Classes, Objects & Encapsulation

Before we start

Why this matters

Outline · Lectures 4–6

Topics in this chapter

Select a topic to jump to the slide that explains it.

1. Class and object
2. Fields and methods
3. Constructors
4. Garbage collection
5. Encapsulation: private fields, getters and setters
6. Instance variable hiding and this

Concept 1 of 6

Class and object

Concept 1 of 6

Class and object (cont.)

public class Student {
    // fields (instance variables)
    private String name;
    private int id;
    private double cgpa;

    // method
    public void printInfo() {
        System.out.println(id + ": " + name + " (" + cgpa + ")");
    }
}

• A class is a blueprint: it declares fields (state) and methods (behaviour).
• An object is an instance created at run time with new; each object has its own copy of the fields.

Concept 2 of 6

Creating objects: new and references

Student s1 = new Student("Ann", 101);
Student s2 = new Student("Bob", 102);
Student s3 = s1;          // no new object!

s3.setCgpa(3.9);          // changes Ann

1. new allocates the object on the heap and sets default field values.
2. The constructor runs to initialise it.
3. new returns a reference, stored in the variable on the stack.

Concept 2 of 6

Creating objects: new and references (cont.)

Step through the code

Trace: constructing objects

Watch the heap: each new creates a fresh Student, then the constructor fills in its fields through this.

Step through the code

Trace: constructing objects (cont.)

Interactive

Object factory

Concept 3 of 6

Encapsulation

Encapsulation

public class Account {
    private double balance;        // hidden

    public double getBalance() { return balance; }

    public void deposit(double amt) {
        if (amt <= 0) {
            System.out.println("Invalid amount");
            return;
        }
        balance += amt;
    }
}
// acc.balance = -500;   ✗ compile error: private

• Make fields private; provide public getters and setters only where needed.
• Setters can validate, so an object can never get into an invalid state.
• You can change the inside later without breaking code that uses the class.

Concept 4 of 6

Constructors

public class Student {
    private String name;
    private int id;

    public Student() {                 // no-arg
        name = "Unknown";
    }
    public Student(String n, int i) {  // parameterized
        name = n;
        id = i;
    }
}

Concept 5 of 6

Instance variable hiding and this

class Box {
    int width;
    Box(int width) {
        width = width;       // ✗ assigns parameter to itself
    }
}
class Box2 {
    int width;
    Box2(int width) {
        this.width = width;  // ✓ field = parameter
    }
}

Step through the code

Trace: the hiding bug

Both constructors receive 7. Watch which object’s field actually becomes 7.

Step through the code

Trace: the hiding bug (cont.)

Concept 6 of 6

Garbage collection

• Java has no delete: the garbage collector (GC) frees memory automatically.
• An object becomes eligible when nothing reachable refers to it: s = null;, reassigning the variable, or the variable going out of scope.
• You cannot control when GC runs; System.gc() is only a request.
• finalize() is deprecated — use try-with-resources to release files and connections.

Step through the code

Trace: which objects become garbage?

Objects turn grey with a dashed border as soon as no reference reaches them.

Step through the code

Trace: which objects become garbage? (cont.)

Code example

Putting it together: a complete class

Nabila (3.75)

Worked example 1 of 3

Worked example 1

public class Rectangle {
    private double width, height;
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
    public double area()      { return width * height; }
    public double perimeter() { return 2 * (width + height); }
}

Worked example 2 of 3

Worked example 2

Worked example 3 of 3

Worked example 3

Practice

Practice (1 of 2)

Try each one on paper first, then check your answer.

public boolean withdraw(double amt) {
    if (amt <= 0 || amt > balance) return false;
    balance -= amt;
    return true;
}

Practice

Practice (2 of 2)

Try each one on paper first, then check your answer.

Check yourself

Quiz

Summary

Summary

• A class defines fields and methods; objects are created with new and reached through references.
• Assigning a reference copies the arrow, not the object — two variables can alias one object.
• Encapsulation: private fields + public methods that validate.
• Constructors initialise objects; the default no-arg constructor exists only if you write none.
• this refers to the current object and resolves field/parameter name clashes.
• Unreachable objects are garbage-collected automatically.