safe.
- Serialization.
Syntax
public enum Singleton { INSTANCE; private singleton() { } }
Example
public enum EnumSingleton { INSTANCE; private String name; private int age; private void build(SingletonBuilder builder) { this.name = builder.name; this.age = builder.age; } public static EnumSingleton getSingleton() { // static getter return INSTANCE; } public void print() { System.out.println("Name: "+name + ", age: "+age); } public static class SingletonBuilder { private final String name; private int age; private SingletonBuilder() { name = null; } public SingletonBuilder(String name) { this.name = name; } public SingletonBuilder age(int age) { this.age = age; return this; } public void build() { EnumSingleton.INSTANCE.build(this); } } public static void main(String[] args) { new SingletonBuilder("Adithya").age(25).build(); EnumSingleton.getSingleton().print(); } }
Output
Name: Adithya, age: 25