java: getting the class of the components of an array

Question

If I have

public <T> doSomething(T[] array) { } 

how can I get T.class from array?

If I do array.getClass() that gets me T[].class instead.

Answer 1

Component Type

Use this:

array.getClass().getComponentType() 

Returns the Class representing the component type of an array. If this class does not represent an array class this method returns null.

Reference:

• Class.getComponentType()

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Safe / Unsafe casting

Is there a way I can cast to Class from Class returned by getComponentType() without getting a compiler warning?

take this method:

public <T> void doSomething(final T[] array) throws Exception{     final Class<? extends Object[]> arrayClass = array.getClass();     final Class<?> componentType = arrayClass.getComponentType();     final T newInstance = (T) componentType.newInstance(); } 

Here's the generated byte code:

public void doSomething(java.lang.Object[] array) throws java.lang.Exception;      0  aload_1 [array]      1  invokevirtual java.lang.Object.getClass() : java.lang.Class [21]      4  astore_2 [arrayClass]      5  aload_2 [arrayClass]      6  invokevirtual java.lang.Class.getComponentType() : java.lang.Class [25]      9  astore_3 [componentType]     10  aload_3 [componentType]     11  invokevirtual java.lang.Class.newInstance() : java.lang.Object [30]     14  astore 4 [newInstance]     16  return 

As you can see, the parameter type is erased to Object[], so the compiler has no way to know what T is. Yes, the compiler could use array.getClass().getComponentType(), but that would sometimes fail miserably because you can do stuff like this:

Object[] arr = new String[] { "a", "b", "c" }; Integer[] integerArray = (Integer[]) arr; doSomething(integerArray); 

(In this case array.getClass().getComponentType() returns String.class, but T stands for Integer. Yes, this is legal and does not generate compiler warnings.)

Answer 2

If you want to do this for multi-dimensional arrays the following recursive code will work

    Class<?> getArrayType(Object array) {         Object element = Array.get(array, 0);         if (element.getClass().isArray()) {             return getArrayType(element);         } else {             return array.getClass().getComponentType();         }     }