Java继承，泛型和字节码

Question

When a class in Java inherits another, does the inherited class become (physically) part of the inheriting class such that its byte-code is added to the .class file for the inheriting class? Or does the .class file for the inheriting class contain only the part specific to it and it's at run time that object details from the two .class files of both classes are built together into one object? If either or both classes was generic, would it have effect on the process? Thx.

Answer 1

When A extends B, then only that information is written into the class file for A.

There is no copying of code of any form from B.class to A.class. Meaning: when you change B, there is normally no need to recompile A.

And generics are mainly a compile time feature - any generic type turns into Object in bytecode (that is called type erasure ). The class file contains a bit of meta information about which entities were declared as generic type, but thats all. In other words: Java generics are not the same thing as C++ templates.

Answer 2

The second one, for example if you compile the following two classes:

public class Parent {
  public void myMethod(String arg) {
  }
}

public class Child extends Parent {
  public void myMethod(String arg) {
    super.myMethod(arg);
  }
}

Then, after you had compiled both classes, you change the Parent class like:

public class Parent {
  public void myMethod(Integer arg) {
  }
}

You'll get a NoSuchMethodError , this happens because Child call the parent's myMethod at runtime.

I hope it was clear, bye.