With an example program, explain how to store user-defined classes in collections.

Storing User-Defined Classes in Collections (with Example)

In Java, you can store objects of user-defined classes in collections like ArrayList, HashSet, TreeSet, etc. To do this effectively, especially for searching, sorting, or uniqueness, the class should override toString(), equals(), and hashCode() (for Hash-based collections) or implement Comparable (for sorted collections).

Example: Storing Student Objects in an ArrayList

1. User-Defined Class: Student

class Student {
    int id;
    String name;

    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    // Override toString() for readable output
    @Override
    public String toString() {
        return "ID: " + id + ", Name: " + name;
    }
}

2. Main Program Using ArrayList<Student>

import java.util.ArrayList;

public class UserDefinedCollectionExample {
    public static void main(String[] args) {
        // Create a list to store Student objects
        ArrayList<Student> students = new ArrayList<>();

        // Add student objects
        students.add(new Student(101, "Alice"));
        students.add(new Student(102, "Bob"));
        students.add(new Student(103, "Charlie"));

        // Display the list
        System.out.println("Student List:");
        for (Student s : students) {
            System.out.println(s);
        }
    }
}

Output:

Student List:
ID: 101, Name: Alice  
ID: 102, Name: Bob  
ID: 103, Name: Charlie

Notes:

• If you use a HashSet or HashMap, override equals() and hashCode().
• If you use a TreeSet or want to sort the list, implement Comparable or use a Comparator.