2 jfh-yourveryfirstprogram

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This is the implementation with explanations of a Hello World simple program. It is useful to document keyword and Java modifiers, as well as how to execute a program.

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2 jfh-yourveryfirstprogram

  1. 1. Riccardo Rigon Java for Hydrologists Hello World R.Rigon-IltavolodilavorodiRemowolf Tuesday, July 16, 13
  2. 2. “Copy the work of other smart people’ http://lin-ear-th-inking.blogspot.it/2013/03/10-step-program-for-developers.html Tuesday, July 16, 13
  3. 3. Objectives Writing and commenting “Hello World” Introduction •This is kind of a parallel of the section “Building a Java Program” by BE. •Second section of DJE is a little different, but many notions can be useful. R. Rigon Tuesday, July 16, 13
  4. 4. Preliminary operations if you are working within Eclipse •Create a new Java project within Eclipse •Create a new Java package •Create a new Java file For all of these operations, please refer to the Lars Vogel Eclipse Tutorial R. Rigon Work alone Tuesday, July 16, 13
  5. 5. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } this, which is one of the simplest program It is saved in a file whose name must be: HelloWorld.java Copy in the new file R. Rigon First Program Tuesday, July 16, 13
  6. 6. A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time. Programmers also typically use packages to organize classes belonging to the same category or providing similar functionality. Classes in the same package can access each other's package-access members. package org.geoframe.first; R. Rigon Packages Tuesday, July 16, 13
  7. 7. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } This, which is one of the simplest program In Java one file can contain one single public class, which is a construct like the above Copy in the new file R. Rigon Modifiers Tuesday, July 16, 13
  8. 8. public means : is in the programming jargon a modifier. A private class is visible outside the package that contains it. It does not apply only to classes but also to Packages and Worlds. Alternative to public for classes) are: private, protected (an no modifier) •private means that the class is hided from other classes within the package. •protected is a version of public restricted only to subclasses * See also the Chapter “Access Control” in BE book, and for generic information also the Building a Java program in DJE book R. Rigon Modifiers Tuesday, July 16, 13
  9. 9. This, which is one of the simplest program Hello World is the name of the class. In Java you need to build a class to do something Copy in the new file package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } R. Rigon Class Names Tuesday, July 16, 13
  10. 10. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } this, which is one of the simplest program To be executed the class needs to contain a main( ) method (or routine). The argument of the method main is a String[ ] object. args is not used but the compiler still needs it. Copy in the new file R. Rigon main( ) Tuesday, July 16, 13
  11. 11. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } This, which is one of the simplest program The main(String[ ] args ) method has usually 3 modifiers: •public •static •void Copy in the new file R. Rigon Methods’ Modifiers Tuesday, July 16, 13
  12. 12. public modifier has already be seen as in the case of classes here, however is applied to a method, a special method indeed. Since the method is public, it is visible outside the class, and can be shared with other classes. R. Rigon Methods’ Modifiers Tuesday, July 16, 13
  13. 13. static (a few things to explain here): a static method http://www.javatutorialhub.com/java-static-variable-methods.html • It is a method which belongs to the class and not to the object (instance) • A static method can access only static data. It can not access non-static data (instance variables) • A static method can call only other static methods and can not call a non- static method from it. R. Rigon Methods’ Modifiers Tuesday, July 16, 13
  14. 14. static (a few things to explain here): a static method http://www.javatutorialhub.com/java-static-variable-methods.html • A static method can be accessed directly by the class name and doesn’t need any object • Syntax : <class-name>.<method-name> • A static method cannot refer to “this” or “super” keywords in anyway Side Note: • main method is static , since it must be be accessible for an application to run , before any instantiation takes place. R. Rigon Methods’ Modifiers Tuesday, July 16, 13
  15. 15. void: is the return type. Since main( ) does not return anything, the void keyword is used R. Rigon Methods’ Modifiers Tuesday, July 16, 13
  16. 16. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } System.out.println( ) Java comes with a lot of libraries. These do not need to be imported. This, for instance is a method (println) of the class System.out R. Rigon The method and the System Tuesday, July 16, 13
  17. 17. package org.geoframe.first; public class HelloWorld { ! public static void main(String[] args) { System.out.println("Pietro ciao!"); ! } } System.out.println( ) This stackoverflow post clarify better what it does: http://stackoverflow.com/questions/12002170/what-is-system-out-println-in-system-out-println-in-java R. Rigon The method and the System Tuesday, July 16, 13
  18. 18. Execution •Inside Eclipse •Outside Eclipse All of the above information is available from the Vogella website •in OMS3 Please look at the appropriate slides R. Rigon Making it to work Tuesday, July 16, 13
  19. 19. Thank you for your attention. G.Ulrici,2000? It ends here Tuesday, July 16, 13

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