CODEWITHSUNDEEP
javaclassobject
Andreas Sandberg
Andreas Sandberg

Reputation: 261

Converting Class of an Object into a Child Class

Say I have the following classes

public abstract class Parent {

    public Parent() {
    
        //stuff here
    
    }

}

public class Child extends Parent {

    public Child() {
    
        //stuff here
    
    }

    public void doSomething() {
    
    }

}

Then I get an object from an array of Parents

public Parent[] parents = new Parent[5];

Parent tempParent = parents[0]

If this object is of the child class, I want to cast the tempParent into the child class in order to use "doSomething" without having to create a new object

if(tempParent.getClass == Child.class) {

    tempParent = (Child) tempParent;

    tempParent.doSomething();

}

This, however, does not work. I still get an error telling me that doSomething is not a function in the Parent class. Is there a way to do what I'm trying to do?

Upvotes: 2

Views: 2223

Answers (3)

Omegaspard
Omegaspard

Reputation: 1980

I think you're trying to solve the issue from the wrong side. Technically speaking you should do it like this.

public abstract class Parent {

    public Parent() {
    
        //stuff here
    
    }

    public abstract void doSomething();

}

public class Child extends Parent {

    public Child() {
    
        //stuff here
    
    }

    @Override
    public void doSomething() {
        // Write the code of doSomething specific to child.
    }

}

A bit off topic but you might want to consider the following:

I think that the class representation of your code is bad.

I think that you should have 2 concrete classes Parent and Child that are composed or either inherit from a common interface. Even better use composition over inheritance.

https://en.wikipedia.org/wiki/Composition_over_inheritance

Upvotes: 3

mre
mre

Reputation: 44250

if you are able to modify the parent class, I would recommend that you make doSomething an abstract method and have the child class override it. that way, there is no need to cast in order to invoke it.

https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html

Upvotes: 0

GhostCat
GhostCat

Reputation: 140623

You need to use the correct TYPE on the LEFT hand side:

Child someChild = (Child) tempParent;

Note: this does not create a new Object.

There is no way of having

Parent whatever = (Child) tempParent;

do what you want to do (invoke a Child class method)!

You see, the compiler only cares about the "defined" type of your reference, it does not take into account that you most likely, at runtime, just "put" something into that reference that in deed is a Child instance!

to do what you want to do.

And note: do not compare objects using ==. You should prefer to call equals() instead. For Class objects == should work (most of the time), but still: get used to check for equality using equals().

Upvotes: 5

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