Casting class to interface in java

Question

Why interface allow all classes?

For example I have the below code :

interface I1 { }
class C1 { }

public class Test{
     public static void main(String args[]){
        C1 o1 = new C1();
        I1 x = (I1)o1; //compiler compile its successfully but failed in run time
     }
}

I know why its failed in runtime because C1 class does not implements I1 interface. If Class C1 implements I1 then above code will work successfully.

Can someone explain why interface allow all class casting?

Here you find your answer http://stackoverflow.com/questions/10388136/java-casting-resulting-in-run-time-error-instead-of-compilation-error — bpoiss, Jan 27 '16 at 13:40
@Satya, he knows that. He's asking why the problem appears at run-time, instead of at compile-time. — ROMANIA_engineer, Jan 27 '16 at 13:41

score 5 · Answer 1 · answered Jan 27 '16 at 13:41

5

There could be a

class C2 extends C1 implements I1 { ... }

An object of this class could be assigned to o1 and the cast would be perfectly OK also at runtime.

answered Jan 27 '16 at 13:41

Henry

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score 4 · Answer 2 · answered Jan 27 '16 at 13:41

The issue is that the cast happens at runtime. This is because there might be a class C2 that derives from C1 and implements I1. So the object might actually be castable to I1 at runtime, but in general you can't tell at compilation time (except for super trivial examples like the one provided).

score 3 · Answer 3 · answered Jan 27 '16 at 13:40

It's because Interface can be implemented by more than one class. On the other hand class can inherit only from one class. In class case compiler can determine at compile time if cast can be accomplished, for interfaces compiler can't figure it out for 100%.

score 2 · Answer 4 · answered Jan 27 '16 at 13:43

2

It's a bad idea. The point of using an interface is to implement it like this:

    class C2 extends C1 implements I1 {

answered Jan 27 '16 at 13:43

Abdelhak

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score 2 · Answer 5 · answered Jan 27 '16 at 13:44

2

When you are casting, you are actually telling the compiler that "please compile this piece of code, i am sure, this is of correct type".

This is not a property of interface, you can write

C1 o1 = new C1();
Integer i = (Integer) o1;

answered Jan 27 '16 at 13:44

Adil Shaikh

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score 1 · Answer 6 · answered Jan 27 '16 at 13:45

1

Since C1 is not implementing I1, there is NO compromise between those 2, that is the reason of the error.

answered Jan 27 '16 at 13:45

ΦXocę 웃 Пepeúpa ツ

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Casting class to interface in java

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