Understanding Java Program Execution: Output, Comments & JVM Basics

1. A program that says more

From now on, every example sits inside the frame from Session 1. Here's a program with several output lines:

public class Intro {    public static void main(String[] args) {        System.out.println("My name is Aarav.");        System.out.println("I am learning Java.");        System.out.println("Today is day 2.");    }}

Output

My name is Aarav.I am learning Java.Today is day 2.

Remember that the file must be called Intro.java because the class is Intro. In IntelliJ, the simplest habit is one class per exercise: right-click src → New → Java Class.

2. print vs println

• println prints, then moves the cursor to a new line (ln = line).

• print prints and stays on the same line.

public class Main {    public static void main(String[] args) {        System.out.print("Good ");        System.out.print("morning");        System.out.println("!");        System.out.println("Next line");    }}

Output

Good morning!Next line

3. Escape sequences

How do you print a double quote, when the double quote itself marks the start and end of text? You escape it with a backslash \.

public class Main {    public static void main(String[] args) {        System.out.println("Name\tMarks");        System.out.println("Riya\t92");        System.out.println("She said \"Java is fun\"");        System.out.println("Line one\nLine two");    }}

Output

Name    MarksRiya    92She said "Java is fun"Line oneLine two

4. Joining text with +

The + sign can join (concatenate) text with other values:

public class Main {    public static void main(String[] args) {        System.out.println("Age: " + 21);        System.out.println("Java " + 21 + " is LTS");    }}

Output

Age: 21Java 21 is LTS

We'll see some surprising behaviour of + with numbers in Session 4.

5. Comments

Comments are notes for humans; the compiler ignores them completely.

public class Main {    public static void main(String[] args) {        // Single-line comment: everything after // is ignored        /*           Multi-line comment:           useful for longer explanations        */        System.out.println("Comments don't print"); // can also sit at the end of a line    }}

There is a third kind, /** ... */, called a Javadoc comment. It is used to document code for other developers, and tools turn it into web pages. We'll use it once we write our own methods and classes.

Good comments explain why, not what. // add 1 to count above count = count + 1; is useless. // skip the header row is useful.

6. How code executes: top to bottom

Every instruction ending in ; is a statement. Inside main, Java runs statements one after another, from top to bottom, exactly once each (until we learn decisions and loops).

A few formatting rules:

• Whitespace and blank lines don't matter to Java, but they matter a lot to humans. Indent the code inside braces by 4 spaces.

• You can put two statements on one line, but don't.

• Press Ctrl + Alt + L (Cmd + Option + L on Mac) in IntelliJ to auto-format.

Interview questions

Practice

1. easy Print a restaurant bill with a tab between item and price for 3 items, with a line of dashes above and below.

2. easy Print: The file is at C:\java\notes.txt exactly.

3. easy Using only print (not println) and \n, print your name and city on two lines.

4. medium Print this pattern using one single println statement:

******

5. medium Predict the output, then run it:

System.out.print("A");System.out.println("B");System.out.print("C\n");System.out.print("D\tE");