懒汉式单例模式

public class Singleton {
private static Singleton instance;

    private Singleton() {}
    
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

饿汉式单例模式

public class Singleton {
private static Singleton instance = new Singleton();

    private Singleton() {}
    
    public static Singleton getInstance() {
        return instance;
    }
}