Constructors ICSE Class 10 Computer Applications

What Constructors is about

Weightage: CISCE publishes no chapter weightage. Section A (40 marks) is compulsory: 1-mark MCQs and 2-mark short answers including output and error questions; in Section B you answer any four of six 15-mark programming questions written in Java.Source: CISCE ICSE Computer Applications (661) specimen paper structure for 2027

Key concepts

Definition and characteristics

A constructor is a special member method used to initialise the member variables of an object when it is created.

Characteristics:
(1) Its name is exactly the same as the class name (case sensitive: for class Yellow, the constructor is Yellow).
(2) It has no return type, not even void.
(3) It is called automatically when an object is created with new; it cannot be called like an ordinary method.
(4) It is usually declared public so objects can be created from anywhere.
(5) It can be overloaded (several constructors with different parameter lists).
(6) If a class has no constructor, Java supplies an invisible default constructor that sets members to default values (0, 0.0, false, null).

class Student
{
String name;
int marks;
Student()
{
name = "Unknown";
marks = 0;
}
}

Worked example

(1 mark) If the name of the class is Yellow, a constructor can be named:
(a) yellow
(b) YELLOW
(c) Yell
(d) Yellow

Try it first, then show the solutionSolution

1. The constructor name must match the class name exactly, including case.

Answer: (d) Yellow

Default (non-parameterised) constructor

A default constructor takes no parameters. It gives the same starting values to every object.

class Lamp
{
int watts;
Lamp()
{
watts = 60;
}
}
Lamp a = new Lamp(); → a.watts is 60.

If the programmer writes no constructor at all, Java provides an implicit default constructor that sets members to 0, 0.0, false or null. But once the programmer writes any constructor (for example a parameterised one), Java no longer supplies the default one; then new Lamp() is an error unless a no-argument constructor is written too.

Worked example

(2 marks) class Pen { int ink; Pen(int i) { ink = i; } } Why does Pen p = new Pen(); fail to compile? Fix it.

Try it first, then show the solutionSolution

1. Once a parameterised constructor is written, Java does not supply a default constructor - 1 mark.
2. Add Pen() { ink = 100; } or call new Pen(100) - 1 mark.

Answer: There is no no-argument constructor, because writing Pen(int i) removes the implicit default one. Fix: add Pen() { ink = 100; } to the class, or create the object with new Pen(100).

Parameterised constructor

A parameterised constructor takes arguments, so each object can be given its own starting values at creation.

class Rect
{
int l, b;
Rect(int x, int y)
{
l = x;
b = y;
}
}
Rect r = new Rect(5, 3); → l = 5, b = 3.

The arguments in the new statement must match the constructor's parameters in number, type and order: an object sifra of class Robot whose constructor takes three double values is created as Robot sifra = new Robot(2.5, 1.0, 0.75);

Worked example

(2 marks) Write the statement to create an object sifra of class Robot whose constructor takes three double parameters.

Try it first, then show the solutionSolution

1. Class name on both sides, object name sifra, new - 1 mark.
2. Three double arguments in the brackets - 1 mark.

Answer: Robot sifra = new Robot(2.5, 1.0, 0.75);

Constructor overloading

Constructor overloading means having more than one constructor in a class, each with a different parameter list. Java calls the one whose parameters match the arguments given with new.

class report
{
int a, b;
report()
{
a = 10; b = 15;
}
report(int x, int y)
{
a = x; b = y;
}
void print()
{
System.out.println(a * b);
}
static void main()
{
report r = new report();
r.print();
report p = new report(4, 5);
p.print();
}
}
Output (in order of the calls):
150
20

Worked example

(2 marks) Give the output:
class temp
{
int a;
temp() { a = 10; }
temp(int z) { a = z; }
void print() { System.out.println(a); }
void main()
{
temp t = new temp();
temp x = new temp(30);
t.print();
x.print();
}
}

Try it first, then show the solutionSolution

1. t uses the default constructor: a = 10; x uses temp(int): a = 30 - 1 mark.
2. t.print() then x.print() - 1 mark.

Answer: 10
30

Constructor vs method

(1) Name: a constructor has the same name as the class; a method can have any valid name.
(2) Return type: a constructor has none (not even void); a method must have one (void or a type).
(3) Calling: a constructor is called automatically when an object is created with new; a method is called explicitly, as ob.method().
(4) Purpose: a constructor initialises a new object; a method performs operations (behaviour).
(5) Number of calls: a constructor runs once per object, at creation; a method can be called any number of times.
(6) Default: Java supplies a default constructor if none is written; it never supplies methods.

Worked example

(2 marks) Write two differences between a constructor and a method.

Try it first, then show the solutionSolution

1. Constructor: same name as class, no return type; method: any name, has a return type - 1 mark.
2. Constructor runs automatically at object creation; method is called explicitly - 1 mark.

Answer: (1) A constructor has the same name as its class and no return type, whereas a method can have any name and must have a return type (void or a data type). (2) A constructor is invoked automatically when an object is created with new, whereas a method is called explicitly through an object or the class.

Constructors in real life

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

Topics in this chapter

• Constructor Programs and Output Tracing

Constructors: syntax and key facts

Constructor syntax

ClassName(parameters) { initialise members; }

No return type, not even void.

Object creation

ClassName ob = new ClassName(arguments);

Arguments choose which constructor runs.

Types

Default (no arguments) and parameterised; overloading = several with different parameter lists

Writing any constructor removes Java's implicit default.

