NullPointerException：覆盖Derived类中的Base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

我有这段代码：

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();
    }
}

执行代码时我得到了一个异常：

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 和异常的 stackTrace 。任何人都可以帮助我理解？

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（）创建派生 object，这意味着首先调用它的超类构造函数，然后调用方法 - 但是你已经覆盖方法所以它是被调用的那个方法的子版本。在该方法中，您调用 bar.length 尚未初始化。

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：几乎从不一个好主意是在构造函数中调用一个可覆盖的方法。

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

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