NullPointerException:覆盖派生类中基类的构造函数调用方法 [英] NullPointerException : Overriding constructor calling method of Base class in Derived class

问题描述

我有这个代码片段:

class Base {
    public Base() {
        method();
    }

    void method() {
        System.out.println("In Base");
    }
}

class Derived extends Base {
    private String bar;

    public Derived() {
        bar="bar";
    }

    public void method() {
        System.out.println(bar.length());
    }

    public static void main(String[] args) {
        Base base=new Derived();
        base.method();
    }
}

在执行代码时出现异常:

On executing the code I got an exception:

Exception in thread "main" java.lang.NullPointerException
    at Derived.method(Main.java:22)
    at Base.<init>(Main.java:5)
    at Derived.<init>(Main.java:17)
    at Derived.main(Main.java:27)

我无法理解为什么有 NullPointerException 和 stackTrace 异常.谁能帮我理解一下?

I'm unable to understand why there is NullPointerException and the stackTrace of the exception. Anyone could help me to understand?

您可以在此处查看代码.

推荐答案

new Derived() 创建一个 Derived 对象，这意味着首先调用其超类构造函数，即反过来调用 method - 但是你已经覆盖了 method 所以它是被调用的方法的子版本.在该方法中，您调用尚未初始化的 bar.length.

new Derived() creates a Derived object, which implies calling its super class constructor first, which in turn calls method - but you have overriden method so it is the child version of that method which is called. In that method, you call bar.length which has not been initialised yet.

结论:在构造函数中调用可重写方法几乎从来都不是一个好主意.

Conclusion: it is almost never a good idea to call an overridable method in a constructor.

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