CODEWITHSUNDEEP
javaarraysgenericsprimitive
Jin Kwon
Jin Kwon

Reputation: 21978

How to create generic primitive arrays?

This is a following question coming from Two methods for creating generic arrays.

With given two methods,

@SuppressWarnings("unchecked")
static <T> T[] array1(final Class<T> elementType, final int size) {

    return (T[]) Array.newInstance(elementType, size);
}

static <T> T[] array2(final Class<T[]> arrayType, final int size) {

    return arrayType.cast(Array.newInstance(arrayType.getComponentType(), size));
}

Both methods work fine for Object type.

final Integer[] objectArray1 = array1(Integer.class, 0);
final Integer[] objectArray2 = array2(Integer[].class, 0);

When it comes to primitives, both invocation don't compile.

// array1
final int[] primitiveArray1 = array1(int.class, 0);

GenericArray.java:12: error: incompatible types
        final int[] primitiveArray1 = array1(int.class, 0);
                                            ^
  required: int[]
  found:    Integer[]
1 error

// array2
final int[] primitiveArray2 = array2(int[].class, 0);

GenericArray.java:13: error: method array2 in class GenericArray cannot be applied to given types;
        final int[] primitiveArray2 = array2(int[].class, 0);
                                      ^
  required: Class<T[]>,int
  found: Class<int[]>,int
  reason: inferred type does not conform to declared bound(s)
    inferred: int
    bound(s): Object
  where T is a type-variable:
    T extends Object declared in method <T>array2(Class<T[]>,int)
1 error

How can I do with primitive types?

Upvotes: 4

Views: 7240

Answers (6)

newacct
newacct

Reputation: 122439

This is the normal way:

int[] foo = (int[])Array.newInstance(int.class, 5);

As ruakh mentioned, you cannot make something work generically for primitive types, so you're gonna have to cast the result somehow.

Upvotes: 1

Eugene
Eugene

Reputation: 9474

This works and creates int[]:

final Object instance = Array.newInstance(Integer.TYPE, 0);

Upvotes: 0

ZhongYu
ZhongYu

Reputation: 19682

Try this instead

static <A> A array3(final Class<A> arrayType, final int size) 
{    impl omitted... }

final int[] primitiveArray3 = array3(int[].class, 0);

Upvotes: 5

ruakh
ruakh

Reputation: 183290

Primitives are incompatible with generics; for example, you can't create a List<int>, and the type of int.class is Class<Integer> rather than Class<int>. So what you describe is not possible.

Upvotes: 5

Steve P.
Steve P.

Reputation: 14699

You cannot use primitives with generics, only Objects; however, you can use their wrappers: Integer, Character, etc.

Upvotes: 0

Code-Apprentice
Code-Apprentice

Reputation: 83527

You cannot use primitives with generics. On the other hand, you can use Integer[] in place of int[] and rely on auto(un)boxing to convert between int and Integer when necessary.

Upvotes: 0

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