Constructor Programs and Output Tracing in Java (ICSE Class 10)

Understand Constructor Programs and Output Tracing

Constructor Programs and Output Tracing: key facts

Constructor rules

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

With void added it becomes a method.

Compiler default constructor

• •Supplied only if the class has no constructor
• •Sets default values 0, 0.0, false, null
• •Vanishes once any constructor is written

Then new ClassName() needs a no-argument version of your own.

Choosing the constructor

• •Match the number and types of arguments in new
• •Widening of types is allowed
• •No match: compile-time error

Same rules as method overloading.

Initialising with same names

this.variable = parameter; | The plain variable = variable; sets the parameter to itself

The most common constructor bug.

How to answer Constructor Programs and Output Tracing questions

1. 1List the constructors of the class with their parameter lists.
2. 2For each new statement, match the arguments to one constructor.
3. 3Run that constructor: write the values it assigns to the instance variables of the new object.
4. 4Execute the method calls in order using the stored values.
5. 5Check for traps: a void before the constructor name, a missing no-argument constructor, or a parameter that hides the variable.

Constructor Programs and Output Tracing 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 Constructor Programs and Output Tracing

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

• Same name, no type

  A constructor wears the class name and carries no return type badge.

  Use it when: Identification.

• Write one, lose the free one

  The moment you write any constructor, the free default one disappears.

  Use it when: Compile-time error questions.

• this = the one being built

  this.bal = bal means: my bal gets the value from the parameter.

  Use it when: Same-name parameters.

Constructor Programs and Output Tracing: solved examples

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

class Pen {
    String brand;
    int price;
    Pen() { brand = "Unknown"; price = 10; }
    Pen(String b, int p) { brand = b; price = p; }
    void show() { System.out.println(brand + " " + price); }
}
public class Test {
    public static void main(String[] args) {
        Pen a = new Pen();
        Pen b = new Pen("Gel", 25);
        a.show();
        b.show();
    }
}

class Q { int x; double y; }
// in main
Q q = new Q();
System.out.println(q.x + " " + q.y);

class Room {
    int l, b, h;
    Room() { l = 2; b = 2; h = 3; }
    Room(int a) { l = a; b = a; h = a; }
    Room(int x, int y, int z) { l = x; b = y; h = z; }
    int volume() { return l * b * h; }
}
public class Test {
    public static void main(String[] args) {
        Room p = new Room();
        Room q = new Room(3);
        Room r = new Room(4, 5, 2);
        System.out.println(p.volume() + " " + q.volume() + " " + r.volume());
    }
}

class Acc {
    int bal;
    Acc(int bal) { bal = bal; }
}
// in main
Acc a = new Acc(500);
System.out.println(a.bal);

Where students lose marks in Constructor Programs and Output Tracing

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

• ✗Mistake: Writing void before a constructor

  It becomes a method and the object is created with the default constructor instead.

  Wrong working

  void Box(int x) { ... }

  Right working

  Box(int x) { ... }

  ✓Do this: No return type at all.

  Usually costs: 1 mark

• ✗Mistake: Calling new Pen() when only a parameterized constructor exists

  The compiler stops supplying the default constructor once you write any constructor.

  Wrong working

  Pen p = new Pen(); // with only Pen(String, int)

  Right working

  Add Pen() { ... } or write new Pen("Gel", 25)

  ✓Do this: Add a no-argument constructor.

  Usually costs: 1 mark

• ✗Mistake: Assigning bal = bal in the constructor

  The parameter hides the instance variable.

  Wrong working

  Acc(int bal) { bal = bal; }

  Right working

  Acc(int bal) { this.bal = bal; }

  ✓Do this: Use this. or different names.

  Usually costs: 1 to 2 marks

Practice questions on Constructor Programs and Output Tracing

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

   A constructor has the same name as:
   (a) the package (b) the class (c) the object (d) the main method

   Hint

   Look at the first line of a constructor.

   Answer

   (b) the class

2. Q2 · 1 mark · Very short answerBoard level

   What is the return type of a constructor?

   Hint

   Not even void.

   Answer

   A constructor has no return type.

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   class C {
       int n;
       C() { n = 5; }
       C(int k) { n = k * 2; }
   }
   // in main
   System.out.println(new C().n + " " + new C(4).n);

   Hint

   Two objects are created.

   Answer

   5 8

4. Q4 · 4 marks · Long answerHarder board level

   Write a class Student with instance variables name (String) and marks (int), a parameterized constructor that sets them, and a method show() that prints "name: marks". Create an object for Asha with 92 and call show().

   Hint

   Use this to avoid hiding the variables.

   Answer

   class Student {
   String name;
   int marks;
   Student(String name, int marks) {
   this.name = name;
   this.marks = marks;
   }
   void show() {
   System.out.println(name + ": " + marks);
   }
   }
   // in main
   Student s = new Student("Asha", 92);
   s.show();
   Output: Asha: 92

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): A constructor is invoked automatically when an object is created using new.
   Reason (R): A constructor has the same name as its class and has no return type.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Does the name rule explain the automatic call?

   Answer

   (b) Both are true, but R does not explain A: the automatic call is the language rule for new; the name rule only identifies the constructor.

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.When is a constructor called?Show answer

   Automatically, when an object is created with new.

2. 2.Can a constructor have a return type?Show answer

   No.

3. 3.What is constructor overloading?Show answer

   Several constructors in one class with different parameter lists.

4. 4.What does the compiler supply if no constructor is written?Show answer

   A default constructor with no parameters.

5. 5.What is this?Show answer

   A reference to the current object.

6. 6.What is a parameterized constructor?Show answer

   A constructor that takes arguments to initialise the object.

Constructor Programs and Output Tracing: frequently asked questions