Lecture 2(2)

Chapter 2: Elementary Programming

Instructor Information

• Instructor: Chris Franson

• UID: [UID details not provided]

Identifiers

• Definition: Identifiers are names that represent elements: variables, constants, methods, classes, and packages in a program.

• Character Composition: Composed of letters, digits, underscores (_), and dollar signs ($).

• Rules:

  • Must start with a letter, underscore, or dollar sign.

  • Cannot start with a digit.

  • Cannot be a reserved word.

  • Cannot be true, false, or null.

  • Can be of any length.

  • Case Sensitivity: Java is case sensitive; e.g., Area, area, and AREA are different identifiers.

Case Sensitivity in Java

• Java differentiates between upper and lower case:

  • Examples:

    • "John" vs "john"

    • "car" vs "CAR"

    • "int" vs "Int"

    • "if" vs "If", where if is a valid Java keyword.

Declaring Variables

• Variables store data used throughout a program.

• Purpose: To hold user input, e.g., reading the radius from the user.

• Naming: Choose descriptive names (e.g., radius for radius).

• Declaration Syntax: datatype variableName;

Variables: Overview

• Definition: A variable is a symbol that can hold/change values.

• Variable Example Syntax:

  • int radius; // Declare an integer variable

  • double area; // Declare a double variable

Data Types in Java

• Primitive Data Types (8 total):

  • boolean: true or false.

  • char: 16-bit character.

  • Integer types:

    • byte: -128 to 127.

    • short: -32768 to 32767.

    • int: -2147483648 to 2147483647.

    • long: -9223372036854775808 to 9223372036854775807.

  • Floating-Point Types:

    • float: 32-bit floating-point.

    • double: 64-bit floating-point.

Declaring and Initializing Variables

• Single Declaration Example:

  • int count; // Declare count to be an integer variable

• Multiple Declaration Example:

  • double radius, area; // Declare multiple variables of the same type.

Assignment Statements

• Definition: Assigns a value to a variable after declaration.

• Syntax: variable = expression; // = is the assignment operator.

• Examples:

  • int x; x = 1;

  • double radius; radius = 1.0;

  • char a; a = 'A';

Declaring and Initializing in One Step

• Example: int radius = 1; // Declare and initialize in one step.

• Others: char firstLetter = 'A';, String aString = "A";

Character Data Type

• Represents a single character in Java (ASCII or Unicode).

• Character Literal: Enclosed in single quotes.

• Examples:

  • char aLetter = 'A';

  • char aDigit = '8';

  • char uLetter = '\u0041';

Writing a Simple Program: Algorithm Example

• Step 1: Read the circle's radius from user input.

• Step 2: Compute area: area = π * radius * radius.

• Step 3: Display the result.

Important Points on Standard Input and Output

• System.out: Refers to output device (monitor).

• System.in: Refers to input device (keyboard).

• Scanner Class: Use to read input; necessitates import java.util.Scanner;

• Example Creation: Scanner input = new Scanner(System.in);

Common Exercises

• Write a program to:

  • Convert Celsius to Fahrenheit.

  • Compute area from radius and length of a cylinder.

  • Find averages from keyboard inputs.

Common Errors

• Undeclared Variables: Accessing variables before declaration/initialization.

• Integer Overflow: Exceeding the max value for a type, e.g., int value = 2147483647 + 1;

• Round-off and Division Errors: Example with 1.0 - 0.1 * 5. Check the output to notice inaccuracies in floating-point operations.

Numeric Type Conversion Rules

• Larger data type takes precedence in operations.

• Examples: If one operand is double, other will be converted to double.

Type Casting in Java

• Implicit Casting: Automatic conversion from a smaller to a larger type.

• Explicit Casting: For converting a larger type to a smaller or specific type, e.g., truncating a double.

• Example: int i = (int) 3.9; // Result is 3.