用旧的覆盖新的软件包

Question

Considering a java application with a embedded X impl/jar file, for example com.test package.
and we cannot change and modify the jar file.
Now suppose I have a X1.jar file which is modified version of exist X(com.test) package.

Question:
Is it possible to override the X1.jar implementation with embedded(exist) implementation just before the library gets loaded? and again, assume we cannot change the package anyway.
I'd like that to redirect all the types under com.test package to the new one.
Thanks in advance.

Answer 1

As per m0skit0's suggestion:

Orig class:

package example.x
public class SomeClass
{
    public SomeClass(String neededVar)
    {...}

    public String someMethod(String someVar)
    {
        // original implementation
    }
}

Custom class:

import example.x.SomeClass
public class MySomeClass extends SomeClass
{
    public MySomeClass(String neededVar)
    {
        super(neededVar);
        // anything else
    }

    @Override
    public String someMethod(String someVar)
    {
        // implement differently
        super.someMethod(someVar);
        // or add to it
    }
}