How Java Runs and the Structure of a Java Class (ISC Class 12 Computer Science)

Understand How Java Runs and the Structure of a Java Class

import java.util.Scanner;

public class Name
{
    int a;                 // data member (instance variable)
    static int count;      // class variable

    Name()                 // constructor
    {
        a = 0;
    }

    int twice(int x)       // method
    {
        return x * 2;
    }

    public static void main(String[] args)
    {
        Name ob = new Name();
        System.out.println(ob.twice(6));
    }
}

How Java Runs and the Structure of a Java Class: key facts

Execution path

• •Source (.java)
• •javac
• •Bytecode (.class)
• •JVM
• •Machine code

Platform independence from bytecode.

main method header

• •public
• •static
• •void
• •main
• •String array args

Every word has a reason.

Identifier rules

• •Start with letter or underscore
• •No keywords
• •Case-sensitive
• •No spaces or hyphens

Digits allowed after the first character.

How to answer How Java Runs and the Structure of a Java Class questions

1. 1For execution questions, describe the order: source, compiler, bytecode, JVM.
2. 2For identifiers, test first character, keywords and special characters.
3. 3For a small class, start at main( ) and follow the constructor and calls in order.
4. 4State the output exactly with its type, for example 12 or 12.0.

How Java Runs and the Structure of a Java Class 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 How Java Runs and the Structure of a Java Class

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

• S-B-J

  Source, Bytecode, JVM.

  Use it when: Order of execution.

• JDK builds, JRE runs

  The JDK contains tools to build; the JRE contains the JVM to run.

  Use it when: JDK versus JRE.

How Java Runs and the Structure of a Java Class: solved examples

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

Where students lose marks in How Java Runs and the Structure of a Java Class

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

• ✗Mistake: Saying the JVM compiles source code

  javac compiles; the JVM executes bytecode.

  Wrong working

  The JVM converts the .java file into bytecode.

  Right working

  The compiler javac converts .java to bytecode; the JVM runs the bytecode.

  ✓Do this: javac compiles, JVM runs.

  Usually costs: 1 mark.

• ✗Mistake: Writing main without static

  The JVM needs it without an object.

  Wrong working

  public void main(String[] args).

  Right working

  public static void main(String[] args).

  ✓Do this: Memorise the header.

  Usually costs: 1 mark.

• ✗Mistake: Treating Total and total as the same

  Java is case-sensitive.

  Wrong working

  Total and total are the same variable.

  Right working

  They are two different identifiers.

  ✓Do this: Check capitals.

  Usually costs: 1 mark.

Practice questions on How Java Runs and the Structure of a Java Class

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 · Very short answerBoard level

   What does JVM stand for?

   Hint

   Virtual.

   Answer

   Java Virtual Machine.

2. Q2 · 2 marks · Short answerBoard level

   State two differences between the JDK and the JRE.

   Hint

   Build versus run.

   Answer

   The JDK contains the compiler and development tools, whereas the JRE contains the JVM and libraries needed only to run programs.

3. Q3 · 2 marks · Short answerHarder board level

   Why is main( ) declared public?

   Hint

   Who calls it?

   Answer

   The JVM calls main( ) from outside the class, so it must be publicly accessible.

4. Q4 · 4 marks · Short answerHarder board level

   Predict the output: class P { int v; P() { v = 5; } int f(int k) { return v * k + 1; } public static void main(String[] a) { P q = new P(); System.out.println(q.f(3)); System.out.println(q.f(q.v)); } }

   Hint

   v is 5 after the constructor.

   Answer

   f(3) = 5 x 3 + 1 = 16. f(q.v) = f(5) = 5 x 5 + 1 = 26. Output lines: 16 and 26.

5. Q5 · 4 marks · Short answerHOTS / competency

   A student says, 'Because Java is compiled, a .class file will not run on a different operating system.' Evaluate the statement.

   Hint

   What does the compiler produce?

   Answer

   The statement is wrong for Java. The compiler produces bytecode, not machine code for one processor. Each operating system has its own JVM that executes the bytecode, so the same .class file runs on any system with a JVM, which is platform independence.

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 does javac produce?Show answer

   Bytecode in a .class file.

2. 2.Which contains the JVM: JDK or JRE?Show answer

   The JRE (the JDK includes it as well).

3. 3.May an identifier start with a digit?Show answer

   No.

4. 4.How many public classes per file?Show answer

   One, with a matching file name.

How Java Runs and the Structure of a Java Class: frequently asked questions