《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();
    }
}