Objects in Java: State, Behaviour and Message Passing (ICSE Class 10)

Understand Objects: State, Behaviour and Message Passing

Objects: State, Behaviour and Message Passing: key facts

Object

• •Object = state (variables) + behaviour (methods)
• •Created by new
• •Has its own copy of instance variables

Identity is separate from state: two objects may hold equal values yet be different objects.

Creating and using an object

• •ClassName ref = new ClassName();
• •ref.member
• •ref.method(arguments);

The dot operator reaches members.

Default values

int	0
double	0.0
boolean	false
char	\u0000
reference types	null

Applies to instance variables only; local variables must be assigned before use.

Class as

• •Blueprint
• •Object factory
• •User-defined data type

Any one of these in a 1-mark definition.

How to answer Objects: State, Behaviour and Message Passing questions

1. 1List the objects created with new, and note which reference points to which object.
2. 2Write the starting (default) values of each object's variables.
3. 3Execute the statements in order, applying each method call to the object named before the dot.
4. 4If a reference is copied (b = a), remember both names refer to one object.
5. 5Print in the required format, remembering that println ends the line.

Objects: State, Behaviour and Message Passing 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 Objects: State, Behaviour and Message Passing

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

• Nouns and verbs

  The nouns an object has (colour, balance) are variables; the verbs it does (switch, deposit) are methods.

  Use it when: Splitting a description into state and behaviour.

• new = new house

  Each new builds a new house from the plan; the plan is the class.

  Use it when: Counting objects in output questions.

• Arrow copy

  b = a copies the arrow, not the house.

  Use it when: Reference assignment questions.

Objects: State, Behaviour and Message Passing: solved examples

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

class Tally {
    int c;
    void inc() { c++; }
    int get() { return c; }
}
public class Test {
    public static void main(String[] args) {
        Tally a = new Tally();
        Tally b = new Tally();
        a.inc();
        a.inc();
        b.inc();
        System.out.println(a.get() + " " + b.get());
    }
}

Tally p = new Tally();
Tally q = p;
q.inc();
q.inc();
p.inc();
System.out.println(p.get() + " " + q.get());

class Account {
    double bal;
    void deposit(double a) { bal = bal + a; }
    double getBal() { return bal; }
}
public class Bank {
    public static void main(String[] args) {
        Account s = new Account();
        s.deposit(500);
        s.deposit(250.5);
        System.out.println(s.getBal());
    }
}

Where students lose marks in Objects: State, Behaviour and Message Passing

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

• ✗Mistake: Treating a reference copy as a second object

  Bulb c = b; creates no new object; both names refer to one object.

  Wrong working

  c is a separate bulb, so b stays off when c is switched on.

  Right working

  c and b refer to the same bulb; switching on through c also shows it on through b.

  ✓Do this: Count the new keywords: that is the number of objects.

  Usually costs: 1 to 2 marks

• ✗Mistake: Writing the class name instead of the object name before the dot

  Instance members are used through an object, not through the class name.

  Wrong working

  Tally.inc();

  Right working

  a.inc(); (a is an object of Tally)

  ✓Do this: Create an object first and call the method on it.

  Usually costs: 1 mark

• ✗Mistake: Forgetting default values

  Instance variables not given a value are not blank; numbers are 0.

  Wrong working

  a.get() before any inc() is not defined.

  Right working

  a.get() before any inc() returns 0.

  ✓Do this: Write the starting values first in every trace.

  Usually costs: 1 mark

Practice questions on Objects: State, Behaviour and Message Passing

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

   How many objects are created by:
   Dog a = new Dog(); Dog b = a; Dog c = new Dog();
   (a) 1 (b) 2 (c) 3 (d) 0

   Hint

   Count the new keywords.

   Answer

   (b) 2: only the two new operations create objects; b is another name for the first object.

2. Q2 · 1 mark · Very short answerBoard level

   What is the default value of an uninitialised String instance variable?

   Hint

   A reference with no object.

   Answer

   null

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   class Dog {
       String name;
       void bark() { System.out.println(name + " says woof"); }
   }
   public class Test {
       public static void main(String[] args) {
           Dog d1 = new Dog();
           Dog d2 = new Dog();
           d1.name = "Tommy";
           d2.name = "Rex";
           d2.bark();
           d1.bark();
       }
   }

   Hint

   Follow the order of the calls, not of the creation.

   Answer

   Rex says woof
   Tommy says woof

4. Q4 · 2 marks · Short answerHarder board level

   The following fragment compiles. What kind of error occurs, and when?

   Dog d = null;
   d.bark();

   Hint

   The reference exists; does the object?

   Answer

   A run-time error (NullPointerException) occurs when the program runs, because d refers to no object, so the message bark() has no receiver.

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

   Assertion (A): Two objects of the same class can hold different values in their instance variables.
   Reason (R): Every object created with new gets its own copy of the instance variables.
   (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 can the values differ?

   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 state in an object?Show answer

   The values stored in its variables (attributes).

2. 2.What is behaviour in an object?Show answer

   The actions it can perform, written as its methods.

3. 3.What is message passing?Show answer

   Calling a method on an object so that it performs an action.

4. 4.What is the starting value of an instance variable of type double?Show answer

   0.0

5. 5.What does Bulb c = b; do?Show answer

   Makes c refer to the same object as b; no new object is created.

6. 6.Which operator joins an object to its method?Show answer

   The dot operator.

Objects: State, Behaviour and Message Passing: frequently asked questions