java - 请问下，这里为何会引发空指针异常？

Question

有人跟我讲，将name进行静态赋值即可，但实例变量不也是属于成员变量吗，也是在整个类实例化的时候产生的啊，跟静态赋值有何区别吗？

Answer 1

The constructor of the parent class is executed before the constructor of the subclass. Base()->test()-->name.length()相当于null.length()。
将nameIf it is written as static, of course it is OK, because the initialization of static members precedes the initialization of instance members.

The order is roughly as follows:

• Parent classstatic{...}

• Static members of parent class

• Parent class construction method

• Subclassstatic{...}

• Subclass static members

• Subclass construction method

Answer 2

First throw out a concept, the initialization process of an object:
Static variable> Static initialization block> Instance variable> Constructor静态变量 > 静态初始化块 > 实例变量 > 构造器
而存在父子类关系的对象，又存在一个嵌套的初始化流程
父类初始化流程 > 子类初始化流程And objects with a parent-child class relationship also have a nested Initialization process

Parent class initialization process> Subclass initialization process

test()方法时，子类的name还没有赋值，仍然是nullSo during your instantiation process, if you call the parent class constructor and call

, a null pointer will naturally be reported. 🎜

Answer 3

You take name放在父类Base中定义就不会报错。 因为你实例化Sub时，会调用默认的构造函数，默认的构造函数会调用父类的构造函数，在父类的构造函数中，你使用了test()方法，而你在子类中重写了该方法，子类的test方法内使用了name，但是这时候name还没有完成初始化。所以会报NullPointerException.