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Java Basics - kirankiran09 - 10-04-2017 Java Basics Contents Hello World Program Statements Explained Java Program Structure in General Java Classes and Static Methods Data Types, Variables and Constants Java Comments and Documents Control Flow Reading from Keyboard Command Line Arguments Processing Summary and References Hello World // HelloWorld.java: Hello World program import java.lang.*; class HelloWorld { public static void main(String args[]) { System.out.println( Hello World ); } } Hello World: Java and C // HelloWorld.java: Hello World program import java.lang.*; class HelloWorld { public static void main(String args[]) { System.out.println( Hello World ); } } Program Processing Compilation # javac HelloWorld.java results in HelloWorld.class Execution # java HelloWorld Hello World Closer Look at - Hello World The class has one method main() public static void main(String args[]) System.out.println( Hello World ) Command line input arguments are passed in the String array args[] e.g java HelloWorld John Jane args[0] John args[1] Jane Closer Look at - Hello World import java.lang.*; Java allows grouping of related classes into a package. It allows different companies can develop different packages, may even have same class and method names, but they differ by package name: ibm.mathlib.* microsoft.mathlib.* Helps in managing name clash problems. Think of this package as library. import statement somewhat serves similar purpose as C s #include Java imports java.lang.* by default So, You don't need to import java.lang.* That means, you can invoke services of java s lang package classes/entities, you don t need to use fully qualified names. We used System.out.println() instead of java.lang. System.out.println() public static void main(String args[]) public: The keyword public is an access specifier that declares the main method as unprotected. static: It says this method belongs to the entire class and NOT a part of any objects of class. The main must always be declared static since the interpreter users this before any objects are created. void: The type modifier that states that main does not return any value. More Java: Classes and static methods // SquareRoot.java: compute square root of number import java.lang.Math; class SquareRoot { public static void main(String args []) { double x = 4; double y; y = Math.sqrt(x); System.out.println("y= "+y); } } Basic Data Types Types boolean either true or false char 16 bit Unicode 1.1 byte 8-bit integer (signed) short 16-bit integer (signed) int 32-bit integer (signed) long 64-bit integer (singed) float 32-bit floating point (IEE 754-1985) double 64-bit floating point (IEE 754-1985) String (class for manipulating strings) Java uses Unicode to represent characters internally Variables Local Variables are declared within the block of code Variable has a type preceding the name Initial value is set by initialization expressions. type variableName = initialValue; e.g. int x = 1; Variables can be defined just before their usage (unlike C) e.g., for( int i = 0; i < 10; i++) Constants Constants are similar to variables except that they hold a fixed value. They are also called READ only variables. Constants are declared with the reserved word final . final int MAX_LENGTH = 420; final double PI = 3.1428; By convention upper case letters are used for defining constants. Declaring Constants - example class CircleArea { public static void main(String args[]) { final double PI = 3.1428; double radius = 5.5; // in cms double area; area = PI * radius * radius; System.out.println("Circle Radius = "+radius+" Area="+area); } } Comments English text scattered through the code are comments JAVA supports 3 types of comments /* */ - Usually used from multi-line comments // - Used for single line comments /** */ - Documentation comments Javadoc Effort to make Java self-documenting True OOP style, encapsulate documentation within code Comments beginning with /** and ending with */ can be extracted and turned into html documentation Control Flow Control Flow Statements in JAVA while loop for loop do-while loop if-else statement switch statement JAVA does not support a goto statement |