Java单例设计模式-确保一个类只有一个对象

// 饿汉式单例
public class A {
    private static A a = new A();

    private A(){

    }

    public static A geObject(){
        return a;
    }

}

// 懒汉式单例
public class B {
    private static B b;

    private B(){

    }

    public static B geObject(){
        if(b == null){
            b = new B();
        }
        return  b;
    }
}