In the tutorial, we will discuss how to Sort an Array with Java in ascending and descending order. java.util.Arrays class provides a lot of Methods to sort an Array with difference types:
- With Primitives Array, We use the method such as:
static void sort(int[] a). - With String or Objects Array that implement
Comparable, We use method:static void sort(Object[] a). - With Custom Object Array that no implement
Comparable, We use method:sort(T[] a, Comparator c).
Let’s do more details with Java Array Sorting.
Sort with Primitives Java Array
We define a simple Int Java Array:
int[] numbers = new int[] { -9, 5, 4, 8, 11, -2, 2 };
Sorting in Ascending Order
We use the method static void sort(int[] a) to sort the specified array into ascending numerical order.
– The method returns nothing
– The Sorting algorithm is a Dual-Pivot Quicksort
– This algorithm offers O(n log(n)) performance so it is typically faster than traditional (one-pivot) Quicksort implementations.
Practice:
package com.grokonez.sortarray;
import java.util.Arrays;
public class JavaArraySortingExample {
public static void main(String[] args) {
int[] numbers = new int[] { -9, 5, 4, 8, 11, -2, 2 };
// Sorting
Arrays.sort(numbers);
// Print Sorted-Array after Sorting
for(int i=0; i
// Output
-9
-2
2
4
5
8
11
Sorting in Descending Order
Java doesn’t support the use of Comparators on primitive types so sorting a primitive array in descending order is not easy.
We have 2 approaches:
Use Guava lib
- Sorting Java Array in ascending order. Then use Guava lib to convert Array to List, reverse the List and finally convert the reversed-list to Java Array.
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.List;
import com.google.common.collect.Lists;
import com.google.common.primitives.Ints;
public class JavaArraySortingExample {
public static void main(String[] args) {
int[] numbers = new int[] { -9, 5, 4, 8, 11, -2, 2 };
// Sort array in ascending order
Arrays.sort(numbers);
// Use Guava lib to convert Array to List -> then reverse the List
List list = Ints. asList(numbers);
List reverseList = Lists.reverse(list);
// Use Guava lib to Convert List to Array
numbers = Ints.toArray(reverseList);
for(int i=0; i
// Output
11
8
5
4
2
-2
-9
Use Java 8
- We transform our Array to a Stream and then map it back to an Int array:
package com.grokonez.sortarray;
import java.util.Comparator;
import java.util.stream.IntStream;
public class JavaArraySortingExample {
public static void main(String[] args) {
int[] numbers = new int[] { -9, 5, 4, 8, 11, -2, 2 };
numbers = IntStream.of(numbers).boxed().sorted(Comparator.reverseOrder()).mapToInt(i->i).toArray();
for(int i=0; iSorting Sub-Array
We can sort a part of Array by using below method interface:
public static void sort(int[] a,
int fromIndex,
int toIndex)
It sorts the specified range of the array into ascending order. If fromIndex == toIndex, the range to be sorted is empty.
- fromIndex - the index of the first element, inclusive
- toIndex - the index of the last element, exclusive
Exception:
- IllegalArgumentException - if fromIndex > toIndex
- ArrayIndexOutOfBoundsException - if fromIndex < 0 or toIndex > a.length
package com.grokonez.sortarray;
import java.util.Arrays;
public class JavaArraySortingExample {
public static void main(String[] args) {
int[] numbers = new int[] { -9, 5, 4, 8, 11, -2, 2 };
// Sorting
Arrays.sort(numbers, 1, 6);
for(int i=0; i
-9
-2
4
5
8
11
2
Sort with String/Objects Array that implement Comparable
Sorting in Ascending Order
With String or Object Array that implement Comparable, we can use the method:
public static void sort(Object[] a)
- Sorts the specified array of objects into ascending order, according to the natural ordering of its elements
- All elements in the array must implement the Comparable interface.
- According to Oracle: "This implementation is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons."
Exception:
- ClassCastException - if the array contains elements that are not mutually comparable (for example, strings and integers)
- IllegalArgumentException - if the natural ordering of the array elements is found to violate the Comparable contract (optional).
