Complete Java + OOPS revision

Revision day

1 Learn

Revision day. Go through each block and say the idea in your own words.

Java basics: a class holds code; main is the entry point; variables have types (int, double, char, boolean, String); operators calculate and compare; if/else and switch decide; for/while/do-while repeat; methods take parameters and return values; arrays store lists from index 0; Strings are immutable objects compared with equals().

OOPS: a class is a blueprint and an object is an instance made with new; constructors set up objects; this is the current object; encapsulation = private fields + methods; inheritance = extends (is-a); super reaches the parent; polymorphism = overloading (compile time) + overriding (run time); abstraction = abstract classes and interfaces; access modifiers control visibility; packages organize classes.

2 See the Code

Java

// One program, many concepts
abstract class Shape {                       // abstraction
    private final String name;               // encapsulation
    Shape(String name) { this.name = name; } // constructor + this
    String getName() { return name; }
    abstract double area();
}

class Rect extends Shape {                   // inheritance
    private final double w, h;
    Rect(double w, double h) { super("Rect"); this.w = w; this.h = h; }
    double area() { return w * h; }          // overriding
}

class Sq extends Rect {
    Sq(double s) { super(s, s); }
}

public class Revise {
    public static void main(String[] args) {
        Shape[] list = { new Rect(2, 3), new Sq(4) };     // array + polymorphism
        for (Shape s : list) {                             // loop
            String size = s.area() > 10 ? "big" : "small"; // decision
            System.out.println(s.getName() + " " + s.area() + " " + size);
        }
    }
}

Match every comment to the concept you learned.
Sq reuses Rect's constructor through super(s, s), and its name is still Rect.

Output

Rect 6.0 small
Rect 16.0 big

3 Practice

Write one sentence and one tiny example for each: class, object, constructor, encapsulation, inheritance, polymorphism, abstraction, interface.
Revisit any Day where you got a Quick Check question wrong.

Try it in your own Java setup or write it in the Practice Notebook.

4 Quick Check

1. Integer division: 9 / 2 = ?
2. Which is decided at run time?
3. A class can extend how many classes directly?
4. Private fields + public methods is called...

Step 5

Answer and check the Quick Check above to unlock this.

Day 28