Singleton Pattern in Java

The singleton pattern ensures that there is exactly one instance of a class.

There are multiple ways to implement the Singleton Pattern in Java, here is my personal favorite:

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public final class SomeClass {
 
  private static final class SingletonHolder {
    private static final SomeClass INSTANCE = new SomeClass();
    private SingletonHolder() {}
  }
 
  private SomeClass() {
    // Add code if needed
  }
 
  /**
    * @return the Singleton Instance.
    */
  public static SomeClass getInstance() {
    return SomeClass.SingletonHolder.INSTANCE;
  }
 
  // add other methods
 
}

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Notes:

• Replace SomeClass with the class name, keep all other names exactly as given.
• Class is final to ensure its constructor cannot be made public by subclassing.
• The SingletonHolder nested class enables lazy loading and ensures synchronization without performance loss.
• Constructor must be made private.
• getInstance returns the singleton instance of this class.