Java array reflection: isArray vs. instanceof

Question

In most cases, you should use the instanceof operator to test whether an object is an array.

Generally, you test an object's type before downcasting to a particular type which is known at compile time. For example, perhaps you wrote some code that can work with a Integer[] or an int[]. You'd want to guard your casts with instanceof:

if (obj instanceof Integer[]) {
    Integer[] array = (Integer[]) obj;
    /* Use the boxed array */
} else if (obj instanceof int[]) {
    int[] array = (int[]) obj;
    /* Use the primitive array */
} else ...

At the JVM level, the instanceof operator translates to a specific "instanceof" byte code, which is optimized in most JVM implementations.

In rarer cases, you might be using reflection to traverse an object graph of unknown types. In cases like this, the isArray() method can be helpful because you don't know the component type at compile time; you might, for example, be implementing some sort of serialization mechanism and be able to pass each component of the array to the same serialization method, regardless of type.

There are two special cases: null references and references to primitive arrays.

A null reference will cause instanceof to result false, while the isArray throws a NullPointerException.

Applied to a primitive array, the instanceof yields false unless the component type on the right-hand operand exactly matches the component type. In contrast, isArray() will return true for any component type.

In the latter case, if obj is null you won't get a NullPointerException but a false.

If obj is of type int[] say, then that will have an array Class but not be an instance of Object[]. So what do you want to do with obj. If you are going to cast it, go with instanceof. If you are going to use reflection, then use .getClass().isArray().

getClass().isArray() is significantly slower on Sun Java 5 or 6 JRE than on IBM.

So much that using clazz.getName().charAt(0) == '[' is faster on Sun JVM.

I recently ran into an issue upgrading a Groovy application from JDK 5 to JDK 6. Using isArray() failed in JDK6:

MissingMethodException:
No signature of sun.reflect.generics.reflectiveObjects.GenericArrayTypeImpl.isArray() ...

Changing to instanceof Object[] fixed this.

If you ever have a choice between a reflective solution and a non-reflective solution, never pick the reflective one (involving Class objects). It's not that it's "Wrong" or anything, but anything involving reflection is generally less obvious and less clear.

Java array reflection is for cases where you don't have an instance of the Class available to do "instanceof" on. For example, if you're writing some sort of injection framework, that injects values into a new instance of a class, such as JPA does, then you need to use the isArray() functionality.

I blogged about this earlier in December. http://blog.adamsbros.org/2010/12/08/java-array-reflection/