在构造函数中初始化一个静态 final 字段 [英] Initialize a static final field in the constructor

问题描述

public class A 
{    
    private static final int x;

    public A() 
    {
        x = 5;
    }
}

• final 表示变量只能赋值一次(在构造函数中).
• static 表示它是一个类实例.
• final means the variable can only be assigned once (in the constructor).
• static means it's a class instance.

我不明白为什么禁止这样做.这些关键字在什么地方相互干扰?

I can't see why this is prohibited. Where do those keywords interfere with each other?

推荐答案

每次创建类的实例时都会调用一个构造函数.因此，上面的代码意味着每次创建实例时都会重新初始化 x 的值.但是因为变量被声明为final(和static)，你只能这样做

A constructor will be called each time an instance of the class is created. Thus, the above code means that the value of x will be re-initialized each time an instance is created. But because the variable is declared final (and static), you can only do this

class A {    
    private static final int x;

    static {
        x = 5;
    }
}

但是，如果您删除静态，则可以这样做:

But, if you remove static, you are allowed to do this:

class A {    
    private final int x;

    public A() {
        x = 5;
    }
}

或者这个:

class A {    
    private final int x;

    {
        x = 5;
    }
}

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