Interfaces

Comparable interface

Comparable interface is used to order the objects of the user-defined class. 

This interface is found in java.lang package and contains only one method named compareTo(Object). 

It provides a single sorting sequence only, i.e., you can sort the elements on the basis of single data member only. For example, it may be rollno, name, age or anything else.

We can sort the elements of:

1. String objects
2. Wrapper class objects
3. User-defined class objects

Collections class

Collections class provides static methods for sorting the elements of collections. If collection elements are of Set or Map, we can use TreeSet or TreeMap. However, we cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements.

Method of Collections class for sorting List elements

public void sort(List list): It is used to sort the elements of List. List elements must be of the Comparable type.

Note: String class and Wrapper classes implement the Comparable interface by default. So if you store the objects of string or wrapper classes in a list, set or map, it will be Comparable by default.

Java Comparable Example

Let's see the example of the Comparable interface that sorts the list elements on the basis of age.

File: Student.java

1. class Student implements Comparable<Student>{  
2. int rollno;  
3. String name;  
4. int age;  
5. Student(int rollno,String name,int age){  
6. this.rollno=rollno;  
7. this.name=name;  
8. this.age=age;  
9. }  
10.   
11. public int compareTo(Student st){  
12. if(age==st.age)  
13. return 0;  
14. else if(age>st.age)  
15. return 1;  
16. else  
17. return -1;  
18. }  
19. }  

TestSort1.java

1. import java.util.*;  
2. public class TestSort1{  
3. public static void main(String args[]){  
4. ArrayList<Student> al=new ArrayList<Student>();  
5. al.add(new Student(101,"Vijay",23));  
6. al.add(new Student(106,"Ajay",27));  
7. al.add(new Student(105,"Jai",21));  
8.   
9. Collections.sort(al);  
10. for(Student st:al){  
11. System.out.println(st.rollno+" "+st.name+" "+st.age);  
12. }  
13. }  
14. }  

105 Jai 21
101 Vijay 23
106 Ajay 27

Java Comparable Example: reverse order

Let's see the same example of the Comparable interface that sorts the list elements on the basis of age in reverse order.




Student.java

1. class Student implements Comparable<Student>{    
2.  int rollno;    
3.  String name;    
4.  int age;    
5.  Student(int rollno,String name,int age){    
6.  this.rollno=rollno;    
7.  this.name=name;    
8.  this.age=age;    
9.  }    
10.      
11.  public int compareTo(Student st){    
12.  if(age==st.age)    
13.  return 0;    
14.  else if(age<st.age)    
15.  return 1;    
16.  else    
17.  return -1;    
18.  }    
19.  }    

TestSort2.java

1. import java.util.*;    
2. public class TestSort2{    
3. public static void main(String args[]){    
4. ArrayList<Student> al=new ArrayList<Student>();    
5. al.add(new Student(101,"Vijay",23));    
6. al.add(new Student(106,"Ajay",27));    
7. al.add(new Student(105,"Jai",21));    
8.     
9. Collections.sort(al);    
10. for(Student st:al){    
11. System.out.println(st.rollno+" "+st.name+" "+st.age);    
12. }    
13. }    
14. }    

106 Ajay 27
101 Vijay 23
105 Jai 21

Comparator interface

Comparator interface is used to order the objects of a user-defined class.

This interface is found in java.util package and contains 2 methods compare(Object obj1,Object obj2) and equals(Object element).

It provides multiple sorting sequences, i.e., you can sort the elements on the basis of any data member, for example, rollno, name, age or anything else.

Methods of Java Comparator Interface

Methods of Java Comparator Interface

Method	Description
public int compare(Object obj1, Object obj2)	It compares the first object with the second object.
public boolean equals(Object obj)	It is used to compare the current object with the specified object.
public boolean equals(Object obj)	It is used to compare the current object with the specified object.

Collections class

Collections class provides static methods for sorting the elements of a collection. If collection elements are of Set or Map, we can use TreeSet or TreeMap.

However, we cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements also.

Method of Collections class for sorting List elements

public void sort(List list, Comparator c): is used to sort the elements of List by the given Comparator.

