一：

public class A {
    private static A instance=null;

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

二:

public class B {
    private B() {
    }

    private static class BHolder {
        private final static B instance = new B();
    }

    public static B getInstance() {
        return BHolder.instance;
    }
}