website/content/ta/fall2017/cpsc220/nov13.md

Lecture for November 13

File IO (Cont.)

Last class we talked about reading from files, we can also write to files.

Import necessary libraries

First you must import all of the necessary libraries

// To read 
import java.util.Scanner; 
import java.io.FileOutputStream;
// To Write
import java.io.FileReader;
import java.io.PrintWriter;
// For Exception Handling
import java.io.IOException;

Then in your main, declare a FileOutputStream and PrintWriter

FileOutputStream file;
PrintWriter print;

Try-Catch-Finally

Create a try block to open a file for writing

try {
      // If the file doesn't exist, it'll create it
      file = new FileOutputStream("output.txt"); 
      print = new PrintWriter(file);
} catch (IOException except) {
      // Prints out the error message
      System.out.println("File error " + except.getMessage()); 
} 

Adding a finally block allows the program to clean up before it closes

    try {
      file = new FileOutputStream("output.txt"); 
      print = new PrintWriter(file);
    } catch (IOException except) {
      System.out.println("File error " + except.getMessage()); 
    } finally { // It starts here!
      delete file;
      delete print;
      file.close();
      return;
    }

Write to the file :)

Then you can write the to file!

// Do you notice the following methods?
  print.println("Your number is");
  print.print("My name is..\n");
  print.printf("%s %d", "Hello ", 5);
  print.flush(); //Clears the output stream
  file.close(); //Closes the file

Extra Note: Disk fragmentation is a way of cleaning up memory that isn't being used by any of the code on your computer.

Swing Graphics

Importing Necessary Libraries

You need to import all the necessary libraries first

import javax.swing.*;
import java.awt.FlowLayout;
import java.awt.event.ActionListener;

Changing the class header

Your class file needs to extend JFrame that way it can use a bunch of the already existent code written for Swing applications

public class firstGUi extends JFrame {
  //....

Swing Components

Java Swing makes use of what is called Swing Components. These are basic building blocks of GUI items you can add to the screen. For example, a checkbox, a radio button, text field. These are all swing components.

I wrote a blog post back in the summer which is an overview of them. You can check it out here: https://brandonrozek.com/2017/06/java-swing-components/

Inside your firstGUI class, declare some Swing components you would like to use in the program

public class firstGUI extends JFrame {
  JButton button1;
  JTextArea area;
  JTextField text;
  // ....

Constructor

You need to create a constructor for this class that way you can initiate all of the swing component values.

// ...
JButton button1;
JTextArea area;
JTextField text;

// Begin Constructor
firstGUI() {
  // Define the components
  JLabel name = new JLabel("Enter in your name:");
  text = new JTextField("Jennifer", 20); // 20 characters long, default value: Jennifer
  area = new JTextArea(10, 10); //Width and Height is 10 characters big
  JScrollPane sp = new JScrollPane(area); //Adds a scroll bar for the text area
  button1 = new JButton("Press Me");
  
  // Set the Layout
  // FlowLayout organizes each of the components top to bottom, left to right
  setLayout(new FlowLayout()); 
  
  // Add the components to the screen
  add(name);
  add(text);
  add(sp); // never add the textarea when surrounded by a ScrollPane
  add(button1);
}

New Main Method

Finally, you need to create the Main method which will initiate it all

public static void main(String[] args) {
  firstGUI myFrame = new firstGUI();
  
  // End the program when the 'x' button (not keyboard) is pressed
  myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  
  myFrame.setTitle("My title"); // Titles the window
  myFrame.pack(); // Packs it all into the frame
  myFrame.setVisible(true); // Makes the frame appear on the screen
}

Making it interactive

You need to change your class header to the following

public class firstGUI extends JFrame implements ActionListener {
  // ...

Then in your class, add the following method

@Override public void actionPerformed(ActionEvent event) {
  // Do stuff as a result of an event here
  area.append("You Pressed the Button");
}

To make it actually activate as a result of an event. You need to attach it to a swing component.

For example, I want the code in actionPerformed to activate in result of a button press.

Add the following line to your code in the constructor.

//...
button1 = new JButton("Press Me");
button1.addActionListener(this); // New Code
//....

Identifying Button Pressed

How do you know which button was pressed in the actionPerformed method?

You can use event.getSource() to find out.

Example:

@Override public void actionPerformed(ActionEvent event) {
  if (event.getSource() == button1) { // Replace button1 with appropriate variable name
    // Do things as a result of a specific button being pressed
  }
}

Summary

To use Swing, do the following steps

1. Import Libraries
2. Declare J___ variables
3. New the J___ variables
4. Add the J___ variables to the frame
5. Add the ActionListener to the components you wish to monitor