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Reading and writting

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IO Java CST200 Week 1: Basic Reading and Writing in Java Instructor: Andreea Molnar
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  • 1. IO Java CST200 Week 1: Basic Reading and Writing in Java Instructor: Andreea Molnar

2. Outline Aims Writing to the display Reading from the keyboard 3. Introduction Input/Ouput (IO) operations in Java are more complex than described here. You will need the information presented here to complete your assignments and for the exams 4. Writing System.out.println() prints a new line System.out.println(Hello World); prints Hello World and then moves to the new line 5. Writing You can have an expression that will be printed. Check the definition of an expression!!! System.out.println(3+7+8); //prints 18 and then moves to a new line System.out.println(My student ID is + 3456E); //prints My student ID is 3456E 6. Writing int ID = 3456; System.out.println(My student ID is + ID); //prints My student ID is 3456 7. Writing String firstName = Andreea; String lastName = Molnar; System.out.println(firstName + lastName); //prints AndreeaMolnar 8. Writing String firstName = Andreea; String lastName = Molnar; System.out.println(firstName + + lastName); //prints Andreea Molnar 9. Writing String firstName = Andreea; String lastName = Molnar; System.out.println(firstName.charAt(0) + + lastName.substring(1, 3)); //prints A ol 10. Writing String firstName = Andreea; String lastName = Molnar; System.out.println(firstName.charAt(0) + + lastName.substring(1, 3)); //prints A ol If you do not understand how the above output is generated check the String API, Introduction to Java.ppt or Annex 1 at the end of this presentation. If any of these do not work for you, please ask. 11. Writing System.out.println(firstName + n" + lastName); //will print: //Andreea //Molnar Escape sequence Meaning b backspace t tab n newline r return double quote single quotebackslash You can use escape sequences. 12. Writing System.out.println(Hello World); prints Hello World and then moves to the new line System.out.print(Hello World); - does the same thing as System.out.println but it does not move to a new line 13. Writing System.out.println(Hello World); System.out.println(I am here); System.out.print(Hello World); System.out.print(I am here); 14. Reading This section will provide you with an example on how to read from the keyboard using BufferedReader. You should find the code used attached under this presentation. 15. Reading You will need to import the following classes, in order to make use of them. import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; I have added a few more explanations on Annex 2. 16. Reading Change the main method to throw an exception: public static void main(String[] args) throws IOException { } Exception handling is also not covered by this course. You can find more details at: http://math.hws.edu/javanotes/c3/s7.html 17. Reading Change the main method to throw an exception: public static void main(String[] args) throws IOException { } Exception handling are also not covered by this course. 18. Reading Use BufferedReader for reading lines of text from standard input (i.e. keyboard) BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); reader.readLine(); //reads a line of text as a String IO in Java is not covered in details in this course, and this example is provided just to help you write algorithms 19. Reading Use the Integer and Double static functions to convert a number from a String to an Integer (int) or Double (double). 20. Reading To convert a String to an Integer you can use: Integer.parseInt(String s); int number = Integer.parseInt(23); http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html 21. Reading To convert a String to a Double you can use: Integer.parseInt(String s); int number = Integer.parseInt(23); http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html 22. Reading To convert a String to a Double you can use: Double.parseDouble(String s); double number = Double.parseDouble (23); http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html 23. Reading If you want to read multiple numbers or strings on the same line, you can use regular expressions to extract the numbers. 24. Reading String line = reader.readLine(); //assuming that the read line provides strings or numbers or characters separated by space String [] input = line.split( ); http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#split(ja va.lang.String) 25. Reading //+ is used as a separator String [] line = line.split(+); //* is used as a separator String [] line = line.split(*); 26. Reading String line = reader.readLine(); //assuming that the user introduces the following string: my first program, the line variable will contain the value my first program Check the definition of a variable if you do not know it !!! My first program line 27. Reading String line = reader.readLine(); String [] input = line.split( ); First value in an array starts at index 0!!! my first program line my first program 0 1 2input index 28. Reading String line = reader.readLine(); String [] input = line.split( ); String firstString = input[0]; my first program line my first program 0 1 2input index my firstString 29. Reading String line = reader.readLine(); String [] input = line.split( ); String secondString= input[1]; my first program line my first program 0 1 2input index first secondString 30. Reading String line = reader.readLine(); String [] input = line.split( ); String thirdString= input[2]; my first program line my first program 0 1 2input index program thirdString 31. Reading String line = reader.readLine(); //assuming now that the user introduces the following string: 45*90*78 String [] input = line.split(*); 45*90*78 line 45 90 78 0 1 2input index 32. Reading String line = reader.readLine(); String [] input = line.split(*); int no1 = Integer.parseInt(input[0]); Although input looks like it contains numbers, the numbers are in fact represented as strings, therefore they need to be converted to int. 45*90*78 line 45 90 78 0 1 2input index 33. Reading String line = reader.readLine(); String [] input = line.split(*); int no1 = Integer.parseInt(input[0]); 45*90*78 line 45 90 78 0 1 2input index 45 no1 34. Reading String line = reader.readLine(); String [] input = line.split(*); int no1 = Integer.parseInt(input[1]); 45*90*78 line 45 90 78 0 1 2input index 90 no2 35. Reading String line = reader.readLine(); String [] input = line.split(*); int no1 = Integer.parseInt(input[2]); 45*90*78 line 45 90 78 0 1 2input index 78 no3 36. Summary Writing: use System.out.println() and System.out.print(); Reading: use BufferedReader class 37. Annex 1 String firstName = Andreea; char firstCharacter= firstName.charAt(0); A n d r e e a 0 1 2 3 4 5 6 index 38. Annex 1 String firstName = Andreea; char firstCharacter= firstName.charAt(0); char secondCharacter = firstName.charAt(1); char thirdCharacter = firstName.charAt(2); char fourthCharacter = firstName.charAt(3); // A n d r e e a 0 1 2 3 4 5 6 index 39. Annex 1 System.out.println(firstCharacter); //will print A System.out.println(secondCharacter); //will print n A n d r e e a 0 1 2 3 4 5 6 index 40. Annex 1 String lastName = Molnar; String subString= lastName.substring(1, 3)); M o l n a r 0 1 2 3 4 5 index 41. Annex 1 String lastName = Molnar; String subString= lastName.substring(1, 3)); System.out.println(lastName); //will print ol M o l n a r 0 1 2 3 4 5 42. Annex 2 When you use an already implemented class from the Java framework, you are basically using a class from a package. java.lang package is automatically imported this is why for the HelloWorld program you didnt need to import anything. To import an entire package you can write import package.*. For example: import java.util.*; 43. Annex 2 Importing a package allows you to use the class from a library without fully using its fully qualified name. For example in the case of BufferedReader, without importing java.io.BufferedReader you would have to write: java.io.BufferedReader reader = new java.io.BufferedReader (); http://docs.oracle.com/javase/7/docs/api/java/io/BufferedReader.html


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