How to emulate C# as-operator in Java

Question

There are situations, where it is practical to have a type-cast return a null value instead of throwing a ClassCastException. C# has the as operator to do this. Is there something equivalent available in Java so you don't have to explicitly check for the ClassCastException?

Answer 1

You can use the instanceof keyword in place of C#'s is, but there is nothing like as.

Example:

if(myThing instanceof Foo) {
   Foo myFoo = (Foo)myThing; //Never throws ClassCastException
   ...
}

Answer 2

I'd think you'd have to roll your own:

return (x instanceof Foo) ? (Foo) x : null;

EDIT: If you don't want your client code to deal with nulls, then you can introduce a Null Object

interface Foo {
    public void doBar();
}
class NullFoo implements Foo {
    public void doBar() {} // do nothing
}
class FooUtils {
    public static Foo asFoo(Object o) {
        return (o instanceof Foo) ? (Foo) o : new NullFoo();
    }
}
class Client {
    public void process() {
        Object o = ...;
        Foo foo = FooUtils.asFoo(o);
        foo.doBar(); // don't need to check for null in client
    }
}

Answer 3

I'm speculating you could propably creas an as operator

something like

as<T,Type> (left, right)  
which evaluates to 
if (typeof(left) == right)
   return (right)left
else
    return null

I'm not sure how you'd do it, I'm a c# at the moment and my Java hat has gotten a bit dusty since I left university.

Answer 4

You could write a static utility method like this. I don't think it's terribly readable, but it's the best approximation of what you're trying to do. And if you use static imports it wouldn't be too bad in terms of readability.

package com.stackoverflow.examples;
public class Utils {
    @SuppressWarnings("unchecked")
    public static <T> T safeCast(Object obj, Class<T> type) {
        if (type.isInstance(obj)) {
            return (T) obj;
        }
        return null;
    }
}

Here's a test case that demonstrates how it works (and that it does work.)

package com.stackoverflow.examples;
import static com.stackoverflow.examples.Utils.safeCast;
import static junit.framework.Assert.assertNotNull;
import static junit.framework.Assert.assertNull;

import org.junit.Test;

public class UtilsTest {

    @Test
    public void happyPath() {
        Object x = "abc";
        String y = safeCast(x, String.class);
        assertNotNull(y);
    }

    @Test
    public void castToSubclassShouldFail() {
        Object x = new Object();
        String y = safeCast(x, String.class);
        assertNull(y);
    }

    @Test
    public void castToUnrelatedTypeShouldFail() {
        Object x = "abc";
        Integer y = safeCast(x, Integer.class);
        assertNull(y);
    }
}

Answer 5

Here's an implementation of as, as suggested by @Omar Kooheji:

public static <T> T as(Class<T> clazz, Object o){
 if(clazz.isInstance(o)){
  return clazz.cast(o);
 }
 return null;
}

as(A.class, new Object()) --> null
as(B.class, new B())  --> B