Example with String Array Sorting:
package com.grokonez.sortarray;
import java.util.Arrays;
public class JavaArraySortingExample {
public static void main(String[] args) {
String[] strings = new String[] { "elasticsearch", "vue", "python", "angular", "java", "grokonez", "javasampleapproach" };
// Sorting
Arrays.sort(strings);
for(int i=0; i
// output
angular
elasticsearch
fruit
grokonez
java
javasampleapproach
python
vue
Sorting in Descending Order
- With String or Object that implement Comparable, for Sorting in Descending Order, we can use above approach with the supporting of Guave lib. Or using the advantage of Java 8 with Comparator.reverseOrder().
Using Guava lib
- Sorting Java String/Object Array in ascending order. Then use Guava lib to convert Array to List, reverse the List and finally convert the reversed-list to Java Array.
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.List;
import com.google.common.collect.Lists;
public class JavaArraySortingExample {
public static void main(String[] args) {
String[] strings = new String[] { "elasticsearch", "vue", "python", "angular", "java", "grokonez", "javasampleapproach" };
// Sorting
Arrays.sort(strings);
// Use Guava lib to convert Array to List -> then reverse the List
List list = Arrays.asList(strings);
List reverseList = Lists.reverse(list);
// Convert List to Array
strings = reverseList.toArray(new String[0]);
for(int i=0; i
// output
vue
python
javasampleapproach
java
grokonez
elasticsearch
angular
Use Java 8
Java 8 provides the advantage with Comparator.reverseOrder():
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
public class JavaArraySortingExample {
public static void main(String[] args) {
String[] strings = new String[] { "elasticsearch", "vue", "python", "angular", "java", "grokonez", "javasampleapproach" };
// Sorting
Arrays.sort(strings, Comparator.reverseOrder());
for(int i=0; i
// Output
vue
python
javasampleapproach
java
grokonez
elasticsearch
angular
Sorting a part of String Array
For sorting a Sub-Array of String/Object(that implements Comparable) we use the below method:
public static void sort(Object[] a,
int fromIndex,
int toIndex)
- fromIndex - the index of the first element (inclusive) to be sorted
- toIndex - the index of the last element (exclusive) to be sorted
Exception:
- IllegalArgumentException - if fromIndex > toIndex or (optional) if the natural ordering of the array elements is found to violate the Comparable contract
- ArrayIndexOutOfBoundsException - if fromIndex < 0 or toIndex > a.length
- ClassCastException - if the array contains elements that are not mutually comparable (for example, strings and integers).
Example:
package com.grokonez.sortarray;
import java.util.Arrays;
public class JavaArraySortingExample {
public static void main(String[] args) {
String[] strings = new String[] { "elasticsearch", "vue", "python", "angular", "java", "grokonez", "javasampleapproach" };
// Sorting
Arrays.sort(strings, 1, 5);
for(int i=0; i
elasticsearch
angular
java
python
vue
grokonez
javasampleapproach
- For descending sorting a sub-array String, we use Comparator.reverseOrder() of Java 8:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
public class JavaArraySortingExample {
public static void main(String[] args) {
String[] strings = new String[] { "elasticsearch", "vue", "python", "angular", "java", "grokonez", "javasampleapproach" };
// Sorting
Arrays.sort(strings, 1, 5, Comparator.reverseOrder());
for(int i=0; i
// Output
elasticsearch
vue
python
java
angular
grokonez
javasampleapproach
Sort with Custom Object Java Array
For sorting Objects Array that does NOT implement Comparable,we use below method:
public static void sort(T[] a,
Comparator c)
- It sorts the specified array of objects according to the order induced by the specified comparator.
- The comparator is used to determine the order of the array.
- According to Oracle: "This implementation is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons."
