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Abstraction in Java

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Abstraction in Java

Abstraction — show what, hide how. abstract class + abstract method = must implement in child.

You cannot write new Shape() if Shape is abstract.

Interface is also abstraction (can-do contract).

Write one tiny example for Abstraction. Compile, print one result, and say the output out loud before you type.

Use Abstraction only when the problem needs it. Don’t force it into every program. Know one beginner trap.

Let's take this on the board with one tiny Main class — no extra files. Abstraction in Java — output: 12.56. Shape has no body for area(); Circle supplies it. new Shape() would not compile. Start from main, go line by line, and stop at each print. That output is the proof for Abstraction.

Exam tip

Say: new Shape() fails; new Circle(2).area() works.

Example

abstract class Shape {
  abstract double area();
}
class Circle extends Shape {
  double r;
  Circle(double r) { this.r = r; }
  double area() { return 3.14 * r * r; }
}
public class Main {
  public static void main(String[] args) {
    Shape s = new Circle(2);
    System.out.println(s.area());
  }
}

Abstraction in Java — output: 12.56. Shape has no body for area(); Circle supplies it. new Shape() would not compile.

Short notes

DefAbstraction is a Java idea you should explain with one small example.
RuleKnow the keyword / API used for Abstraction. Type a tiny Main and print one result.
RememberFor Abstraction, say the output before you code.
TrapAbstraction — only memorising the heading. No example, no trap.
Hide how, show what
abstract class

abstract class Shape { abstract double area(); } class Circle extends Shape { double r; Circle(double r) { this.r = r; } double area() { return 3.14 * r * r; } } public class Main { public static void main(String[] args) { Shape s = new Circle(2); System.out.println(s.area()); } }

Abstraction in Java

Example

Short notes

Questions

Abstraction in Java

Example

Short notes

Questions