Java Programming Concepts and Structures

Programming Languages in Java

• Java is an object-oriented programming language.
• Object-oriented programming (OOP) is based on the concept of objects having properties and behaviors.
• Objects interact with each other using methods (functions).

Java Classes and Objects

• A class is a blueprint for an object.
• An object is an instance of a class.
• Classes define properties (variables) and methods (functions).

Java Access Modifiers

• Public: Accessible from anywhere.
• Protected: Accessible within the same package and by subclasses.
• Default (Package-Private): Accessible within the same package only.
• Private: Accessible only within the same class.

Control Structures

• if-else: Conditional statements.
  • if(condition) { statement1; } // Execute statement1 if the condition is true
  • else { statement2; } // Execute statement2 if the condition is false
• if-else if-else: Multiple conditions
  • if(condition1) { //Execute st1 if cond1 is true
  • else if(condition2) { // Execute st2 if cond2 is true
  • else { // Execute else part if none of the conditions are true. }
• switch: A choice structure used to select among multiple blocks of code.

switch (expression) {
case value1: // execute if expression matches the value 1
statement1;
break;  
case value2: //execute if expression matches value2
statement2;
break;
default: //execute if expression matches none of the values
defaultStatement;
break;
}

• for loop: Repeated execution of a block of code.
  • for(initialization; condition; increment/decrement){ //block of statements}
• while loop: Repeated code execution as long as a condition is true.
  • while(condition){ //statements }
• do-while loop: Repeated code block at least once then repeats as long as a condition is true.
  • do { //statements } while (condition);

Java Scanner

• The Scanner class is used to read input from various sources.
  • Scanner scanner = new Scanner(System.in); creates a Scanner object to read from the keyboard, scanner = new Scanner(file); to read from a file, scanner = new Scanner(str); to read from a string .
• Use methods next(), nextInt(), nextDouble(), nextLine(). for different data types.

Java Constructors

• Constructors are special methods that are invoked when an object of a class is created.
• They have the same name as the class.
• They initialize object variables.

Example Code (from the documents)

import java.util.Scanner;
public class Test {
public static void main(String[] args) {
String s = "Hello World! 3 + 3.0 = 6";
Scanner scanner = new Scanner(s);
System.out.println(scanner.nextLine()); // Prints the entire line
}}

import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("enter the first Num ");
int x = scanner.nextInt();
System.out.print("enter the second Num ");
int y = scanner.nextInt();
int z = x + y;
System.out.println("the result is " + z);
}}