Chapter 33: Class and Object in Java

This chapter is only about class (the blueprint) and object (an instance in memory). Constructors get their own chapter next (34). Inheritance comes later—keep models flat here.

1. Topic title

Class and object: class defines shape; new builds one instance in the heap

2. What it means

class Lamp {
    boolean isOn;
 
    void flip() {
        isOn = !isOn;
    }
}
 
Lamp desk = new Lamp();
desk.flip();

• class Lamp — defines a type with a field isOn and a method flip.
• new Lamp() — allocates a Lamp object on the heap, runs the constructor (compiler adds a default one here), returns a reference.
• desk — reference variable pointing at that object.
• desk.flip() — sends message flip to the object desk refers to.

3. Why it is used

Programs model the world as things with state and behavior. Classes bundle data + operations so you can create many independent lamps (many objects) from one class.

4. Real-world example

A BankAccount class (toy): fields like String owner and double balance, methods like deposit and withdraw. Each customer gets new BankAccount()—separate balances, same code.

5. Java code example

class Counter {
    int value;
 
    void increment() {
        value++;
    }
 
    void reset() {
        value = 0;
    }
 
    int read() {
        return value;
    }
}
 
public class ClassObjectDemo {
    public static void main(String[] args) {
        Counter a = new Counter();
        Counter b = new Counter();
        a.increment();
        a.increment();
        b.increment();
        System.out.println("a reads " + a.read());
        System.out.println("b reads " + b.read());
    }
}

Output: a reads 2, b reads 1 — two objects, two value fields.

6. Explanation of code

• Counter a and Counter b are separate references; unless assigned same reference, they point to different objects.
• Instance fields (value) live per object. Instance methods run on a specific object; value inside increment means this object’s field (implicit this—Chapter 35).

7. Common mistakes

Thinking static fields are per-object — they are per-class; not this chapter’s focus, but do not mix them up when you add counters.

Null reference — Counter c = null; c.increment(); → NullPointerException.

Forgetting new — Counter c = Counter(); is invalid Java syntax for construction.

8. Best practices

One public class per file for public top-level classes; filename matches public class.

Initialize fields in constructors or at declaration when you have a sensible default (Chapter 34).

Prefer private fields once you learn encapsulation (Chapter 38); public fields are okay in tiny learning snippets only.

9. Small practice task

Define class Point with int x and int y, and methods void move(int dx, int dy) and String describe() returning "(" + x + "," + y + ")". In main, create two Point objects, move them differently, print describe() for each.

Beginner tip

Say “object in the heap, reference is the remote control”—two remotes can point at one TV if you assign references that way.