Topic 5: Java Basics PDF

Topic 5: Java Basics Reference: Introduction to programming Java: With a problem-solving approach, 2nd Edition – Chapter 3: pages 62-101 Class Heading Class: is simply a container for the program code. It is the file name. Class heading: public class Hello public: class is acces...

Topic 5: Java Basics Reference: Introduction to programming Java: With a problem-solving approach, 2nd Edition – Chapter 3: pages 62-101 Class Heading Class: is simply a container for the program code. It is the file name. Class heading: public class Hello public: class is accessible by other classes or programs class: identifies the beginning of class code Hello: class name. (class name describes the task of class) main Method Heading main Method: is where the computer starts execution of code. main Method Heading: public static void main(String[] args) public: everyone can access the method static: method is accessible from any class void: method doesn’t return a value main: reserved name (keyword) String[] args: arguments for main method Class and main Method Headings Braces { } : to denote a block of statements. Opening brace { determines the beginning of a class or a method Closing brace } determines the end of a class or a method Comments Comments: Text within the code that is intended for human readers and is ignored by the compiler. Types of comments: One-line: // Block-comment: Comments Example: public class Hello { public static void main(String[] args) { System.out.println("Hello World!"); //print on screen } } Readability Increase the program readability by: Comments: add explanations to the code. Blank lines: makes the program easy to read and understand. Hard to understand Easy to understand public class Hello{public static void public class Hello main(String[] args) { {System.out.println("Hello World! ");}} public static void main(String[] args) { System.out.println("Hello World! "); } } Getting Inputs Scanner stdIn = new Scanner(System.in): takes inputs from keyboard. stdIn is the name of the scanner Note: the statement “import java.util.Scanner;” should be used to use the scanner. Examples: int x = stdIn.nextInt(); float y = stdIn.nextFloat(); double w = stdIn.nextDouble(); char r = stdIn.next().charAt(0); String s = stdIn.nextLine(); Giving Outputs System.out.println( ) statement gives output on the screen. Examples: Printing a single message: ;System.out.println("Hello World! ") Printing two messages one after another: ;System.out.println("I am " + "a programmer") Printing the value of the variable “result”: ;System.out.println(result) Keywords Keywords: are words that have already been defined for Java compiler. They have special meaning for the compiler and cannot be used as a variable, class or a method name. Examples: public,static,class,import (keywords for defining parts of code) int, float, double, char (keywords for data types) if, else, switch (keywords for conditions) for, while (keywords for loops) Identifiers Identifier: a name of any component in the program (a class, method, or variable) Examples: Hello , main, x, a2. Letters (A-Z) , numbers (0-9), _ , $ can be used (use any combination) Don’t start with a number Cannot use keywords as names Recommendation: The identifier should be descriptive. Class name should start with an uppercase e.g. Hello Method name should start with a lowercase e.g. printResult Variables Variable: is a reserved memory location to store a value. Variable can only hold one type of value. Variable Datatypes: here are some datatypes: int stores whole numbers up to 231-1. float stores decimal numbers up to 7 digits. double stores decimal numbers up to 16 digits. char stores a symbol (letter, keyboard symbols, digits …). String stores text (words, sentences, paragraphs …). Variables Declaration Declaration of variables: allocates a memory location for a variable with a specific name and datatype. Examples: int a1; float dist; double result; char op; String msg; Variables Assignment Assignment: stores a value to a memory location of a variable. Examples: a1 = 25; dist = 103.25f ; result = 9.38524; Op = ‘*’; msg = “Welcome”; Declaration and Initialization Declaration and Initialization: allocates a memory location for a variable with a specific name and datatype and stores a value in that location in one command. Examples: int a1 = 25; float dist = 103.25f; double result = 9.38524; char op =‘*’; String msg = “Welcome"; Constants Constant: a variable with a fixed value that cannot be changed in the program. Examples: final int SPEEDOFSOUND = 343; final float PI = 3.1415926f; Recommendation: Variable’s name should start with a lowercase e.g. firstName Constant’s name is all capitalized e.g. NUMBER Variables and Type Casting Type casting: to convert the value from one data type to another. Conversion from smaller to larger datatypes is done automatically, but some precision could be lost. ------range increase----> int < float < double Use the cast operator (int), (float), (double) to convert between different datatypes. Variables and Type Casting Examples for automatic type casting: From integer to float: float x = 5; From integer to double: double r = 92; From float to double: double r = 3.78f; Examples for manual type casting: From float to integer: int x = (int) 5.25f; From integer to float: float r = (float) 41/3; From integer to double: double r = (double) 41/3; Arithmetic Operations Addition: a + b Subtraction: x – y Multiplication: d * r Division: s / h Modulus: k % q Operator precedence order: Parenthesis: () Exponent: XY Multiplication and Division: *,/ Addition and Subtraction: +,- Relational Operations Greater than: a > 6 Less than: x < 15 Greater than or equal: c >= d Less than or equal: p