Instance and Static Variables, Primitive and Composite Types (ICSE Class 10 Java)

Understand Instance and Static Variables, Primitive and Composite Types

Instance and Static Variables, Primitive and Composite Types: key facts

Instance vs static

Instance: one copy per object, no static keyword | Static: one copy for the class, declared with static, shared by all objects

Access static as ClassName.variable.

Default values

• •int 0
• •double 0.0
• •boolean false
• •reference null

Applies to member variables, not local variables.

Primitive vs composite

Primitive	holds the value
Composite	holds a reference to an object
Copy of primitive	independent
Copy of reference	same object

String and arrays are composite.

Class members

• •Member variables = state
• •Member methods = behaviour
• •Object = instance of the class

Use real-life examples to explain.

How to answer Instance and Static Variables, Primitive and Composite Types questions

1. 1List the objects created and write their instance variables (with default values) in a table.
2. 2Keep one separate cell for each static variable, shared by all objects.
3. 3Execute each call on the object named before the dot, updating only that object's cells (and the static cell if the method changes it).
4. 4For reference assignments, note that both names point to the same table.
5. 5Evaluate the print statements with the final values.

Instance and Static Variables, Primitive and Composite Types 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 Instance and Static Variables, Primitive and Composite Types

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

• Static = Single copy

  static starts with S like single: one copy for all.

  Use it when: Instance vs static.

• Photocopy vs arrow

  Copying a primitive photocopies the value; copying an object variable copies the arrow to the same object.

  Use it when: Primitive vs composite.

• Class = cookie cutter

  The cutter (class) makes many cookies (objects); each cookie has its own decoration (instance variables).

  Use it when: Explaining class and object.

Instance and Static Variables, Primitive and Composite Types: solved examples

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

class Emp {
    static String company = "ABC";
    String name;
}
public class Test {
    public static void main(String[] args) {
        Emp e1 = new Emp();
        Emp e2 = new Emp();
        e1.name = "Ravi";
        e2.name = "Sita";
        Emp.company = "XYZ";
        System.out.println(e1.company + " " + e2.name + " " + e2.company);
    }
}

class Box {
    int v;
}
public class Test {
    public static void main(String[] args) {
        int a = 5;
        int b = a;
        b = 10;
        Box x = new Box();
        x.v = 5;
        Box y = x;
        y.v = 10;
        System.out.println(a + " " + x.v);
    }
}

class Rectangle {
    int l, b;
    void setValues(int x, int y) {
        l = x;
        b = y;
    }
    int area() {
        return l * b;
    }
    int perimeter() {
        return 2 * (l + b);
    }
}
public class Test {
    public static void main(String[] args) {
        Rectangle r1 = new Rectangle();
        Rectangle r2 = new Rectangle();
        r1.setValues(4, 3);
        r2.setValues(10, 2);
        System.out.println(r1.area() + " " + r1.perimeter());
        System.out.println(r2.area() + " " + r2.perimeter());
    }
}

class Data {
    int n;
    double d;
    boolean f;
    String s;
}
public class Test {
    public static void main(String[] args) {
        Data x = new Data();
        System.out.println(x.n + " " + x.d + " " + x.f + " " + x.s);
    }
}

Where students lose marks in Instance and Static Variables, Primitive and Composite Types

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

• ✗Mistake: Treating a static variable as a separate copy per object

  There is only one copy; every object sees the same value.

  Wrong working

  a.total = 1 and b.total = 1 after each add()

  Right working

  total is 1 after a.add() and 2 after b.add() for both objects

  ✓Do this: Keep one cell for each static variable.

  Usually costs: 1 to 2 marks

• ✗Mistake: Thinking a reference assignment makes a second object

  Only new creates an object.

  Wrong working

  Box y = x; then y.v = 10 leaves x.v = 5

  Right working

  x.v becomes 10 because x and y are one object

  ✓Do this: Count the new keywords.

  Usually costs: 1 mark

• ✗Mistake: Forgetting default values in a trace

  Member variables start at 0, 0.0, false or null.

  Wrong working

  x.n is undefined

  Right working

  x.n is 0

  ✓Do this: Write the defaults first.

  Usually costs: 1 mark

Practice questions on Instance and Static Variables, Primitive and Composite Types

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 String variable in Java is an example of a:
   (a) primitive type (b) composite type (c) keyword (d) literal

   Hint

   String is a class.

   Answer

   (b) composite type

2. Q2 · 1 mark · Very short answerBoard level

   Which keyword declares a class variable shared by all objects?

   Hint

   One copy for the class.

   Answer

   static

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   class Cnt {
       static int n;
       void hit() { n++; }
   }
   public class Test {
       public static void main(String[] args) {
           Cnt p = new Cnt();
           Cnt q = new Cnt();
           p.hit();
           q.hit();
           q.hit();
           System.out.println(Cnt.n + " " + p.n);
       }
   }

   Hint

   How many copies of n exist?

   Answer

   3 3 (one shared n, increased three times)

4. Q4 · 3 marks · Short answerHarder board level

   Distinguish between primitive and composite data types with three points and an example of each.

   Hint

   Value or reference?

   Answer

   Primitive: holds a single simple value; copying copies the value; examples int, double, char. Composite: defined by a class; holds a reference to an object; copying copies the reference, so both names refer to one object; examples String, arrays and user-defined classes such as Box.

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

   Assertion (A): A static variable is shared by all objects of its class.
   Reason (R): Only one copy of a static variable exists for the whole class.
   (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

   Why is it shared?

   Answer

   (a) Both are true and R explains A.

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 another name for a static variable?Show answer

   Class variable.

2. 2.How many copies of an instance variable exist?Show answer

   One for every object.

3. 3.What is the starting value of a boolean member variable that is never assigned?Show answer

   false

4. 4.What does a composite variable hold?Show answer

   A reference to an object.

5. 5.Which defines the state of an object: variables or methods?Show answer

   Variables.

6. 6.How is a static variable accessed?Show answer

   With the class name, ClassName.variable.

Instance and Static Variables, Primitive and Composite Types: frequently asked questions