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

You can use instanceof.

JLS 15.20.2 Type Comparison Operator instanceof

 RelationalExpression:
    RelationalExpression instanceof ReferenceType

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

That means you can do something like this:

Object o = new int[] { 1, 2 };
System.out.println(o instanceof int[]); // prints "true"        

You'd have to check if the object is an instanceof boolean[], byte[], char[], double[], float[], int[], long[], short[], or Object[], if you want to detect all array types.

Also, an int[][] is an instanceof Object[], so depending on how you want to handle nested arrays, it can get complicated.

For the toString, java.util.Arrays has a toString(int[]) and other overloads you can use. It also has deepToString(Object[]) for nested arrays.

public String toString(Object arr) {
   if (arr instanceof int[]) {
      return Arrays.toString((int[]) arr);
   } else //…
}

It's going to be very repetitive (but even java.util.Arrays is very repetitive), but that's the way it is in Java with arrays.

See also

• Managing highly repetitive code and documentation in Java
• Java Arrays.equals() returns false for two dimensional arrays.

One can access each element of an array separately using the following code:

Object o=...;
if ( o.getClass().isArray() ) {
    for(int i=0; i<Array.getLength(o); i++){
        System.out.println(Array.get(o, i));
    }
}

Notice that it is unnecessary to know what kind of underlying array it is, as this will work for any array.

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) == '[';
  }
  

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

You can create a utility class to check if the class represents any Collection, Map or Array

  public static boolean isCollection(Class<?> rawPropertyType) {
        return Collection.class.isAssignableFrom(rawPropertyType) || 
               Map.class.isAssignableFrom(rawPropertyType) || 
               rawPropertyType.isArray();
 }

Simply obj instanceof Object[] (tested on JShell).