Sorting Example with Custom Object
Create a Comparator class
We define an SortbyAge class that implements Comparator:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
class SortbyAge implements Comparator {
// Used for sorting in ascending order of
// AGE
public int compare(Customer a, Customer b) {
return a.getAge() - b.getAge();
}
}
public class JavaArraySortingExample{
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.sort(customers, new SortbyAge());
for(int i=0; i-> Output:
id = 9, name = Peter, age = 19
id = 1, name = Jack, age = 23
id = 2, name = Bob, age = 27
id = 7, name = Mary, age = 29
id = 3, name = Joe, age = 31
Use Java 8
We can do more simple with the advantage of Java 8 function:
Arrays.sort(customers, Comparator.comparing(Customer::getAge));
Example Code:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
public class JavaArraySortingExample {
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.sort(customers, Comparator.comparing(Customer::getAge));
for(int i=0; i-> Output:
id = 9, name = Peter, age = 19
id = 1, name = Jack, age = 23
id = 2, name = Bob, age = 27
id = 7, name = Mary, age = 29
id = 3, name = Joe, age = 31
Descending Sorting
Define a Descending-Comparator
We just re-define an Descending-Comparator sorting:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
class DescendingSortbyAge implements Comparator {
// Used for sorting in ascending order of
// AGE
public int compare(Customer a, Customer b) {
return b.getAge() - a.getAge();
}
}
public class JavaArraySorting {
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.sort(customers, new DescendingSortbyAge());
for(int i=0; i
id = 3, name = Joe, age = 31
id = 7, name = Mary, age = 29
id = 2, name = Bob, age = 27
id = 1, name = Jack, age = 23
id = 9, name = Peter, age = 19
Use Java 8
With Java 8, we use: Comparator.comparing(Customer::getAge).reversed().
- Full Example:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
public class JavaArraySorting {
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.sort(customers, Comparator.comparing(Customer::getAge).reversed());
for(int i=0; i
id = 3, name = Joe, age = 31
id = 7, name = Mary, age = 29
id = 2, name = Bob, age = 27
id = 1, name = Jack, age = 23
id = 9, name = Peter, age = 19
Sorting Sub-Array of Custom Object Java Array
For sorting a Sub-Array of Java Object Array, we use below method:
public static void sort(T[] a,
int fromIndex,
int toIndex,
Comparator c)
- fromIndex - the index of the first element (inclusive) to be sorted
- toIndex - the index of the last element (exclusive) to be sorted
- c - the comparator to determine the order of the array.
Example:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
public class JavaArraySortingExample {
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.sort(customers, 0, 4, Comparator.comparing(Customer::getAge).reversed());
for(int i=0; i
id = 3, name = Joe, age = 31
id = 7, name = Mary, age = 29
id = 1, name = Jack, age = 23
id = 9, name = Peter, age = 19
id = 2, name = Bob, age = 27
Java 8 Arrays.parallelSort
Java 8 provides a new API interface:
public static void parallelSort(T[] a, int fromIndex, int toIndex,
Comparator cmp)
- The sorting algorithm is a parallel sort-merge that breaks the array into sub-arrays that are themselves sorted and then merged. When the sub-array length reaches a minimum granularity, the sub-array is sorted by Arrays.sort method.
- The ForkJoin common pool is used to execute any parallel tasks.
Example:
package com.grokonez.sortarray;
import java.util.Arrays;
import java.util.Comparator;
class Customer {
private int id;
private String name;
private int age;
public Customer(int id, String name, int age){
this.id = id;
this.name = name;
this.age = age;
}
public String toString() {
return String.format("id = %d, name = %s, age = %d", this.id, this.name, this.age);
}
public int getAge() {
return age;
}
}
public class JavaArraySorting {
public static void main(String[] args) {
Customer[] customers = new Customer[] {
new Customer(1, "Jack", 23),
new Customer(7, "Mary", 29),
new Customer(3, "Joe", 31),
new Customer(9, "Peter", 19),
new Customer(2, "Bob", 27)
};
// Sorting
Arrays.parallelSort(customers, 0, 4, Comparator.comparing(Customer::getAge).reversed());
for(int i=0; i-> Output:
id = 3, name = Joe, age = 31
id = 7, name = Mary, age = 29
id = 1, name = Jack, age = 23
id = 9, name = Peter, age = 19
id = 2, name = Bob, age = 27