How the board asks Constructors

The question patterns to expect, and the method that works for each.

Tricks to remember Constructors

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

• SNAP

  Same name as class, No return type, Automatic call, Purpose: initialise.

  Use it when: Characteristics of constructors.

• Write one, lose the free one

  Once you write any constructor, Java stops giving the free default constructor.

  Use it when: Errors with new ClassName().

Constructors: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 2 + 3 + 5 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Constructors

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

• ✗Mistake: Wrong case in constructor name

  The 2024 examiners reported confusion over case (yellow vs Yellow) and unclear capital letters.

  ✓Do this: Write the class name exactly, with a clear capital letter.

  Usually costs: 1 mark.

• ✗Mistake: Outputs in reverse order

  In 2025 candidates wrote 20 before 150 for the report class.

  ✓Do this: Print in the order of the method calls in main().

  Usually costs: 1 mark.

• ✗Mistake: Skipping one of the print calls

  In 2024 some gave only x.print()'s output for the temp class.

  ✓Do this: Trace every call in main().

  Usually costs: 1 mark.

• ✗Mistake: Omitting constructor arguments

  In 2025 candidates left out the three double arguments when creating sifra.

  ✓Do this: Match the constructor's parameter list in new ClassName(...).

  Usually costs: 1 mark.

• ✗Mistake: Giving a constructor a return type

  void Name() turns it into an ordinary method.

  ✓Do this: Constructors never have a return type.

  Usually costs: 1 to 2 marks.

Practice questions on Constructors

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 is invoked:
   (a) when the class is compiled
   (b) when an object is created
   (c) only when called by name
   (d) at the end of the program

   Hint

   new.

   Answer

   (b) when an object is created

2. Q2 · 1 mark · MCQBoard level

   The return type of a constructor is:
   (a) void
   (b) int
   (c) the class name
   (d) none

   Hint

   Not even void.

   Answer

   (d) none

3. Q3 · 2 marks · Very short answerBoard level

   Define constructor overloading with an example.

   Hint

   Several constructors.

   Answer

   Constructor overloading is having more than one constructor in a class with different parameter lists, e.g. Point() { x = 0; y = 0; } and Point(int a, int b) { x = a; y = b; }; the arguments in new decide which one runs.

4. Q4 · 2 marks · Very short answerBoard level

   Write a statement creating an object k of class Kite whose constructor takes a String colour and an int length.

   Hint

   Order of arguments.

   Answer

   Kite k = new Kite("Red", 75);

5. Q5 · 2 marks · Short answerHOTS / competency

   Give the output:
   class N
   {
   int v;
   N() { v = 1; }
   N(int a) { v = a * a; }
   void add(N o) { v = v + o.v; }
   public static void main(String args[])
   {
   N x = new N(3);
   N y = new N();
   x.add(y);
   y.add(x);
   System.out.println(x.v + " " + y.v);
   }
   }

   Hint

   Order matters.

   Answer

   10 11 (x.v = 9 + 1 = 10; then y.v = 1 + 10 = 11)

6. Q6 · 2 marks · Very short answerHarder board level

   What does an implicit default constructor do?

   Hint

   Default values.

   Answer

   When a class has no constructor, Java supplies one with no parameters that initialises the member variables to default values: 0 for numbers, 0.0 for decimals, false for boolean and null for objects such as String.

7. Q7 · 15 marks · Long answerHarder board level

   Define a class Interest with members p, r, t and si (double). Write a parameterised constructor to set p, r and t, void calc() to compute simple interest si = p × r × t / 100, and void show() to print all. Create an object with p = 5000, r = 8, t = 3 in main().

   Hint

   Constructor sets inputs.

   Answer

   class Interest
   {
   double p, r, t, si;
   Interest(double a, double b, double c)
   {
   p = a;
   r = b;
   t = c;
   }
   void calc()
   {
   si = p * r * t / 100;
   }
   void show()
   {
   System.out.println("P = " + p + ", R = " + r + ", T = " + t);
   System.out.println("SI = " + si);
   }
   public static void main(String args[])
   {
   Interest ob = new Interest(5000, 8, 3);
   ob.calc();
   ob.show();
   }
   }
   Output:
   P = 5000.0, R = 8.0, T = 3.0
   SI = 1200.0

8. Q8 · 2 marks · Short answerHOTS / competency

   Can a constructor be private? What would happen to new from another class?

   Hint

   Access specifiers.

   Answer

   Yes, a constructor can be private, but then objects cannot be created with new from another class; only code inside the class can create them. That is why constructors are usually public.

9. Q9 · 2 marks · Very short answerBoard level

   State two characteristics of a constructor.

   Hint

   SNAP.

   Answer

   (1) It has the same name as the class. (2) It has no return type, not even void (also: it is called automatically when an object is created).

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.Constructor name rule?Show answer

   Same as the class name, same case.

2. 2.Return type of a constructor?Show answer

   None, not even void.

3. 3.Two types of constructors?Show answer

   Default (no-argument) and parameterised.

4. 4.When does Java supply a default constructor?Show answer

   Only when the class has no constructor at all.

5. 5.What decides which overloaded constructor runs?Show answer

   The arguments given with new.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Constructor: same name as class, no return type, automatic with new, initialises members.
• ☐Default: no parameters; Java gives one only if none is written.
• ☐Parameterised: values passed in new ClassName(args).
• ☐Overloading: several constructors, different parameter lists.
• ☐Method: any name, return type, called explicitly, any number of times.

Constructors: frequently asked questions