Singleton

public class  {
    private volatile static Singleton uniqueInstance;
    
    private () {}
    
    private static Singleton getInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }  
            }
        }
        return uniqueInstance;
    }
}

//急切模式
public class  {
    private static Singleton uniqueInstance = new Singleton();
    
    private () {}
    
    private static Singleton getInstance() {
        return uniqueInstance;
    }
}