当您调用构造函数（新类）时，实例初始化程序块首先运行会发生什么事情？ [英] What exaclty happens when a you call a constructor(new Class),do instance initializer blocks runs first?

问题描述

我是Java尤其是OOP的一个完整的初学者，所以请原谅我的天真，但是最近在经历Head First Java时，他们说，构造函数是当有人说 new 时运行的代码
但是我尝试了这段代码以查看实际发生了什么，但是令我惊讶的是输出完全不同。

Im a complete beginner in the Java especially OOP so please pardon my naivety but recently while going through Head First Java,they said,A constructor is a code that runs when somebody says new on a class type. But then i tried this code to see what actually happened but to my surprise the output was totally different.

public class Test {
    private int n;
    {System.out.println("Out1 "+n);}
    public Test() {
       n=10;
       System.out.println("Inside Constructor");
    }

    public static void main(String args[]) {
        System.out.println("Hello World!");
        Test obj=new Test();
        }
    {System.out.println("Out2 "+n);}
}

输出：

Hello World！

Out1 0

Out2 0

内部构造函数

我的问题：为什么？我调用构造函数初始化特定的实例变量后，实例变量是否应立即初始化？这是否意味着构造函数的全部目的要在制造对象之前运行！

The Output:
Hello World!
Out1 0
Out2 0
Inside Constructor
My question:WHY?Shouldnt the instance variable get initialized as soon as i call the contructor to initialize the specific instance variable?Isnt that the whole purpose of constructors?to run before making the object!

推荐答案

构造函数按以下顺序执行：

The constructor executes in the following order:

1. 调用超类的构造函数。


2. 运行
   
3. 执行构造函数的其余部分。

即，两个sysout语句都在赋值 n = 10; 之前执行。那就是应该的。请参见 JLS§12.5。

I.e., both of your sysout statements execute just before the assignment n=10;. That's how it should be. See JLS §12.5.

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