Arrays.sort()

Primitives

针对原始类型（primitive data types），Arrays.sort()用到了Quick Sort算法。

int[] numbers = {4, 9, 1, 3, 2, 8, 7, 0, 6, 5};
System.out.println(Arrays.toString(numbers));
java.util.Arrays.sort(numbers);
System.out.println(Arrays.toString(numbers));

输出：

Before sorting: [4, 9, 1, 3, 2, 8, 7, 0, 6, 5]
After sorting: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Objects

针对对象类型（objects），Arrays.sort()用到了修改版的Merge Sort算法。

对象implements Comparable

public class Employee implements Comparable<Employee> {
    private String name;
    private int age;
    private int salary;
 
    public Employee(String name, int age, int salary) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }
 
    @Override
    public int compareTo(Employee employee) {
        return this.salary - employee.salary;
    }
}

Some core classes in the JDK already implemented the Comparable interface (e.g. String, Date and primitive wrappers: Integer, Long, Double, etc), so that we can sort array of these types naturally.

String[] fruits = {"Orange", "Grape", "Apple", "Lemon", "Banana"};
System.out.println("Before sorting: " + Arrays.toString(fruits));
Arrays.sort(fruits);
System.out.println("After sorting: " + Arrays.toString(fruits));

输出为：

Before sorting: [Orange, Grape, Apple, Lemon, Banana]
After sorting: [Apple, Banana, Grape, Lemon, Orange]

对象implements Comparator

import java.util.Comparator;

public class EmployeeSalaryComparator implements Comparator<Employee> {
    @Override
    public int compare(Employee emp1, Employee emp2) {
        return emp1.getSalary() - emp2.getSalary();
    }
}

public class Employee {
    private String name;
    private int age;
    private int salary;
 
    public Employee(String name, int age, int salary) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }
    
    public static void main(String[] args) {
        Employee[] newEmployees = new Employee[4];
 
        newEmployees[0] = new Employee("Tom", 45, 80000);
        newEmployees[1] = new Employee("Sam", 56, 75000);
        newEmployees[2] = new Employee("Alex", 30, 120000);
        newEmployees[3] = new Employee("Peter", 25, 60000);

        System.out.println("Before sorting: " + Arrays.toString(newEmployees));
        Arrays.sort(newEmployees, new EmployeeSalaryComparator());
        System.out.println("After sorting: " + Arrays.toString(newEmployees));
    }
}

使用Comparator，就可以对Employee对象进行不同方式的排序。

Collections.sort()

Collections.sort()用法上面跟Arrays.sort()基本差不多。它们两个的区别就在于，Arrays.sort()是用于array，Collections.sort()是用于list，且Arrays.sort()可以用于primitive data types。

Ascending Order

// Java program to demonstrate working of Collections.sort()
import java.util.*;

public class Collectionsorting
{
	public static void main(String[] args)
	{
		// Create a list of strings
		ArrayList<String> al = new ArrayList<String>();
		al.add("Geeks For Geeks");
		al.add("Friends");
		al.add("Dear");
		al.add("Is");
		al.add("Superb");

		/* Collections.sort method is sorting the
		elements of ArrayList in ascending order. */
		Collections.sort(al);

		// Let us print the sorted list
		System.out.println("List after the use of Collections.sort():\n" + al);
	}
}

Descending Order

// Java program to demonstrate working of Collections.sort()
// to descending order.
import java.util.*;

public class Collectionsorting
{
	public static void main(String[] args)
	{
		// Create a list of strings
		ArrayList<String> al = new ArrayList<String>();
		al.add("Geeks For Geeks");
		al.add("Friends");
		al.add("Dear");
		al.add("Is");
		al.add("Superb");

		/* Collections.sort method is sorting the
		elements of ArrayList in ascending order. */
		Collections.sort(al, Collections.reverseOrder());

		// Let us print the sorted list
		System.out.println("List after the use of Collections.sort():\n" + al);
	}
}