Clasificación de elementos de arrays y clases contenedoras que ya implementan Comparable en Java

Java proporciona la interfaz Comparable para ordenar objetos usando miembros de datos de la clase. La interfaz Comparable contiene solo un método compareTo() que compara dos objetos para imponer un orden entre ellos. Devuelve un entero negativo, cero o un entero positivo para indicar si el objeto de entrada es menor, igual o mayor que el objeto actual. Se utiliza principalmente para ordenar las arrays o listas de objetos personalizados.

Dado que todas las clases Wrapper ya implementan la interfaz Java Comparable, proporciona una implementación predeterminada de compareTo() , y es por eso que las funciones Collections.sort() y Arrays.sort() se pueden usar en estos objetos. Ordenar elementos de arrays y listas que contienen clases Wrapper como objetos que ya implementan la interfaz Comparable.

Ilustración:

Input  :  {8 , 9 , 1 , 5 , 3 , 0}
Output :  {0 , 1 , 3 , 5 , 8 , 9}


Input  :  {"Ankit", "Parul" , "Swati" , "Tarun", "Akshat"}
Output :  {"Akshat" , "Ankit" , "Parul" , "Swati" , "Tarun"}

Implementación:

Ejemplo

// Java Program to Sorting Elements illustrating
// Array.srt) and Collection.sort() method
// Naive approach
 
// Importing all classes of
// java.util package
import java.util.*;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Case 1: Array of Integer using sort()
 
        // Sorting array of integers
        // using Arrays.sort()
        // Custom input entries
        int[] a = { 8, 9, 1, 5, 3, 0 };
 
        // Print the array before sorting
        System.out.println("Before Sorting: "
                           + Arrays.toString(a));
 
        // By default sorting is in ascending order
        Arrays.sort(a);
 
        // Print the array after sorting
        System.out.println("After Sorting: "
                           + Arrays.toString(a));
 
        // Case 2: Array of string using sort()
 
        // Sorting array of Strings
        // using Arrays.sort()
        String[] str = { "Ankit", "Parul", "Swati", "Tarun",
                         "Akshat" };
 
        // Print the input array of string before sorting
        System.out.println("Before Sorting: "
                           + Arrays.toString(str));
 
        // Sort() method
        Arrays.sort(str);
 
        // Print the input array of string before sorting
        System.out.println("After Sorting: "
                           + Arrays.toString(str));
 
        // Case 3: Collections.sort
 
        // Sorting List of String Collections.sort()
 
        // Creating an list object of string type
        // Custominput elements in object
        List<String> lt = Arrays.asList("Red", "Blue",
                                        "Green", "Black");
 
        // Print the elements before sorting
        System.out.println("Before Sorting:  " + lt);
 
        Collections.sort(lt);
 
        // Print the elements after sorting
        System.out.println("After Sorting:: " + lt);
    }
}

Before Sorting: [8, 9, 1, 5, 3, 0]
After Sorting: [0, 1, 3, 5, 8, 9]
Before Sorting: [Ankit, Parul, Swati, Tarun, Akshat]
After Sorting: [Akshat, Ankit, Parul, Swati, Tarun]
Before Sorting:  [Red, Blue, Green, Black]
After Sorting:: [Black, Blue, Green, Red]

Ahora, si queremos ordenar una clase definida por el usuario en un orden específico, entonces tenemos que implementar la interfaz Comparable que está presente en el paquete java.lang y proporcionar la implementación del método compareTo( ). El método compareTo también genera una excepción NullPointerException o ClassCastException si el valor especificado es nulo o si el tipo del objeto especificado impide que se compare con el objeto.

Implementación:

Ejemplo 

// Java Program to Sorting Elements of Arrays and Wrapper
// Classes that Already Implements Comparable
 
// Importing all classes from
// java.util package
import java.util.*;
 
// Class 1
// Helper class
class Student implements Comparable<Student> {
 
    // Member variables of this class
    private int rollno;
    private String name;
    private Double marks;
 
    // Constructor of this class
    public Student(int rollno, String name, Double marks)
    {
        // This keyword refers to
        // current object itself
 
        this.rollno = rollno;
        this.name = name;
        this.marks = marks;
    }
 
    // Sorting based on marks of Students
    public int compareTo(Student s)
    {
        return this.marks.compareTo(s.marks);
    }
 
    public String toString()
    {
        return "Student{"
            + "RollNo=" + rollno + ", Name='" + name + '\''
            + ", Marks=" + marks + '}';
    }
}
 
// Class 2
// Main class
class GFG {
 
    // main driver method
    public static void main(String args[])
    {
        // Creating an ArrayList of user-defined
        // type(Student)
        ArrayList<Student> arr = new ArrayList<>();
 
        // Adding elements to object created above
        // Custom input entries
        arr.add(new Student(1, "Ankush", 98.0));
        arr.add(new Student(2, "Akshat", 99.0));
        arr.add(new Student(3, "Parul", 87.0));
        arr.add(new Student(4, "Tarun", 78.0));
        arr.add(new Student(5, "Swati", 90.0));
 
        // Iterating over the above ArrayList
        for (int i = 0; i < arr.size(); i++)
 
            // Print the ArrayList elements as
            // in order added
            System.out.println(arr.get(i));
 
        // Calling the Collection.sort() method
        // to sort elements od ArrayList
        Collections.sort(arr);
 
        // Printing the ArrayList after sorting
        System.out.println("\nAfter Sorting :\n");
        for (int i = 0; i < arr.size(); i++)
            System.out.println(arr.get(i));
    }
}

Student{RollNo=1, Name='Ankush', Marks=98.0}
Student{RollNo=2, Name='Akshat', Marks=99.0}
Student{RollNo=3, Name='Parul', Marks=87.0}
Student{RollNo=4, Name='Tarun', Marks=78.0}
Student{RollNo=5, Name='Swati', Marks=90.0}

After Sorting :

Student{RollNo=4, Name='Tarun', Marks=78.0}
Student{RollNo=3, Name='Parul', Marks=87.0}
Student{RollNo=5, Name='Swati', Marks=90.0}
Student{RollNo=1, Name='Ankush', Marks=98.0}
Student{RollNo=2, Name='Akshat', Marks=99.0}