1. Comments

Java supports three types of comments:

• Single-line: // comment text
• Multi-line: /* comment text */
• Documentation: /** comment text */ — used for generating API documentation.

2. Keywords

Keywords are reserved words with predefined meanings in Java and cannot be used as identifiers (e.g., variable names). Examples include:

abstract, boolean, break, byte, case, catch, char, class, continue, default, do, double, else, enum, extends, final, finally, float, for, if, implements, import, int, interface, instanceof, long, native, new, package, private, protected, public, return, short, static, super, switch, synchronized, this, throw, throws, transient, try, void, volatile, while

3. Literals

4. Variables

A variable represents a named memory location that stores data which may change during program execution.

Syntax: dataType variableName = initialValue;

Example: int salary = 15000;

Rules for using variables:

• Variable names must be unique within the same scope.
• Multiple variables of the same type can be declared in one statement: int x = 1, y = 2, z = 0;
• A variable must be initialized before use.

public static void main(String[] args) {
    int value = 1;
    System.out.println(value); // Valid: initialized before use
    int a = 2, b = 3, result = 0; // Multiple declarations
}

5. Identifiers

Identifiers name variables, methods, classes, etc. Rules:

• Can contain letters, digits, underscores (_), and dollar signs ($).
• Cannot start with a digit.
• Cannot be a keyword.
• Case-sensitive (e.g., count ≠ Count).

6. Data Types

Java has eight primitive data types:

Usage guidelines:

• Prefer int for integers; use long only when necessary (append L or l: long num = 100L;).
• Perfer double for decimals; use float only if needed (append F or f: float val = 3.14f;).

7. Keyboard Input with Scanner

To read user input, use the Scanner class:

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner inputReader = new Scanner(System.in);
        // Use inputReader.nextInt(), nextLine(), etc.
    }
}

8. Operators and Expressions

Operators perform operatoins on operands (variables or literals). An expression combines operands and operators.

Example: Extract digits from a 3-digit number n:

• Units digit: n % 10
• Tens digit: (n / 10) % 10
• Hundreds digit: n / 100

9. Increment and Decrement Operators

• ++: increments a variable by 1 (e.g., count++)
• --: decrements a variable by 1 (e.g., count--)

10. Type Conversion

Implicit (Widening) Conversion

Automatically converts a smaller type to a larger one:

int a = 10;
double b = a; // Valid: int → double

In arithmetic expressions, byte, short, and char are promoted to int:

byte x = 10, y = 20;
int sum = x + y; // x and y promoted to int before addition

Explicit (Narrowing) Conversion

Manually convert a larger type to a smaller one using casting:

double d = 10.8;
int i = (int) d; // i becomes 10 (fractional part discarded)

Syntax: (targetType) expression