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Java Basics Overview

Jun 22, 2025

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Overview

This lecture introduces Java programming from installation to basic syntax, variables, user input, expressions, GUI basics, useful math methods, and generating random values.

Getting Started with Java

  • Java is a top programming language used in enterprise, Android apps, and more.
  • Computers read machine code (binary), but humans write source code.
  • Java source files end with .java; they are compiled to bytecode (.class), making them cross-platform.
  • The JVM (Java Virtual Machine) translates bytecode to machine code.
  • Download the JDK (Java Development Kit) which includes the JVM and JRE (Java Runtime Environment).
  • Use an IDE (recommended: Eclipse or IntelliJ IDEA) for easier coding, compiling, and running.

Writing and Running Programs

  • Create a Java project, then add a class (e.g., Main) containing the main method.
  • The main method: public static void main(String[] args) { } is the entry point for every Java program.
  • Use System.out.print() and System.out.println() to output text to the console.
  • println adds a new line after printing, while print does not.
  • Escape sequences (e.g., \n for a new line, \t for tab, \" for double quotes, \\ for backslash) format output.
  • Comments: // for single-line, /* ... */ for multi-line.

Variables and Data Types

  • Variables store values; their type must be declared.
  • Eight primitive types: boolean, byte, short, int, long, float, double, char.
  • String is a reference type for text.
  • Declaration: int x; Assignment: x = 5; Initialization: int x = 5;
  • Use long (add L at the end) for very large integers, float (add f at the end) for decimals, double for more precision.
  • Characters use single quotes, e.g., char letter = 'A';, strings use double quotes.

Swapping Variables

  • Swapping variable values requires a temporary variable:
    • temp = x; x = y; y = temp;

User Input with Scanner

  • Import the Scanner: import java.util.Scanner;
  • Create a Scanner: Scanner scanner = new Scanner(System.in);
  • Read input: scanner.nextLine() for strings, scanner.nextInt() for integers, scanner.nextDouble() for decimals.
  • After reading numbers, use an extra scanner.nextLine() to consume the leftover newline.

Expressions and Operators

  • Expressions combine operands (variables/values) and operators (+, -, *, /, %).
  • Increment: x++; Decrement: x--;
  • Integer division truncates decimals (e.g., 10/3 gives 3).
  • Cast to double to get precise results (e.g., double result = (double)x / y;).*

Basic GUI Applications with JOptionPane

  • Import: import javax.swing.JOptionPane;
  • Show input dialog: JOptionPane.showInputDialog("Enter your name");
  • Show message dialog: JOptionPane.showMessageDialog(null, "Hello " + name);
  • Convert input for numbers: Integer.parseInt(inputStr) or Double.parseDouble(inputStr).

Useful Math Methods

  • Use Math.max(a, b), Math.min(a, b), Math.abs(x), Math.sqrt(x), Math.round(x), Math.ceil(x), Math.floor(x).
  • Example: Find hypotenuse: z = Math.sqrt(x*x + y*y);

Generating Random Values

  • Import: import java.util.Random;
  • Create a Random object: Random random = new Random();
  • Generate random int: random.nextInt(n) (returns 0 to n-1).
  • Generate random double: random.nextDouble() (returns 0 to 1).
  • Generate random boolean: random.nextBoolean().

Key Terms & Definitions

  • Source Code — Human-readable program text (.java).
  • Bytecode — Compiled cross-platform code (.class) for the JVM.
  • JDK — Java Development Kit: tools for coding, compiling, running Java.
  • JVM — Java Virtual Machine: runs bytecode on any machine.
  • IDE — Integrated Development Environment: coding tool (e.g., Eclipse).
  • Primitive Data Type — Basic data type (int, double, boolean, etc.).
  • Reference Data Type — Stores addresses; e.g., Strings and objects.
  • Scanner — Class used to accept user input.
  • Expression — Combination of operands and operators.
  • JOptionPane — GUI class for dialog boxes.
  • Random — Class for generating pseudo-random numbers.
  • Escape Sequence — Special character in strings (e.g., \n, \t).

Action Items / Next Steps

  • Download and install the JDK and an IDE (Eclipse or IntelliJ).
  • Practice creating Java projects, classes, and basic output.
  • Experiment with variables, user input, and simple expressions.
  • Try swapping variables and using math/random classes.
  • Attempt to build a basic GUI using JOptionPane.
  • Complete assignment: code to swap two variables and post in the comments.

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