Abstract Windows Toolkit

The Abstract Windows Toolkit libraries are identified mainly by the import statements of "import java.awt.*;" and
"import java.awt.event.*;". Where "import java.awt.*;" refers to the components that are displayable on the screen
and "import java.awt.event.*;" refers to the events that can be triggered by user interaction with those components.

The most commonly used components are:

The most commonly used layouts:

The most commonly used event listeners:

Here is an example:
import java.awt.*; import java.awt.event.*; import java.awt.image.BufferedImage; class AWTExample extends Frame implements ActionListener { Menu menuFile=new Menu("File"); MenuItem menuFileExit=new MenuItem("Exit"); TextField txtColor=new TextField(); Button btnAddColor=new Button("Add Color"); List lstColors=new List(5); Button btnRemoveColor=new Button("Remove Color"); Label lblMostRecentFavoriteColor=new Label(""); TextArea txtMyFavoriteColors=new TextArea(4, 100); Button btnChooseFavoriteColor=new Button("Choose Favorite Color"); public static void main(String args[]) { AWTExample awtExample=new AWTExample("AWT Example"); Toolkit toolkit=Toolkit.getDefaultToolkit(); Dimension dimScreen=toolkit.getScreenSize(); awtExample.setSize(dimScreen.width, dimScreen.height-40); awtExample.setVisible(true); } AWTExample(String strTitle) { super(strTitle); setIconImage(new BufferedImage(1, 1, BufferedImage.TYPE_INT_RGB)); addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent we) { System.exit(0); } }); Panel tempPan=new Panel(); tempPan.setLayout(new BorderLayout()); tempPan.add("West", new Label("Color: ")); tempPan.add("Center", txtColor); tempPan.add("East", btnAddColor); btnAddColor.addActionListener(this); add("North", tempPan); Panel tempPan2=new Panel(); tempPan2.setLayout(new BorderLayout()); tempPan2.add("North", new Label("List of Colors:")); tempPan2.add("Center", lstColors); tempPan2.add("South", btnRemoveColor); btnRemoveColor.addActionListener(this); add("Center", tempPan2); Panel tempPan3=new Panel(); tempPan3.setLayout(new BorderLayout()); Panel tempPan3A=new Panel(); tempPan3A.setLayout(new GridLayout(2, 1)); Panel tempPan3A1=new Panel(); tempPan3A1.setLayout(new GridLayout(1, 2)); tempPan3A1.add(new Label("Most Recent Favorite Color: ")); tempPan3A1.add(lblMostRecentFavoriteColor); tempPan3A.add(tempPan3A1); tempPan3A.add(new Label("Past Favorite Colors:")); tempPan3.add("North", tempPan3A); tempPan3.add("Center", txtMyFavoriteColors); Panel tempPan3B=new Panel(); tempPan3B.add(btnChooseFavoriteColor); btnChooseFavoriteColor.addActionListener(this); tempPan3.add("South", tempPan3B); add("South", tempPan3); menuFile.add(menuFileExit); menuFileExit.addActionListener(this); MenuBar mBar=new MenuBar(); mBar.add(menuFile); setMenuBar(mBar); } public void actionPerformed(ActionEvent ae) { Object evSource=ae.getSource(); if(evSource==menuFileExit) { System.exit(0); } else if(evSource==btnAddColor) { lstColors.add(txtColor.getText()); } else if(evSource==btnRemoveColor) { int intSelectedIndex=lstColors.getSelectedIndex(); if(intSelectedIndex==-1) return; lstColors.remove(intSelectedIndex); } else if(evSource==btnChooseFavoriteColor) { String strItems[]=lstColors.getItems(); ChooseFavoriteColorDialog cDialog=new ChooseFavoriteColorDialog(this, strItems); cDialog.show(); if(cDialog.isCanceled()) return; String strChoice=cDialog.getChoice(); lblMostRecentFavoriteColor.setText(strChoice); txtMyFavoriteColors.append(strChoice+"\n"); } } class ChooseFavoriteColorDialog extends Dialog implements ActionListener, ItemListener { List lstColorChoices=new List(5); Button btnCancel=new Button("Cancel"); boolean blnCanceled=false; ChooseFavoriteColorDialog(Frame parent, String strItems[]) { super(parent, "Choose Favorite Color Dialog", true); for(int i=0;i<strItems.length;i++) { lstColorChoices.add(strItems[i]); } add("North", new Label("Color Choices:")); add("Center", lstColorChoices); lstColorChoices.addItemListener(this); add("South", btnCancel); btnCancel.addActionListener(this); Dimension dimScreen=Toolkit.getDefaultToolkit().getScreenSize(); setLocation(dimScreen.width/4, dimScreen.height/4); setSize(dimScreen.width/2, dimScreen.height/2); } public boolean isCanceled() { return blnCanceled; } public String getChoice() { return lstColorChoices.getSelectedItem(); } public void itemStateChanged(ItemEvent ie) { Object evSource=ie.getSource(); if(evSource==lstColorChoices) { int intSelectedIndex=lstColorChoices.getSelectedIndex(); if(intSelectedIndex==-1) return; blnCanceled=false; dispose(); } } public void actionPerformed(ActionEvent ae) { Object evSource=ae.getSource(); if(evSource==btnCancel) { blnCanceled=true; dispose(); } } } }
The first difference in this source file is the "extends" keyword in the class declaration. When one class "extends" another class it is said that the first class becomes a subclass of the second class which is a superclass of the class extending the second class. The class must extend Frame because we want to use the class as a window to be displayed on the screen. Also, a second difference follows in the keyword of "implements". The "implements" keyword tells the compiler that the class being declared implements an interface. In this case, the interface java.awt.event.ActionListener is being implemented. When a class implements an interface, all functions declared in the interface must by declared and coded in the implementing class. For ActionListener a single function must be declared, and the signature of the function is "public void actionPerformed(ActionEvent ae)". The "actionPerformed" function will be called whenever an event is dispatched by the user by clicking a button or other action generating component. The ActionEvent parameter to "actionPerformed" will contain event specific information for the listener of the event. Mainly, the "getSource" function of ActionEvent is used to determine which button triggered the event. It is possible for a single listener to be listening for events from