对 java 中的 Integer 类型数组进行排序

Question

Cannot use Arrays.sort() for Integer type array if it has null elements. Unlike int type array, null values are not assigned to 0.
How to sort the Integer type array if it has a null element?

Integer[] arr=new Integer[4] {6,7,null,null};

I want to sort this arr array variable.
The error is as follows:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.Comparable.compareTo(Object)" because "a[runHi]" is null
    at java.base/java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
    at java.base/java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
    at java.base/java.util.Arrays.sort(Arrays.java:1040)
    at Example.medicinePriceForGivenDisease(Example.java:41)
    at Example.main(Example.java:21)

Answer 1

Since Integer is a Comparable , you can use the overloaded Arrays.sort(Object[]) . Ie:

Integer[] arr = new Integer[4]; //not "int"
// fill in some data in arr
Arrays.sort(arr);

Answer 2

The java.util.Arrays class has all required methods for sorting, including sort(Object[] a) . Since Integer is an object, it will do nicely:

Integer[] arr = new Integer[] { 32, 12, 9, 77 };
Arrays.sort(arr);
// Array sorted it

Answer 3

The single parameter Arrays.sort(Object[]) sorts elements that must implement the Comparable interface using the compareTo method. Basically it calls e1.compareTo(e2) for elements of the array. This will throw an Exception if either e1 or e2 is null.

Solution: use the 2-parameter Arrays.sort(T[], Comparator<? super t) and provide a Comparator that is able to compare 2 elements even if one or both are null.

Very simple example (compares null as being 0 ):

var comparator = Comparator.comparingInt((Integer n) -> n==null ? 0 : n.intValue());
Arrays.sort(arr, comparator);

A class implementing Comparator can also be used for more complex ordering:

class MyComparator implements Comparator<Integer> {
    @Overwrite
    public int compare(Integer a, Integer b) {
        if (a == null && b == 0) return 0;
        if (a == null) return -1;
        ...  // must be implemented
    }
}


Arrays.sort(arr, new MyComparator());

^{no IDE was used or hurt while composing this post; included code is meant only as idea how to solve the problem - it is incomplete, eventually wrong or even contain compilation errors}

Answer 4

List<Integer> sortedArray = Arrays.stream(arr)
                              .filter(x -> x != null)
                              .sorted().toList();