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How to make a singleton enum in Java?
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
Updated on: 22-Nov-2023
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