A.People who are in the habit of switching from one bank to another. B.Young people who are fond of modern technology. C.Young people who are wealthy and well-educated. D.People who have computers at home.
【单选题】下面的代码实现了设计模式中的什么模式 public class A { private A instance; private A() { } public static A getInstance() { if ( A == null ) instance = new A(); return instance; } }