Java Comparator Example (Non-generic Old Style)

Let's see the example of sorting the elements of List on the basis of age and name. In this example, we have created 4 java classes:

1. Student.java
2. AgeComparator.java
3. NameComparator.java
4. Simple.java

Student.java

This class contains three fields rollno, name and age and a parameterized constructor

1. class Student{  
2. int rollno;  
3. String name;  
4. int age;  
5. Student(int rollno,String name,int age){  
6. this.rollno=rollno;  
7. this.name=name;  
8. this.age=age;  
9. }  
10. }  

AgeComparator.java

This class defines comparison logic based on the age. 

If the age of the first object is greater than the second, we are returning a positive value. 

It can be anyone such as 1, 2, 10. If the age of the first object is less than the second object, 

we are returning a negative value, it can be any negative value, and if the age of both objects is equal, we are returning 0

1. import java.util.*;  
2. class AgeComparator implements Comparator{  
3. public int compare(Object o1,Object o2){  
4. Student s1=(Student)o1;  
5. Student s2=(Student)o2;  
6.   
7. if(s1.age==s2.age)  
8. return 0;  
9. else if(s1.age>s2.age)  
10. return 1;  
11. else  
12. return -1;  
13. }  
14. }  

NameComparator.java

This class provides comparison logic based on the name. 

In such case, we are using the compareTo() method of String class, which internally provides the comparison logic

1. import java.util.*;  
2. class NameComparator implements Comparator{  
3. public int compare(Object o1,Object o2){  
4. Student s1=(Student)o1;  
5. Student s2=(Student)o2;  
6.   
7. return s1.name.compareTo(s2.name);  
8. }  
9. } 

Simple.java

In this class, we are printing the values of the object by sorting on the basis of name and age

1. import java.util.*;  
2. import java.io.*;  
3.   
4. class Simple{  
5. public static void main(String args[]){  
6.   
7. ArrayList al=new ArrayList();  
8. al.add(new Student(101,"Vijay",23));  
9. al.add(new Student(106,"Ajay",27));  
10. al.add(new Student(105,"Jai",21));  
11.   
12. System.out.println("Sorting by Name");  
13.   
14. Collections.sort(al,new NameComparator());  
15. Iterator itr=al.iterator();  
16. while(itr.hasNext()){  
17. Student st=(Student)itr.next();  
18. System.out.println(st.rollno+" "+st.name+" "+st.age);  
19. }  
20.   
21. System.out.println("Sorting by age");  
22.   
23. Collections.sort(al,new AgeComparator());  
24. Iterator itr2=al.iterator();  
25. while(itr2.hasNext()){  
26. Student st=(Student)itr2.next();  
27. System.out.println(st.rollno+" "+st.name+" "+st.age);  
28. }  
29.   
30.   
31. }  
32. }  

       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23
       
       Sorting by age       
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java Comparator Example (Generic)

Student.java





class Student{  
int rollno;  
String name;  
int age;  
Student(int rollno,String name,int age){  
this.rollno=rollno;  
this.name=name;  
this.age=age;  
}  
}  






AgeComparator.java





import java.util.*;  
class AgeComparator implements Comparator<Student>{  
public int compare(Student s1,Student s2){  
if(s1.age==s2.age)  
return 0;  
else if(s1.age>s2.age)  
return 1;  
else  
return -1;  
}  
}  






NameComparator.java




This class provides comparison logic based on the name. 




In such case, we are using the compareTo() method of String class, which internally provides the comparison logic





import java.util.*;  
class NameComparator implements Comparator<Student>{  
public int compare(Student s1,Student s2){  
return s1.name.compareTo(s2.name);  
}  
}  






Simple.java




In this class, we are printing the values of the object by sorting on the basis of name and age





import java.util.*;  
import java.io.*;  
class Simple{  
public static void main(String args[]){  
  
ArrayList<Student> al=new ArrayList<Student>();  
al.add(new Student(101,"Vijay",23));  
al.add(new Student(106,"Ajay",27));  
al.add(new Student(105,"Jai",21));  
  
System.out.println("Sorting by Name");  
  
Collections.sort(al,new NameComparator());  
for(Student st: al){  
System.out.println(st.rollno+" "+st.name+" "+st.age);  
}  
  
System.out.println("Sorting by age");  
  
Collections.sort(al,new AgeComparator());  
for(Student st: al){  
System.out.println(st.rollno+" "+st.name+" "+st.age);  
}  
}  
}  



       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23

       Sorting by age     
       105 Jai 21
       101 Vijay 23
       106 Ajay 27


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