CODEWITHSUNDEEP
javacasting
sumit
sumit

Reputation: 3720

Casting a class into another non hierarchical class form in java

Consider class A

class A {
    int i;
    int j;
    int k;
}

and here is the class B

class B {
    int a;
    int b;
    int c;
}

I was wondering if java has any such ting which allows to write/define our own custom casting logic.

For e.g. to cast class B's object into class A's object where

i -> a // i pointing to value of a

j -> b // j pointing to value of b

k -> c // k pointing to value of c

(I can customize the logic to whatever i wish)

I have a few heavy weight objects to "cast" into some other classes to use and i do not want to write a converter method for this.

(casting operates only on the object in consideration and does not create another object)

any thoughts/suggestions for this ?

Thanks in Anticipation !

Upvotes: 0

Views: 68

Answers (3)

Mateusz Stefek
Mateusz Stefek

Reputation: 3846

No. There's no such thing as unions known from C. You cannot modify the memory directly in JVM. That's the point of Java. It sacrifices some potential for tricks, but in the end the code tends to be less buggy and easier to maintain than the code written in C.

If your concern is memory consumption caused by copying, you can convert A and B into interfaces. The interfaces should expose getters and setters. For example: int getI() and int getA(). Then you can introduce a class implementing both of them. The indirection caused by the interface calls will have some performance cost, but in many cases, it won't be noticable.

Upvotes: 0

user6754053
user6754053

Reputation:

This defeats the safety purposes of Java for two reasons.

Let's consider an updated version of your class code.

class A {
    int i;
    int j;
    int k;
    int l;
}

class B {
    int a;
    int b;
    int c;
}

Now imagine what would happen if you did something like this, assuming the classes were written right above: (Warning:code will not compile)

public static void main(String[] args)
{
    B classTwo = new B();
    A classOne = (A) classTwo;
    classOne.l = 3; // <-- what would happen?
}

As you can see, it won't work, because creating an instance of B will not allocate the variabls properly. This is why Java does not permit casting like that.

What Java does permit is converting between superclasses and subclasses.

For example:

class Fruit {
    int i;
}
class Apple extends Fruit {
    int j;
}

And now inside the main function: Fruit f = new Fruit(); Apple a = (Apple)f; // this compiles, but will raise a ClassCastException

You'll ask, why does it raise a ClassCastException? Apple extends Fruit.

Yes, Apple extends Fruit. But, Fruit does not allocate memory for the j variable.

What is allowed is the other way around, casting an Apple to a Fruit, because all Apples have properties of Fruits (even in real life).

If you want to use your idea of casting, you could try an interface.

interface C {
    public int getVarOne();
    public int getVarTwo();
    // NO variables here, only functions
}

Any class which implements (not extends) an interface must also implement all of the methods defined in it.

Once you've implemented the interface, the behavior is similar to casting to a superclass. There are also other uses for interfaces, but that is beyond the scope of this question.

Upvotes: 1

Darshan Mehta
Darshan Mehta

Reputation: 30819

You can use the frameworks like ModelMapper (documentation) to define your maping logic and use it to convert object from one type to object of another type. E.g. this is how the configuration would be:

//Model Classes
class A{
    int a;
}
class B{
    int d;
}

//Mappings
PropertyMap<A, B> map = new PropertyMap<A, B>() {
    protected void configure() {
        map(source.a, destination.d);
      }
};

//Test Program
public static void main(String[] args) throws Exception {
    ModelMapper mapper = new ModelMapper();

    PropertyMap<A, B> map = new PropertyMap<A, B>() {
        protected void configure() {
            map(source.a, destination.d);
          }
    };
    mapper.addMappings(map);
    A a = new A();
    a.a = 10;
    B b = mapper.map(a, B.class);
    System.out.println(b.d);
}

Upvotes: 2

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