Java Objects: State, Behaviour and the JVM (ISC Class 11 Computer Science)

Understand Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine

class Date {
    int d, m, y;
    int days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};   // non-leap year
    Date(int d, int m, int y) { this.d = d; this.m = m; this.y = y; }
    void next() {
        d++;
        if (d > days[m - 1]) { d = 1; m++; }
        if (m > 12) { m = 1; y++; }
    }
}

class Calculator { double acc;
    void add(double x) { acc += x; }
    void mul(double x) { acc *= x; } }

Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine: key facts

Object

Object = State (attributes) + Behaviour (methods) + Identity | Created by new

Definition.

Compile and run

• •Source file (.java)
• •compiled by javac to bytecode (.class)
• •run by the JVM on the machine

Portability.

Message

obj.method(arguments) | A request to an object

Interaction.

How to answer Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine questions

1. 1List the attributes and their initial values.
2. 2For each call, apply the method to that object's state.
3. 3Write the state after each call.
4. 4Remember objects of the same class do not share instance variables.

Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine 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, Real-world Examples and the Java Virtual Machine

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

• State, behaviour, identity

  What it has, what it does, which one it is.

  Use it when: Object.

• Bytecode travels, JVM runs

  Portability comes from the JVM.

  Use it when: JVM.

• Message = method call

  Objects talk by calling methods.

  Use it when: Interaction.

Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine: solved examples

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

Where students lose marks in Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine

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

• ✗Mistake: Treating a class as an object

  Mixing blueprint and instance.

  Wrong working

  Calculator is an object

  Right working

  Calculator is a class; c is an object

  ✓Do this: Class = type; object = instance.

  Usually costs: 1 mark.

• ✗Mistake: Sharing state across objects

  Assuming one copy.

  Wrong working

  e.acc becomes 17.0

  Right working

  e.acc is its own value

  ✓Do this: Each object has its own state.

  Usually costs: 2 marks.

• ✗Mistake: Saying the JVM compiles source code

  Confusion with javac.

  Wrong working

  JVM compiles .java

  Right working

  javac compiles; the JVM runs bytecode

  ✓Do this: Separate the two.

  Usually costs: 1 mark.

Practice questions on Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine

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

   The Java compiler converts source code into:
   (a) machine code (b) bytecode (c) assembly (d) HTML

   Hint

   JVM input.

   Answer

   (b) bytecode

2. Q2 · 2 marks · Short answerBoard level

   Identify the state and the behaviour of a Bank Account object. (2 marks)

   Hint

   Attributes and methods.

   Answer

   State: account number, holder name, balance. Behaviour: deposit(), withdraw(), displayBalance().

3. Q3 · 4 marks · Short answerHarder board level

   Give the state of a Date object after: Date t = new Date(30, 4, 2025); t.next(); t.next(); (use the Date class above; April has 30 days). Then for Date u = new Date(28, 2, 2025) state the date after three calls. (4 marks)

   Hint

   days[3] = 30.

   Answer

   t: 30/4 becomes 1/5 then 2/5/2025. u: 28/2 becomes 1/3, 2/3, 3/3/2025.

4. Q4 · 3 marks · Short answerHarder board level

   A class Counter has int n; void inc() { n++; } void dec() { n--; } and a method reset(). Trace: Counter p = new Counter(); Counter q = new Counter(); p.inc(); p.inc(); q.dec(); p.dec(); System.out.println(p.n + " " + q.n); (3 marks)

   Hint

   Separate states.

   Answer

   p.n: 0 to 1 to 2 to 1; q.n: 0 to -1. Output: 1 -1

5. Q5 · 3 marks · Short answerHOTS / competency

   Two Calculator objects c1 and c2 are created, and c2 is assigned to c1 with c1 = c2; Then c1.add(10) is called and acc values are printed. Explain the output and the fate of the first c1 object. (3 marks)

   Hint

   References.

   Answer

   After c1 = c2 both names refer to the same object, so c1.add(10) changes the one object: c1.acc and c2.acc both show 10.0 (if c2 started at 0). The original c1 object has no reference left and becomes eligible for garbage collection.

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.Object has?Show answer

   State, behaviour, identity.

2. 2.Behaviour is defined by?Show answer

   Methods.

3. 3.javac output?Show answer

   Bytecode.

4. 4.JVM role?Show answer

   Runs bytecode.

5. 5.Is state shared between objects?Show answer

   No, each has its own.

Objects: State, Behaviour, Real-world Examples and the Java Virtual Machine: frequently asked questions