Classes, Objects and Constructors in Java (ISC Class 12 Computer Science)

Understand Classes, Objects and Constructors in Java

Classes, Objects and Constructors in Java: key facts

Constructor rules

• •Same name as the class
• •No return type, not even void
• •Runs automatically at new
• •Can be overloaded

A class with no constructor gets a default one from the compiler.

Creating an object

• •ClassName ref = new ClassName(arguments);
• •Reference ref holds the address
• •Each object has its own instance variables

Declaring the reference alone creates no object.

this keyword

• •this.field = parameter;
• •this(args) calls another constructor
• •It must be the first statement in the constructor

this is the current object.

Reference versus object

• •q = p copies the reference
• •p == q is true for the same object
• •Two new objects with equal data give == false

Use a method such as equals to compare contents.

How to answer Classes, Objects and Constructors in Java questions

1. 1List the attributes as private or default instance variables with proper types.
2. 2Write the constructor(s): same name as the class, no return type, assign every field.
3. 3Write the methods for the behaviour; use return for computed values.
4. 4In main, create each object with new, call the methods and print the results.
5. 5When tracing, track each reference separately: note which variables name the same object.
6. 6Check the output formatting (spaces, println versus print).

Classes, Objects and Constructors in Java in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Classes, Objects and Constructors in Java

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Class = blueprint, object = house

  Many houses can be built from one blueprint, each with its own colour.

  Use it when: Class versus object.

• Constructor has no type

  No return type, name equals class: it looks like a method with the type missing.

  Use it when: Recognising constructors.

• this = me

  this.x is "my x"; plain x may be the parameter.

  Use it when: Name clash between field and parameter.

• new makes, = shares

  new makes an object; assignment only makes another name for it.

  Use it when: Reference tracing questions.

Classes, Objects and Constructors in Java: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

class Box {
    int l, w;
    Box() { l = 1; w = 1; }
    Box(int x) { l = x; w = x; }
    Box(int x, int y) { l = x; w = y; }
    int area() { return l * w; }
}
public class Test {
    public static void main(String[] args) {
        Box a = new Box();
        Box b = new Box(4);
        Box c = new Box(3, 5);
        System.out.println(a.area() + " " + b.area() + " " + c.area());
    }
}

Box p = new Box(2, 3);
Box q = p;
q.l = 10;
System.out.println(p.l + " " + p.area());
q = new Box(1, 1);
q.l = 7;
System.out.println(p.l + " " + q.l);

class Book {
    String title; int copies;
    Book(String t, int c) { title = t; copies = c; }
    boolean issue() {
        if (copies > 0) { copies--; return true; }
        return false;
    }
}

Book b = new Book("Maths", 2);
System.out.println(b.issue());
b.issue();
System.out.println(b.issue() + " " + b.copies);

Where students lose marks in Classes, Objects and Constructors in Java

The slips examiners see most often, and what to write instead.

• ✗Mistake: Writing a return type on the constructor

  A method with a return type, even void, is an ordinary method, not a constructor.

  Wrong working

  void Box(int x) { l = x; }

  Right working

  Box(int x) { l = x; }

  ✓Do this: No return type for constructors.

  Usually costs: 1 mark

• ✗Mistake: Shadowing the field by the parameter

  qty = qty assigns the parameter to itself and the field remains 0.

  Wrong working

  Item(int qty) { qty = qty; }

  Right working

  Item(int qty) { this.qty = qty; }

  ✓Do this: Use this.field or different parameter names.

  Usually costs: 1 mark

• ✗Mistake: Thinking q = p makes a copy

  Only the reference is copied, so both names change the same object.

  Wrong working

  After q = p; q.l = 10; p.l is still 2

  Right working

  After q = p; q.l = 10; p.l is 10

  ✓Do this: A second object needs new.

  Usually costs: 1 mark

Practice questions on Classes, Objects and Constructors in Java

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which statement creates an object of class Box?
   (a) Box b; (b) Box b = Box(); (c) Box b = new Box(); (d) new Box b;

   Hint

   Look for the new keyword and the constructor call.

   Answer

   (c) Box b = new Box();

2. Q2 · 2 marks · Short answerBoard level

   Distinguish between a class and an object, and between an instance variable and a local variable.

   Hint

   Think blueprint, instance, and where the variable lives.

   Answer

   A class is the template that defines attributes and methods; an object is one instance created from it with new. An instance variable is declared in the class outside any method and each object has its own copy; a local variable is declared inside a method or block and exists only while that code runs.

3. Q3 · 4 marks · Short answerHarder board level

   Write a class Rectangle with a constructor taking length and breadth, and methods area() and perimeter(). Show the output for new Rectangle(6, 4).

   Hint

   Use this.field if the names clash.

   Answer

   class Rectangle {
       int length, breadth;
       Rectangle(int length, int breadth) {
           this.length = length;
           this.breadth = breadth;
       }
       int area() { return length * breadth; }
       int perimeter() { return 2 * (length + breadth); }
   }

   With new Rectangle(6, 4): area() = 24 and perimeter() = 2 x (6 + 4) = 20.

4. Q4 · 3 marks · Short answerHarder board level

   Write the output:

   class Counter {
       int c;
       Counter() { this(5); }
       Counter(int n) { c = n * 2; }
   }
   // in main
   Counter x = new Counter();
   Counter y = new Counter(3);
   System.out.println(x.c + " " + y.c);

   Hint

   this(5) calls the other constructor.

   Answer

   new Counter() runs this(5), so c = 5 x 2 = 10. new Counter(3) gives c = 6. Output: 10 6

5. Q5 · 3 marks · Case-basedHOTS / competency

   Using the Box class from the worked examples, predict the output and justify each value:

   Box a = new Box(2, 2);
   Box b = new Box(2, 2);
   System.out.println((a == b) + " " + (a.area() == b.area()));

   Hint

   What does == compare for objects?

   Answer

   a == b is false because it compares the references and a and b are two different objects. a.area() == b.area() compares the int values 4 and 4, which is true. Output: false true

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is the return type of a constructor?Show answer

   None, not even void.

2. 2.What does new do?Show answer

   Creates an object, allocates memory and calls the constructor.

3. 3.What is constructor overloading?Show answer

   Having several constructors in one class with different parameter lists.

4. 4.What does this refer to?Show answer

   The current object.

5. 5.What is the default value of an int field?Show answer

   0.

6. 6.What does == compare for two object references?Show answer

   Whether they refer to the same object.

Classes, Objects and Constructors in Java: frequently asked questions