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Basic Data Structures in Java

Basic Data Structures in Java. (c) IDMS/SQL News http://www.geocities.com/idmssql. Language Features. Object Oriented Class – the basic entity of OOP Object – instance of a class Methods – process code within class Interpreted Lang – Bytecode

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Basic Data Structures in Java

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  1. Basic Data Structures in Java (c) IDMS/SQL News http://www.geocities.com/idmssql

  2. Language Features • Object Oriented • Class – the basic entity of OOP • Object – instance of a class • Methods – process code within class • Interpreted Lang – Bytecode • Builtin support for threading (concurrent execution different pieces of code) Java - Chapter 1 (c)http://www.geocities.com/idmssql

  3. HelloWorld Again public class HelloWorld { public static void main(String arguments[]) { System.out.println(“Hello World"); } } Note: 1. Any main ‘program’ must have a main method and it must be declared static. No instance is required. JVM can directly run this. 2. If a class has no main method, then the only way to use it is to call from another class. In practice, many classes are defined like this... See next page Java - Chapter 1 (c)http://www.geocities.com/idmssql

  4. /** * This is the template for a class */ public class test{ // instance variables - private int x; /** Constructor for objects of class test */ public test() {// initialise instance variables x = 10; } /** An example of a method – */ public int sampleMethod(int y) {// put your code here return x + y; } } Java - Chapter 1 (c)http://www.geocities.com/idmssql

  5. Instances We have a class test and a method sampleMethod How do we use them from another program? test test1; // declare a new ‘variable’ test1 of type test test1 = new test(); int xyz1; xyz1 = test1.sampleMethod(30);//we get 40 This is the essence of OO programming. Java - Chapter 1 (c)http://www.geocities.com/idmssql

  6. OO features / conventions • Class name starts with Upper case • Variables start with lowercase, each word with uppercase • Method – lowercase • Methods must return something else declared as void Java - Chapter 1 (c)http://www.geocities.com/idmssql

  7. Capitalization Java is picky about capitalization; System.out.println() vs. system.out.println() are not the same HelloWorld and helloworld are not the same! Java - Chapter 1 (c)http://www.geocities.com/idmssql

  8. Datatypes – 8 of them • boolean: true or false. 1 bit size • char: Unicode character. 16 bits. • byte: very small integer number; -128-127, 8 bits • short: a smallish integer number, -32,768 to 32,767 • int: normal integer, 32 bits, -2,147,483,648 to 2,147,483,647 • long: a really big 64 bit integer, -9223372036854775808 to + 9223372036854775807 • float: 32 bit floating point number, -3.40292347E+38 to + 3.40292347E+38 • double: 64 bit long floating point number, • good for -E+308 to +E+308 and about twice the number of digits of precision as a float Java - Chapter 1 (c)http://www.geocities.com/idmssql

  9. Declaring Variables • Variables are declared in the C style; the type first, followed by the variables being declared • Variable names must start with a letter or an underscore character, “_” • Cannot start with a number. InterCap Style int inputCount; int currentCount23, finalValue; Java - Chapter 1 (c)http://www.geocities.com/idmssql

  10. 3 types of variable declarations instance, class and local variables class Variables1 { double salary = 2534.50 ;// instance variable static int counter1; // class variable public static void main (String args[]){ int temp1 = 0; // local variable defined inside a method System.out.println ("counter1=" + counter1 + " temp1=" + temp1 ); } // end of method main } // end of class Variables1 Java - Chapter 1 (c)http://www.geocities.com/idmssql

  11. Initialize You can initialize variables in a declaration. Double initialSpeed =10.0, finalSpeed=60.0, currentSpeed = 0.0; char endChar = ‘a’; Note: Instance and static(=class) variables will be initialized to default values. Local variable will NOT be initialized automatically. Java - Chapter 1 (c)http://www.geocities.com/idmssql

  12. Scope of a Variable • IBM doc says “variable scope can be confusing in Java” • An instance method has access to all (instance, class and local variables) • A class method (= defined static) has access to only class variables and local variables • Local variables are limited to within a method. Outside the method they do not exist. Java - Chapter 1 (c)http://www.geocities.com/idmssql

