Question

I have a JAR file that I’m using and I want to modify one of the files inside it. In a nutshell I have

public class ClassB {
    public void printMethod(){
       ClassA A = new ClassA();
       A.printout();
    }   
}

public class ClassA {
  public void printout(){
    System.out.println("1234");
  }
}

and I want to change ClassA’s printout method to

public class ClassA {
  public void printout(){
    System.out.println("abcd");
  }
}

I know you cannot modify a JAR without unpacking/rebuilding it and for this, let’s say I can’t do that. Is there a way to make modifications to ClassA without actually touching the current ClassA? My approach is to have a new class inherit from ClassA with an overridden method and then have a new class inherit from ClassB that calls the Inherited ClassA

public class InheritedClassA extends ClassA{
  @Override
  public void printout(){
    System.out.println("abcd");
  }
}

public class InheritedClassB extends ClassB{
  @Override
  public void printMethod(){
    InheritedClassA A = new InheritedClassA();
    A.printout();
  }
}

I don’t like this approach though because in my actual JAR, so many classes are using ClassA that its a nightmare trying to correctly do this with all of them, which then all require the same process on them. I know you cannot overload/overwrite a whole class which is basically what I want to do. Is there another way to do this?

EDIT
To make it even harder, I cannot download any new frameworks or software or anything.

Answer 1

One option which may not be feasible would be to create a new version of ClassA, package it up in its own jar file, and put that ahead of the original version in your classpath.

However, this is a pretty odd scenario – why can you not update the existing jar file? Even if that means a bit of extra work, it’s likely to be much cleaner in the long run than any other approach.