How to see if an object is an array without using reflection?

Question

How can I see in Java if an Object is an array without using reflection? And how can I iterate through all items without using reflection?

I use Google GWT so I am n

Answer 1

There is no subtyping relationship between arrays of primitive type, or between an array of a primitive type and array of a reference type. See JLS 4.10.3.

Therefore, the following is incorrect as a test to see if obj is an array of any kind:

// INCORRECT!
public boolean isArray(final Object obj) {
    return obj instanceof Object[];
}

Specifically, it doesn't work if obj is 1-D array of primitives. (It does work for primitive arrays with higher dimensions though, because all array types are subtypes of Object. But it is moot in this case.)

I use Google GWT so I am not allowed to use reflection :(

The best solution (to the isArray array part of the question) depends on what counts as "using reflection".

• In GWT, calling obj.getClass().isArray() does not count as using reflection¹, so that is the best solution.

• Otherwise, the best way of figuring out whether an object has an array type is to use a sequence of instanceof expressions.

  public boolean isArray(final Object obj) {
      return obj instanceof Object[] || obj instanceof boolean[] ||
         obj instanceof byte[] || obj instanceof short[] ||
         obj instanceof char[] || obj instanceof int[] ||
         obj instanceof long[] || obj instanceof float[] ||
         obj instanceof double[];
  }
  

• You could also try messing around with the name of the object's class as follows, but the call to obj.getClass() is bordering on reflection.

  public boolean isArray(final Object obj) {
      return obj.getClass().toString().charAt(0) == '[';
  }
  

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^{1 - More precisely, the Class.isArray method is listed as supported by GWT in this page.}