Is there a form of "type safe" casting in Java?

Question

Suppose I have two classes deriving from a third abstract class:

public abstract class Parent{
    public Parent(){
    }
}

public class ChildA extends Parent {
    public ChildA {
    }
}

public class ChildB extends Parent {
    public ChildB {
    }
}

In C# I could handle casting in a somewhat type safe manner by doing:

ChildA child = obj as ChildA;

Which would make child == null if it wasn't a ChildA type object. If I were to do:

ChildA child = (ChildA)obj;

...in C# this would throw an exception if the type wasn't correct.

So basically, is there a way to to do the first type of casting in Java? Thanks.

Duplicate: http://stackoverflow.com/questions/148828/how-to-emulate-c-sharp-as-operator-in-java — Michał Powaga, Dec 27 '11 at 22:37
Apologies. I searched for "type safe casting java". Didn't occur to me to search for "as keyword java". — garrettendi, Dec 27 '11 at 22:53

score 77 · Accepted Answer · answered Dec 27 '11 at 22:35

77

I can't think of a way in the language itself, but you can easily emulate it like this:

ChildA child = (obj instanceof ChildA ? (ChildA)obj : null);

answered Dec 27 '11 at 22:35

Aleks G

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What about obj instanceof List ? (List)obj : null? You can not use instance of for a generic type, because it will disappear at runtime. – vajanko Nov 16 '14 at 10:46
1

@vajanko You're right, you can't do that with a generic type if you also need to check for type of items. You could still do `List ? (List)obj : null` - but you'll get compile-time warnings. The question didn't say anything about generics. – Aleks G Nov 16 '14 at 18:19

score 27 · Answer 2 · answered Feb 12 '16 at 07:44

27

In java 8 you can also use stream syntax with Optional:

    Object o = new Integer(1);

    Optional.ofNullable(o)
            .filter(Number.class::isInstance)
            .map(Number.class::cast)
            .ifPresent(n -> System.out.print("o is a number"));

answered Feb 12 '16 at 07:44

Dmitry Klochkov

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Number.class::cast is f*** amazing! this one's great! – Martín Zaragoza Sep 10 '19 at 17:08

Fabienr_75 · Answer 3 · 2017-06-21T22:30:03.367

20

You can use this method which is compatible with all java types :

public static <T> T safeCast(Object o, Class<T> clazz) {
    return clazz != null && clazz.isInstance(o) ? clazz.cast(o) : null;
}

Example :

// A given object obj
Integer i = safeCast(obj, Integer.class);

edited Jun 21 '17 at 22:30

answered Jun 21 '17 at 15:40

Fabienr_75

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score 9 · Answer 4 · answered Dec 27 '11 at 22:35

9

You can use the instanceof operator.

if(obj instanceof ChildA){
     final ChildA child = (ChildA) obj;
}

answered Dec 27 '11 at 22:35

asenovm

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score 3 · Answer 5 · answered Mar 22 '22 at 21:59

3

In modern Java you would use

if (obj instanceof ChildA childa) {

}

which is nearly the same in modern C#

if (animal is Dog dog)
{
    // Use dog here
}

answered Mar 22 '22 at 21:59

Martin Meeser

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ghilesZ · Answer 6 · 2022-10-03T11:05:42.087

I think a good way to deal with this is to use generic methods, which is a reusable/safe option as in the following example:

@SuppressWarnings("unchecked")
public <E> E coerceTo(Object o, Class<E> target) throws IllegalArgumentException {
  if(target.isInstance(o)) return (E)o;
  String msg = "expected "+target.getName()+" but was "+o.getClass().getName();
  throw new IllegalArgumentException(msg);
}

Note that here, the cast only occurs when safe, and it's correct to add the suppressWarnings annotation.

Here's an example of how to call the method:

 Object o = 1;
 int a = coerceTo(o, Integer.class);

score 2 · Answer 7 · answered Dec 27 '11 at 22:36

You can always just check first:

if (child instanceof ChildA) {
    ChildA child = (ChildA) child;
    // Do stuff.
}

Or just make a quick method:

public ChildA getInstance(Parent p) {
    if (child instanceof ChildA) {
        return (ChildA) p;
    } else {
        return null;
    }
}

Is there a form of "type safe" casting in Java?

7 Answers7