  13. Assignments • Similar to any other language… • double currentSpeed = 0.0; • int clock = 0; • currentSpeed = currentSpeed + 1.0; • clock++; // known as postincrement • // clock = clock + 1; // same as above • A number like 14.65 is treated as double! • float price1; • price1 = 14.65; Gives compile error • price1 = 14.65f; // ok now Java - Chapter 1 (c)http://www.geocities.com/idmssql

  14. if .. else if (booleanTest) { // code for Value is true. } else { // code for Value is false.} eg: int weight1 = 100, weight2=200; if ( weight1 == weight2) {System.out.print(“weight1 equals weight2 ");} else {System.out.print(“weight1 not equals weight2 ");} • Note the difference between the equality operator (==) and the assignment operator (=) Note! Java - Chapter 1 (c)http://www.geocities.com/idmssql

  15. String Not a basic data type! Used as if it is ! String in Java is implemented as a class Note: not ‘string’, capital ‘S’ is required String str1 = new String(“string value”); String str2 = “ string value”; Char char1 = ‘A’; //String and char are not the same! More on String later Java - Chapter 1 (c)http://www.geocities.com/idmssql

  16. Final Variables (=constants) • A final variable is one that cannot be changed. Any attempt to change it, will result in a compile-time error. This is done with the keyword ‘final’ in the declaration. • Typically they are in CAPITAL letters double final PI = 3.14159265359 ; int final SPEED_LIMIT = 70; Java - Chapter 1 (c)http://www.geocities.com/idmssql

  17. Other Syntax • while • do ... while • for ... loop • Most are simple and straightforward Switch – will be discussed later All others see syntax page at the end ... Java - Chapter 1 (c)http://www.geocities.com/idmssql

  18. Arrays datatype arrayName [] = new datatype[25]; or datatype [] arrayName = new datatype[25]; eg: String ErrorCode [] = new String[50]; The first/last elements are ErrorCode[0] ... ErrorCode[49] Java - Chapter 1 (c)http://www.geocities.com/idmssql

  19. Initializing Arrays Method 1 String names[]; Names = new String[3]; Names[0] = “Ada”; Names[1] = “Byron”; Names[2] = “Napolean”; Method 2 String names[] = {“Ada”; “Byron”; “Napolean”}; Here definition and initialize are done at once. Java - Chapter 1 (c)http://www.geocities.com/idmssql

  20. Hello World Again In the HelloWorld public static void main (String args[]) { ...} The main method takes an array as input arguments. If I say Java HelloWorld John David These names are passed to args[0], args[1]... Here follows the modified program Java - Chapter 1 (c)http://www.geocities.com/idmssql

  21. Example class Hello2 { public static void main (String args[]) { int i; if (args.length == 0) System.out.println("Hello Nobody "); else System.out.println("Arguments length= " + args.length); for (i=0; i < args.length; i = i+1) { System.out.println("Hello " + args[i]); } } } Try Java Hello2 * What do you get? Java - Chapter 1 (c)http://www.geocities.com/idmssql

  22. Javadoc – part of JSDK Input is Java source Output – html files with help information C:\tvg\JCourse1>Javadoc TestGreeting.java Loading source file TestGreeting.java... Constructing Javadoc information... Standard Doclet version 1.4.2_03 Generating constant-values.html... Building tree for all the packages and classes... Building index for all the packages and classes... Generating overview-tree.html... Generating index-all.html... Generating deprecated-list.html... Building index for all classes... Generating allclasses-frame.html... Generating allclasses-noframe.html... Generating index.html...Generating packages.html... Generating TestGreeting.html...Generating package-list... Generating help-doc.html... Java - Chapter 1 (c)http://www.geocities.com/idmssql

  23. Java Buzzwords • Java2 Platforms • - Standard Edition J2SE • - Enterprise Edition – J2EE • - Micro Edition – J2ME (PDA) Every Platform has • Run time – JRE (can be shipped with application) • Dev Kit – JDK (includes JRE) Enterprise Information System • Servlet, JSP, J2EE Client, Applet...Application Server • JDBC, Beans, JavaBeans, EJB ... Java - Chapter 1 (c)http://www.geocities.com/idmssql

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