Classes and Objects

In simple words#

Learning objectives#

• Define a class with fields and methods
• Create objects with new
• Understand references
• Override toString

Explanation#

Fields hold state (balance, owner); methods define behaviour (deposit, withdraw).

new BankAccount(...) creates an object on the heap and returns a reference.

Two variables can reference the same object — changes through one are seen through the other.

Override toString to give objects a readable text form.

Code example: A simple bank account#

1public class BankAccount {
2    String owner;
3    double balance;
4

5    void deposit(double amount) {
6        balance += amount;
7    }
8

9    @Override
10    public String toString() {
11        return owner + ": ₹" + balance;
12    }
13

14    public static void main(String[] args) {
15        BankAccount a = new BankAccount();
16        a.owner = "Asha";
17        a.deposit(5000);
18

19        BankAccount b = a;          // same object, second reference
20        b.deposit(1000);
21

22        System.out.println(a);      // calls toString()
23    }
24}

Asha: ₹6000.0

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The class defines fields and a method.
2. new creates an object.
3. b points to the same object as a.
4. toString controls printing.

Real-world use case#

Common mistakes#

Best practices#

• One class per concept
• Meaningful toString
• Keep fields private
• Small, focused methods

Remember this#

Practice exercise#

1boolean withdraw(double amount) {
2    if (amount > balance) return false;
3    balance -= amount;
4    return true;
5}

Mini quiz#

Want more? Practise all Java MCQs · Java interview questions

Related topics#

My notes#

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