Converting an array of objects to an array of their primitive types

Question

If you have an array of Java objects which have a primitive type (for example Byte, Integer, Char, etc). Is there a neat way I can convert it into an array of the primitive type? In particular can this be done without having to create a new array and loop through the contents.

So for example, given

Integer[] array

what is the neatest way to convert this into

int[] intArray

Unfortunately, this is something we have to do quite frequently when interfacing between Hibernate and some third party libraries over which we have no control. It seems this would be a quite common operation so I would be surprised if there's no shortcut.

Thanks for your help!

Solution 1

Unfortunately, there's nothing in the Java platform that does this. Btw, you also need to explicitly handle null elements in the Integer[] array (what int are you going to use for those?).

Solution 2

Once again, Apache Commons Lang is your friend. They provide ArrayUtils.toPrimitive() which does exactly what you need. You can specify how you want to handle nulls.

Solution 3

With streams introduced in Java 8 this can be done:

int[] intArray = Arrays.stream(array).mapToInt(Integer::intValue).toArray();

However, there are currently only primitive streams for int, long and double. If you need to convert to another primitive type such as byte the shortest way without an external library is this:

byte[] byteArray = new byte[array.length];
for(int i = 0; i < array.length; i++) byteArray[i] = array[i];

Or the for loop can be replaced with a stream if you want:

IntStream.range(0, array.length).forEach(i -> byteArray[i] = array[i]);

All of these will throw a NullPointerException if any of your elements are null.

Solution 4

Using Guava:

int[] intArray = Ints.toArray(Arrays.asList(array));

Documentation:

• Arrays.asList (Core API)
• Ints.toArray (Guava)

Solution 5

In particular can this be done without having to create a new array and loop through the contents.

You can't convert an array of Integer to int (i.e. you can't change the type of the elements of an array) in Java. So you either must create a new int[] array and copy the value of the Integer objects into it or you can use an adapter:

class IntAdapter {
    private Integer[] array;
    public IntAdapter (Integer[] array) { this.array = array; }
    public int get (int index) { return array[index].intValue(); }
}

This can make your code a little more readable and the IntAdapter object will only consume a few bytes of memory. The big advantage of an adapter is that you can handle special cases here:

class IntAdapter {
    private Integer[] array;
    public int nullValue = 0;
    public IntAdapter (Integer[] array) { this.array = array; }
    public int get (int index) { 
        return array[index] == null ? nullValue : array[index].intValue();
    }
}

Another solution is to use Commons Primitives which contains lots of predefined adapters. In your case, have a look at ListIntList.

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Comments

[Il-Bhima]

Good point about the nulls. For my purposes I would have accepted an exception being thrown if one of the entries is null, the same way an NullPointerException is thrown when you unbox an object.

[Lordalcol]

This does not appear to be Java, at least not Java 1.6 or 1.7.

[Jaroslav Záruba]

@LorDalCol Dollar is actually a Java library

[robinst]

This answer is no longer accurate with Java 8, see Alex's answer.

[robinst]

Instead of Integer::intValue, you can also use i -> i (which uses unboxing).

[Andreas]

@robinst And unboxing is the compiler calling Integer::intValue for you, so why create a new lambda, when the method is readily available?

[robinst]

@Andreas Just listing another option, which one you choose is a question of code style/personal preference. I also microbenchmarked (using JMH) the two approaches, and they have the same performance.

[Kartik Chugh]

Using the first code snippet posted was giving me a "Can't use non-static method in static context" error so I instead did: int[] ints = Arrays.stream(objects).mapToInt(i -> Integer.parseInt(i.toString())).toArray(); Hope this is helpful for anyone with the same issue. And if anyone knows a better way please let me know.

[Ole V.V.]

It is possible to name a Java method $! I don’t think it’s encouraged, though…

[Per Lundberg]

This should be the accepted answer nowadays. Thanks Alex.

[Björn Zurmaar]

Arrays.setAll(unboxed, i -> boxed[i]); is a nice alternative to the for loop from my perspective.

[Alex - GlassEditor.com]

@BjörnZurmaar setAll only has overloads for the same primitive types as streams, so you couldn't use it for byte, but you could do Arrays.setAll(array, i -> byteArray[i] = array[i]) which will also write to the original array.