为什么我的子类没有在 java 中正确初始化变量
在下面的代码中，myString 始终初始化为 null。我必须在 init() 或类似的函数中手动初始化。据我所知它与超类/子类有关，但我不明白确切的机制
public class A extends B {
private String myString = "test";
public static void main(String[] args) {
new A();
}
public A() {
super();
}
public void c() {
System.out.println(myString);
}
}
public class B {
public B() {
c();
}
public void c() {
}
}
In the code below, myString is always initialized to null. I have to manually initialize in an init() or similar. As far as I can tell it is related to superclass/subclass but I don't understand the exact mechanism
public class A extends B {
private String myString = "test";
public static void main(String[] args) {
new A();
}
public A() {
super();
}
public void c() {
System.out.println(myString);
}
}
public class B {
public B() {
c();
}
public void c() {
}
}
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评论（3）
您的代码的问题是，
myString在类A的构造函数的开头初始化，但在超级构造函数之后（即类B）。由于您之前从类B的构造函数访问变量（通过调用重写方法c间接），因此您会得到此行为。根据经验：如果您想避免意外行为，请不要在执行构造函数之前调用重写方法。
The issue with your code is, that
myStringis initialized at the begin of the constructor of classAbut right after the super constructor (i.e. classB). Since you access the variable before from the constructor of classB(indirectly via call to overriden methodec) your get this behaviour.As a rule of thumb: if you want to avoid unexpected behavior do not call overriden methods before the constructor has been executed.
在完全创建对象并完成对超类构造函数的调用后，立即添加对
c();覆盖方法的调用。将您的代码更改为此..
Add a call to
c();overidden method right after the object has been fully created and call to superclass constructor is done.Change your code to this ..
《Thinking in Java Second Edition》作者：Bruce Eckel，多态方法的行为
构造函数内部（第 337-339 页）。
Thinking in Java Second Edition by Bruce Eckel, Behavior of polymorphic methods
inside constructors (p. 337-339).