单例模式

饿汉模式

//饿汉模式
class Bank{
    private Bank(){     
    }
    private static Bank bank = new Bank();
    
    public static Bank hugger(){
        return bank;
    }
}

懒汉模式

class Bank1{
    private Bank1(){
        
    }
    private static Bank1 bank = null;
    
    public static  Bank1 lazse(){
        
        if(bank == null){
            synchronized(Bank1.class){
                if(bank == null){
                    bank = new Bank1();
                }
            }
        }
        return bank;
    }