How to override the behavior of a class if I have only its object?

Question

The question may be a stupid one to ask but, kindly help me on this. I need to override the behavior of a class, but I will get only the object of it. Following is the code sample.

I want to use the method Util.newChildComponent(String id) itself in all the places.

class ParentComponent{
    public ParentComponent(String id){}
    protected void someBehavior(){}
}

class ChildComponent extends ParentComponent{
    public ChildComponent(String id){
        super(id);
    }
    protected void childsBehavior(){}
}

public class Util {
    public static ParentComponent newChildComponent(String id)
    {
        ParentComponent fc = new ChildComponent(id);
        // Initialize the object, and performs some common configuration.
        // Performs different operations on the generated child object.
        // The child class has many members apart from childsBehavior().
        return fc;
    }
}


// The above classes belongs to a jar, so I cannot edit the code.
// I need to use Util.newChildComponent(String id)
// But I need to override the behavior as mentioned below.
public void f1()
{
    // TODO: How to override childsBehavior() method using the object 
    // I have it in pc? Is it possible?
    // Util.newChildComponent(id) method decorates the ChildComponent
    // So I need to use the same object and just override childsBehavior() method only.
    ParentComponent pc = Util.newChildComponent("childId");

    // I need to achieve the result below
    /*
    ParentComponent pc = new ChildComponent(id){
            @Override
            protected void childsBehavior(){
                super.someBehavior();
                // Do the stuff here.
            }
        }; */
}

Thanks in advance.

Answer 1

Sounds like you are looking for a DynamicObjectAdaptorFactory .

Here's a use of his excellent object.

static class ParentComponent {

    protected void someBehavior() {
        System.out.println("ParentComponent someBehavior");
    }
}

static class ChildComponent extends ParentComponent {

    private ChildComponent(String id) {

    }

    protected void childsBehavior() {
        System.out.println("childsBehavior");
    }
}

public static class Util {

    public static ParentComponent newChildComponent(String id) {
        ParentComponent fc = new ChildComponent(id);
        // Initialize the object, and performs some common configuration.
        return fc;
    }
}

interface Parent {

    void someBehavior();

}

// The adaptor.
public static class Adapter implements Parent {

    final ParentComponent parent;

    private Adapter(ParentComponent pc) {
        this.parent = pc;
    }

    // Override the parent behaviour.
    public void someBehavior() {
        System.out.println("Adapter invoke someBehaviour()");
        parent.someBehavior();
        System.out.println("Adapter finished someBehaviour()");
    }

}

public void test() {
    ParentComponent pc = Util.newChildComponent("childId");
    Parent adapted = DynamicObjectAdapterFactory.adapt(pc, Parent.class, new Adapter(pc));
    adapted.someBehavior();
}

Prints:

Adapter invoke someBehaviour()
someBehavior
Adapter finished someBehaviour()

Answer 2

Have a look at Java's Dynamic Proxy. This should allow you to change the behavior at runtime by creating invocation handlers to handle a call to a method. You can even then forward it to the original method if you want.

Answer 3

Declare a new class that extends the one with the method you want to change

public class MyClass extends ChildComponent {

    @Override
    protected void childsBehavior()
    {
     .....
    }

}

Then in your code you can use casting, which is a feature we get to enjoy thanks to inheritance

MyClass pc = (myClass) Util.newChildComponent("childId");

But then you would have to create the new object like so

MyClass pc = new MyClass("childId");

and do the configurations by hand that would have been done at Util . Unless you extend Util and crete a method that creates an object of MyClass and copy the configuration code there.

And now pc will have all the methods of ParentComponent and ChildComponent , but the childsBehavior will be what you declared.

Answer 4

If you don't want to change the code that uses Util to instantiate the objects you need, you could rewrite the Util class with the behavior you need (ie instantiating a class that extends ChildComponent with the overridden method), just remember to put it in a package of the same name in your project (so that the import of its clients doesn't change). This way no other code should change at all.