Java OOP Lab Manual

Key ConceptsThis chapter covers Java’s lexical rules: the basic tokens and symbols that form valid source code. These rules matter in exams because errors in names, literals, comments, separators, and keywords can prevent compilation.

Whitespace separates tokens; spaces, tabs, and line breaks are usually ignored.

Identifiers name classes, methods, variables, and packages.

May contain letters, digits, _, and $.

Cannot begin with a digit.

Cannot be a Java keyword.

Java is case-sensitive: total and Total differ.

Literals are fixed values written directly in code, such as 25, 3.14, 'A', "Java", and true.

Comments document code and are ignored by the compiler:

// single-line

/ ... / multi-line

/* ... / documentation comment

Separators organize code: (), {}, [], ;, ,, ., and :.

Keywords are reserved words with predefined meanings, such as class, public, static, void, and return.

Before you continue: Why is 2marks invalid as an identifier, while marks2 is valid?

Key TermsToken - The smallest meaningful unit of Java source code.

Identifier - A programmer-defined name for a program element.

Literal - A fixed value written directly in source code.

Comment - Text ignored by the compiler and used for explanation.

Separator - A symbol that divides or organizes Java code.

Keyword - A reserved word that cannot be used as an identifier.

Whitespace - Spaces, tabs, and line breaks used to separate code elements.

Active RecallToken

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Identifier

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Literal

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Comment

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Separator

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Keyword

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Whitespace

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Worked ExamplesExample 1: Valid and invalid identifiersstudentName is valid because it starts with a letter.

marks2 is valid because digits may follow the first character.

2marks is invalid because an identifier cannot begin with a digit.

class is invalid because it is a keyword.

total_marks is valid; underscore is allowed.

Now you try: Classify value, 7value, static, and value$.

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Example 2: Reading literals and separatorsint count = 10;

char grade = 'A';

String subject = "Java";

System.out.println(subject + ": " + grade);10 is an integer literal.

'A' is a character literal; character values use single quotes.

"Java" is a string literal; strings use double quotes.

; ends each statement.

() contains method arguments, and . accesses a member.

Now you try: Identify the literals and separators in double rate = 2.5;.

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Example 3: Comments and keywordspublic class Demo {

// Program entry point

public static void main(String[] args) {

int number = 5; / integer literal /

System.out.println(number);

}

}public, class, static, void, and int are keywords.

Demo, main, String, args, and number are identifiers.

The two comments are ignored during compilation.

Braces define blocks; brackets define an array type.

Now you try: List the keywords and identifiers in private int total = 20;.

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Practice Questions(Easy) Define whitespace and give two examples.

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(Easy) State whether firstName, 9items, and while are valid identifiers.

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(Medium) Identify the literal types in boolean ready = true; char symbol = '#';.

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(Medium) Explain the difference between // and / ... / comments.

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