《Cracking the Coding Interview》——第14章：Java——题目1

2014-04-26 18:20

题目：从继承的角度，把构造函数设成private有什么意义？

解法：就不能继承了。单体模式里也这么干，目的是为了不让使用者自主生成对象，进行限制。

代码：

 // 14.1 In terms of inheritance, what is the point of declaring the constructor as private?
 // Answer:
 //    1. private member cannot be inherited, thus the class cannot be inherited.
 //    2. constructor is private, so you cannot new an object freely. Only through public static member method can you get an instance of this class.
 //    3. with this you can implement Singleton Pattern, which ensures that a class has at most one instance within an application. You can but you don't have to do it.
 public class TestJava {
     private static TestJava instance = null;

     public static TestJava getInstance() {
         if (instance == null) {
             instance = new TestJava();
         }
         return instance;
     }

     public void f() {
         System.out.println("TestJava::f()");
     }

     private TestJava() {
     }

     public static void main(String[] args) {
         System.out.println("Hello world.");
         TestJava testJava = TestJava.getInstance();
         testJava.f();
     }
 }