Possible Duplicate:
Sort arrays of primitive types in descending order
Java : How to sort an array of floats in reverse order?
How do I reverse an int array in Java?

The following code will sort the array in ascending order :

int a[] = {30,7,9,20}; Arrays.sort(a); System.out.println(Arrays.toString(a));

I need to sort it in descending order. How do I use Comparator to do this?

Please help.

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4 Answers

For primitive array types, you would have to write a reverse sort algorithm:

Alternatively, you can convert your int[] to Integer[] and write a comparator:

public class IntegerComparator implements Comparator<Integer> { @Override public int compare(Integer o1, Integer o2) { return o2.compareTo(o1); } }

or use Collections.reverseOrder() since it only works on non-primitive array types.

and finally,

Integer[] a2 = convertPrimitiveArrayToBoxableTypeArray(a1); Arrays.sort(a2, new IntegerComparator()); // OR // Arrays.sort(a2, Collections.reverseOrder()); //Unbox the array to primitive type a1 = convertBoxableTypeArrayToPrimitiveTypeArray(a2);
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If it's not a big/long array just mirror it:

for( int i = 0; i < ++i ) { temp = arr[i]; arr[i] = arr[arr.length - i - 1]; arr[arr.length - i - 1] = temp; }
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 Comparator<Integer> comparator = new Comparator<Integer>() { @Override public int compare(Integer o1, Integer o2) { return o2.compareTo(o1); } }; // option 1 Integer[] array = new Integer[] { 1, 24, 4, 4, 345 }; Arrays.sort(array, comparator); // option 2 int[] array2 = new int[] { 1, 24, 4, 4, 345 }; List<Integer>list = Ints.asList(array2); Collections.sort(list, comparator); array2 = Ints.toArray(list);
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Guava has a method Ints.asList() for creating a List<Integer> backed by an int[] array. You can use this with Collections.sort to apply the Comparator to the underlying array.

List<Integer> integersList = Ints.asList(arr); Collections.sort(integersList, Collections.reverseOrder());

Note that the latter is a live list backed by the actual array, so it should be pretty efficient.

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