COMP 14 Introduction to Programming Miguel A Otaduy May 17 2004 Last Friday Variables and expressions Input output Writing a whole program Any questions Today Classes and objects Graphical user interface GUI File input output Formatting input and output Classes In the Java programming language a program is made up of one or more classes a class contains one or more methods a method contains program statements Classes What is a class Data Operations Classes allow creation of new data types public class Student Classes Vs Data Types Abstract Descriptors Concrete Entities Data Type Variable Class Object Inside a class Other classes Data types Methods operations public class Student private String name private int age public void ComputeGrade Primitive Variables int x 45 x is associated with a memory location It stores the value 45 When the computer sees x it knows which memory location to look up the value in Reference Variables Integer num The memory location associated with num can store a memory address The computer will read the address in num and look up an Integer object in that memory location Creating Objects We use the new operator to create objects called instantiation parameter Integer num num new Integer 78 Changing the Reference Var num new Integer 50 The address of the newly created object is stored in the already created reference variable num Garbage Collection What happened to the memory space that held the value 78 If no other reference variable points to that object Java will throw it away Using Objects System out object represents a destination to which we can send output Example println method System out println Hello World information provided to the method method parameters dot operator object Questions 1 True or False A primitive variable is a variable that stores the address of a memory space False 2 The operator new is used to create a class object 3 4 In Java the dot operator is used to access members of a class It separates the class or object name from the method name True or False Class objects are instances of that class True The class String String variables are reference variables Given String name Equivalent Statements name new String Van Helsing name Van Helsing name Van Helsing Van Helsing Van Helsing Van Helsing The class String The String object is an instance of class string The value Van Helsing is instantiated The address of the value is stored in name The new operator is unnecessary when instantiating Java strings String methods are called using the dot operator Common String Methods String String str constructor creates and initializes the object char charAt int index returns char at the position specified by index starts at 0 int indexOf char ch returns the index of the first occurrence of ch int compareTo String str returns negative if this string is less than str returns 0 if this string is the same as str returns positive if this string is greater than str Common String Methods boolean equals String str returns true if this string equals str int length returns the length of the string String replace char toBeReplaced char replacedWith returns the string in which every occurrence of toBeReplaced is replaced with replacedWith String toLowerCase returns the string that is the the same as this string but all lower case String toUpperCase returns the string that is the same as this string but all upper case String Examples String str Van Helsing System out println str length 11 System out println str charAt 2 n System out println str indexOf s 7 System out println str toLowerCase van helsing Using Dialog Boxes for I O Use a graphical user interface GUI class JOptionPane Contained in package javax swing showInputDialog allows user to input a string from the keyboard showMessageDialog allows the programmer to display results Program must end with System exit 0 JOptionPane Methods showInputDialog str JOptionPane showInputDialog strExpression stores what the user enters into the String str showMessageDialog JOptionPane showMessageDialog parentComponent strExpression boxTitleString messageType showMessageDialog parentComponent parent of the dialog box we ll use null StrExpression what you want displayed in the box boxTitleString title of the dialog box messageType what icon will be displayed messageType JOptionPane ERROR MESSAGE error icon JOptionPane INFORMATION MESSAGE information icon JOptionPane PLAIN MESSAGE no icon JOptionPane QUESTION MESSAGE question mark icon JOptionPane WARNING MESSAGE exclamation point icon JOptionPane Example JOptionPane showMessageDialog null Hello World Greetings JOptionPane INFORMATION MESSAGE UsingGUI java example Reading From Text Files Similar to reading from the keyboard Create a BufferedReader object but use a FileReader object instead of InputStreamReader Create BufferedReader object inside the main method instead of outside Substitute the name of the file for System in When finished reading from the file we need to close the file BufferedReader close method Reading From Text Files String file data dat BufferedReader inFile new BufferedReader new FileReader file String line inFile readLine inFile close Exceptions FileNotFoundException if the file specified to open was not found IOException some other I O exception public static void main String args throws FileNotFoundException IOException Writing To Text Files Similar to reading from text files Use FileWriter and PrintWriter instead of FileReader and BufferedReader PrintWriter methods include print and println just like those we used in System out Like reading we need to close the file when we re done PrintWriter close method Writing To Text Files String file outfile dat PrintWriter outFile new PrintWriter new FileWriter file outFile print Hi outFile println There outFile close User Input BufferedReader reads everything as a string Integer parseInt only handles one integer in the string How to handle Enter 3 numbers 34 15 75 The StringTokenizer Class tokens elements that comprise a string tokenizing process of extracting these elements delimiters characters that separate one token from another StringTokenizer class defined in the java util package used to separate a string into tokens Tokens and Delimiters The weekend is over delimiter space tokens The weekend is over Bart Lisa Homer Marge delimiter tokens Bart Lisa Homer Marge The StringTokenizer Class Default delimiters space tab carriage return new line Methods StringTokenizer String str StringTokenizer