How to Sort an Array in Java

November 4, 2020 admin

Arrays can be sorted using Arrays class in java.util package.

If you don’t want to write a sorting algorithm yourself to sort an array, the java.util.Arrays class must first be imported. The method sort(Object [] a) is static and must therefore be called with the class name (without creating an object). It has no return value, but only sorts the array passed as a parameter.

Primitive types
Objects
Sorting with a comparator

Primitive types

The primitive data types are sorted in ascending natural order:

Example 1 : Sort An Array Of Integer In Java

import java.util.Arrays;

public class ArraySortierenClass {

    public static void main(String[] args) {

        int[] arr = {9, 3, 1, 6, 2, 5};
        
        Arrays.sort(arr);
        
        System.out.println(Arrays.toString(arr)); 
    }
}

Output:

[1, 2, 3, 5, 6, 9]

Here the natural order of numbers is quite clear. In case of objects, it’s a bit more complicated: Is the “natural order”, e.g. for persons, sorting by last name, first name, or birth date?

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Example 2 : Sort An Array Of String In Java

import java.util.Arrays;

public class ArraySortierenClass {

    public static void main(String[] args) {
        String[] name = new String[7];
        name[0] = "Philpe";
        name[1] = "Emily";
        name[2] = "Alex";
        name[3] = "Thomas";
        name[4] = "Ali";
        name[5] = "Yohan";
        name[6] = "Jean";
        
        Arrays.sort(name);
        
        for (int i = 0; i < name.length; i++) {
            System.out.println(name[i]);
        }
    }
}

Output:

Alex
Ali
Emily
Jean
Philpe
Thomas
Yohan

The Arrays.sort() method is overloaded and can accept different parameters, including of course primitive data types.

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Objects

For objects such an order is only given if java.lang.Comparable interface is implemented. The String class implements the interface, so an array of type String can be sorted easily:

import java.util.Arrays;

public class ArraySortierenClass {

    public static void main(String[] args) {

        String[] name = {"Thomas", "Alex", "Emily"};
        
        Arrays.sort(name);
        
        System.out.println(Arrays.toString(name)); 
    }
}

Output:

[Alex, Emily, Thomas]

But be careful! Natural order does not mean alphabetical at the same time: When uppercase and lowercase letters are mixed, the uppercase letters come first and only then the lowercase letters:

import java.util.Arrays;

public class ArraySortierenClass {

    public static void main(String[] args) {

        String[] name = {"Thomas", "Yohan", "Emily", "alex"};
        
        Arrays.sort(name);
        
        System.out.println(Arrays.toString(name)); 
    }
}

Output:

[Emily, Thomas, Yohan, alex]

It is not compared alphabetically, as expected, but based on the Unicode values of the characters in the string.

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Sorting With Comparator

Alternatively, a comparator can be passed to sort() method. Comparator is an object that contains a method that specifies whether an object of a type is to be regarded as larger, smaller or the same size as another object.

The problem with strings that are not sorted alphabetically can now be solved as follows:

import java.util.Arrays;
import java.util.Comparator;

public class ArraySortierenClass {

    public static void main(String[] args) {

        String[] name = {"Thomas", "Yohan", "Emily", "alex"};
        
        Arrays.sort(name, new Comparator() {
            @Override
            public int compare(String first, String second) {
                return first.toLowerCase().compareTo(second.toLowerCase());
            }
        });
        
        System.out.println(Arrays.toString(name)); 
    }
}

Output: