CSE 205 Exam 2 Review By Cheri Thompson and Katherine Kincade

Interfaces Interfaces are like classes, but they contain groups of methods that have nothing inside. Think of an interface as being a blueprint. Classes implement interfaces. When a class uses an interface, it must declare each method that was outlined in the interface, or the compiler will not allow that class to compile. interface house { //…. void setWindows(int size); //etc... } class Mansion implements House { Int windowSize; //….. void setWindows(int size) { windowSize = size; } //etc…. }

GUI: Your Tools JApplet: “holds” everything desired to be displayed when ran JButton: an object which displays a pressable button JTextField: takes in user input which is typed by user JTextArea: displays plain text JLabel: displays a label message JList: a component used to display an array or vector of Objects JRadioButton: a button which is selected or deselected based on a desired state ButtonGroup: when multiple buttons are in the same ButtonGroup, clicking one button deactivates the others in the group Container: used with getContentPane( ) method of JFrame and JApplet JPanel: Holds other components JTabbedPane: A pane which allows the user to switch between different tabs JScrollPane: Provides a scrollable view to a JTextArea or JTextField ActionEvent: an event that can trigger the ActionListener ActionListener: allows an action to occur when a specific ActionEvent occurs

GUI Graphics: Listening To Buttons ●—When the mouse is inside the window of a GUI program, events occur. ●—If you want your program to react to events, you need to use event listener objects. ●—Event Listeners are instances of classes, they provide instructions in their methods explaining how you want your program to react to events—like button clicks. ●—Without event listeners, your buttons/GUI elements will not respond to user interaction! ○—Users can click all they want, without a listener, your buttons, lists, etc., are not paying attention.

GUI Graphics: Listening To Buttons--Step 1 —You implenent the ActionListener interface to create a listener class that does what you want when the action happens—such as changing text of a JLabel in the example below. —This class will be a nested private class inside the class or code block that contains your elements. In the code to the left, the JLabel named jumpLabel will change text to “Now jumping” when someone interacts with the element that has this listener added to it. private class JumpListener implements ActionListener { public void actionPerformed(ActionEvent event) { jumpLabel.setText(“Now jumping”); } }

GUI Graphics: Listening To Buttons--Step 2 —Just like how you use the Scanner class, after you create your nested class—JumpListener in this case—you must create an object of that class before you can apply it to any elements. —You can create this object any time before its needed. Make sure it’s accessible to the GUI elements that need it. JumpListener jumpListener = new JumpListener(); jumpLabel = new JLabel(“ready to jump”); jumpButton = new JButton(“Click to jump”);

GUI Graphics: Listening To Buttons--Step 3 —GUI elements like Jbutton and Jlabel have an add method so that an ActionListener can pay attention to them. —If you don’t add your ActionListener to your object, they don’t pay any attention to each other. —Make sure to add your ActionListener after your GUI elements have been initialized. jumpButton.addActionListener(jumpListener);

GUI Graphics: Listening To Buttons--Step 4 Test your buttons! —Now you should have everything in place to check if your elements are actually interacting with your ActionListener correctly. —If you get the output you want, congratulations! —If you’re not seeing anything on your screen, check the layouts of your JPanels, and if your elements are all added to your Jpanels.

Exceptions Handling —When your program comes across an error it doesn’t know what to do with, it will stop. —You can avoid the program stopping itself by giving it instructions for when it runs into errors. This is what input/output exception handling is about. —You can even force the program to claim it has found a certain kind of error/exception. —If you want to throw an input/output related exception (like FileNotFoundException), you will need to import java.io.

Throwing Exceptions —If you indicate that your method throws an exception using the throws keyword, the method will end when it runs into that kind of exception. public static void main(String[] args) throws FileNotFoundException { //now main will end rather than giving you an error if a FileNotFoundException happens here }

Catching Exceptions —Use the try and catch keywords. Code inside a try block will attempt to execute as normal, but if an exception detailed in a catch statement is found, the code will stop executing and move to the catch. —If your program throws a FileNotFoundException and has an IOException catch statement, this will execute because a FileNotFoundException is a descendant of IOException. —Catch blocks only execute if the exception identified in the catch statement happens during the try block of code. —The Finally block of code executes after the try and catch statements have executed, regardless of what happened in the try and catch statements.

Checked And Unchecked Exceptions —If you do not indicate how to deal with a checked exception, the program will not compile. Checked exceptions are also sometimes known as compile time exceptions. If you try to read a file, but the file you specified does not exist, the compiler will indicate that you have a FileNotFoundException and refuse to compile. This is a checked exception. An unchecked exception is not caught by the compiler, and will show up while the program is running when unexpected behavior occurs. An example of an unchecked exception is an ArrayIndexOutOfBoundsException. —Want more info? Look to Chapter 7.4 of Java For Everyone 2e.

Common Java Exceptions ArrayIndexOutOfBoundsException: The array index you’re trying to access is either larger than the largest index, or negative. In java.lang. NullPointerException: You’re trying to access an object that holds a null value. In java.lang. StackOverflowError: Your recursive function called itself too many times and the program ran out of room because of it. In java.lang. NumberFormatException: You probably tried to convert a string to a number, but there was a non- number in the string. In java.lang. IllegalArgumentException: An argument you’re trying to pass into your method (the stuff in parenthesis when you call the method) doesn’t match what the method is formatted to receive (the stuff in parenthesis when you create the method). In java.lang.

Text File Read/Write You can read and write files using a BufferedReader and BufferedWriter String str; FileReader fReader = new FileReader(“file.txt”); BufferedReader bReader = new BufferedReader(fReader); while(str = bReader.readLine()) != null){ System.out.println(str); } bReader.close(); fReader.close()

Serialization/ De-Serialization Serialization allows objects to exist outside of a running program Example for serialization. De-serialization is same except you use FileInputStream and ObjectOutputStream Student s = new Student(); FileOutputStream fileout = new FileOutputStream(“students.txt”); ObjectOutputStream output = new ObjectOutputStream(fileout); output.writeObject(s); output.close(); fileout.close();

Algorithms Insertion Sort: Removes one element from the input data and then places it in its correct place within the sorted portion of the list Selection Sort: The algorithm splits an array into two parts: sorted and unsorted. It finds the smallest element in the unsorted portion of the array and then moves it to the end of the sorted portion of the array Merge Sort: Is a recursive algorithm which splits the unsorted array into sublists, each containing one element and then the sublists are merged together until the list is sorted Quick Sort: A pivot is chosen, and then the elements are sorted into two groups: the elements smaller than the pivot and the elements larger than the pivot. This is done recursively until the array is sorted

Sorting —Insertion: O(n 2 ) —Selection: O(n 2 ) —Merge: More efficient than selection sort, O(nlog n ) — Quick: O(n 2 ) —Want more info? Look to Chapter 14 of Java For Everyone 2e.