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Programming Discrete Math Concepts for Beginners

Iterator, Comparator, and Comparable

Explore how to implement and use the Iterator, Comparator, and Comparable interfaces in Java. Understand their roles in sorting and traversing collections, including how to handle common issues with comparison logic. Gain practical experience applying these interfaces to manage ordered data effectively in programming.

We'll cover the following...

The Java Collections Framework provides three important primary interfaces: Comparator, Comparable, and Iterator. Comparison plays a significant role in sorting and shuffling algorithms. Iteration is also crucial.

We’ll see a few code snippets where these three interfaces are implemented.

To sort elements, we can use TreeSet and TreeMap from the Java Collections Framework. But it’s the Comparator or the Comparable interface that precisely defines what sorted order means.

Java code for the Comparator interface

We can have objects that encapsulate ordering by implementing the Comparator and Comparable interfaces. Take a look at the following code snippet:

Java
//How the Comparator and Comparable interfaces are implemented by a class
//to sort String and Integer data types provided by List and
//ArrayList data structures
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
class Account implements Comparator<Account>, Comparable<Account> {
    private String accountHoldersName;
    private int accountNumber;
    Account() {
    }
    Account(String name, int number) {
        accountHoldersName = name;
        accountNumber = number;
    }
    public void setAccountHoldersName(String accountHoldersName) {
        this.accountHoldersName = accountHoldersName;
    }
    public String getAccountHoldersName() {
        return accountHoldersName;
    }
    public void setAccountNumber(int accountNumber) {
        this.accountNumber = accountNumber;
    }
    public int getAccountNumber() {
        return accountNumber;
    }
    //overriding the compareTo() method to sort the name
    //@param account
    //@return
    public int compareTo(Account account) {
        return (this.accountHoldersName).compareTo(account.accountHoldersName);
    }
    //overriding the compare() method to sort the account number
    //@param account
    //@param anotherAccount
    //@return
    public int compare(Account account, Account anotherAccount) {
        return account.accountNumber - anotherAccount.accountNumber;
    }
}
//ComparatorInterfaceExample
class Main{
    public static void main(String[] args) {
        // list of account objects
        List<Account> listOfAccounts = new ArrayList<Account>();
        listOfAccounts.add(new Account("Sanjib", 203));
        listOfAccounts.add(new Account("Jason", 205));
        listOfAccounts.add(new Account("John", 201));
        listOfAccounts.add(new Account("Hicky", 204));
        listOfAccounts.add(new Account("Amubrata", 202));
        // sorting the ArrayList
        Collections.sort(listOfAccounts);
        //printing the sorted names
        System.out.println("Printing the sorted names of account holders: ");
        for(Account account: listOfAccounts){
            System.out.print(account.getAccountHoldersName() + ", ");
        }
        //sorting the ArrayList with the help of Comparator
        Collections.sort(listOfAccounts, new Account());
        System.out.println(" ");
        //sorting based on account numbers
        System.out.println("Printing names of account holders based on sorted account numbers in ascending order: ");
        for(Account account: listOfAccounts)
            System.out.print(account.getAccountHoldersName() + " : "+ account.getAccountNumber() + ", ");
    }
}

First of all, we sort the names of the account holders. Then, we print the names of account holders based on sorted account numbers in ascending order.

Printing the sorted names of account holders: 
Amubrata, Hicky, John, Jason, Sanjib,  
Printing names of account holders based on sorted account numbers in ascending order: 
John : 201, Amubrata : 202, Sanjib : 203, Hicky : 204, Jason : 205,
...