multiple event sources. Next, in the "main" function we construct a new AWTExample object that we want to display on the computer screen. We make use of commonly used Object in the Toolkit class. The dimension that is returned from the Toolkit classes "getScreenSize" function returns the pixels width and height of the computer screen, so we can position components on the screen depending on the width and height of the screen. The reason why I placed "-40" in the height of the "awtExample" frame is that many computer operating systems have a toolbar at the bottom of the screen that can block out components that are inside of the frame near the bottom of the screen. Lastly, the "setVisible" function of Frame is called to make the window visible. In the AWTExample constructor I made use of the "super" constructor of AWTExample's superclass. It is necessary to call the super constructor for most subclass's in order for the built-in integrity of the superclass to be maintained. The super constructor has one parameter, a String that defines the title of the frame that will be displayed in the title bar of the frame. Next, in the constructor the setIconImage function is called to set the icon displayed in the title bar. The BufferedImage is set to a black rectangle that will be displayed. The next line of code is probably the most confusing to explain, but in short it executes code that is placed in the "windowClosing" event when the user clicks the "x" in the upper- right hand corner to close a window. In this example, the only code executed is "System.exit(0);" which will make all windows and executing units(threads) for the current application process exit immediately. Next, Panels are created and set to certain layouts for positioning components on the screen. If a container sets its layout using "setLayout" to BorderLayout then whenever the "add" function of the container is called the first parameter passed in must be a String of one of the values "North", "South", "East", "West", "Center". The "Center" component takes up all the space not claimed by the other components. If a container sets its layout using "setLayout" to GridLayout then the number of rows and columns to use in the container is specified in the constructor of GridLayout. For example, new GridLayout(2, 1) represents a container with 2 rows and 1 column. Another example, new GridLayout(1, 2) represents a container with 1 row and 2 columns. Lastly, new GridLayout(2, 2) represents a container with 2 rows and 2 columns. When components are placed into container using "add" they are placed from left to right then down a component and left to right again, and repeat for as many rows as the layout specifies(similar to reading a book). If a container doesn't set its layout using "setLayout" then it uses the default layout for that data type. For Frame and Dialog, the default layout is BorderLayout. For Panel, the default layout is FlowLayout. After doing some adding to containers you will see a line of code "btnAddColor.addActionListener(this);". Without this line of code, code will not execute when "btnAddColor" is clicked because no event listeners will have been registered for "btnAddColor". The parameter to "addActionListener" is the class instance that should implement the code to be executed when an event is dispatched. Note that only classes that implement ActionListener interface will be valid as a parameter to "addActionListener". Now that the action listener has registered itself, whenever the user clicks on the button "btnAddColor" the code in "actionPerformed" of the action listener will be executed. You will become more adept at deciphering the laying out of components in containers using layout managers with more practice. The menu item "Exit" is added to the menu "File" and an action listener is added to the "Exit" menu item then a MenuBar object is created and the "File" menu is added to it. The "setMenuBar" function of Frame is called to set the menu bar to the object we just created. Skip down to the "actionPerformed" function. The first line of code is "Object evSource=ae.getSource();" which gets an Object which represents the component that triggered the event. Next, some "if" and "else if" conditional blocks are evaluated to determine which button's code should be executed. The first "if" condition checks if the "menuFileExit" was clicked and if it was then the Java runtime environment exits the application. The second condition checks if the "btnAddColor" was clicked and if it was then the line of code "lstColors.add(txtColor.getText());" is executed which adds an item to the list "lstColors" using the text the user typed into the text field "txtColor". The third condition checks if the "btnRemoveColor" was clicked and if it was then the selected index is checked to make sure it is valid(if it isn't valid then its value is "-1") then if it wasn't returned as a result of it being invalid then the item with the selected index is removed from "lstColors". The fourth condition checks if the "btnChooseFavoriteColor" was clicked and if it was then all the items in "lstColors" are assigned to a String array and that array is passed to a class that subclasses java.awt.Dialog called ChooseFavoriteColorDialog. The first parameter for the ChooseFavoriteColorDialog is a java.awt.Frame or a subclass of java.awt.Frame that is necessary when making a call to the super constructor inside of ChooseFavoriteColorDialog. Next, the "show" function is called, and once the "show" function is called the execution blocks(or is haulted) until the java.awt.Dialog class instance is disposed. After the java.awt.Dialog is disposed, the execution resumes with "if(cDialog.isCanceled())" to check the state of the ChooseFavoriteColorDialog's "blnCanceled" boolean value. If its btnCancel was clicked then the "actionPerformed" function will return; without executing any further code. Otherwise, if the "btnCancel" wasn't clicked then the execution wasn't canceled and the code that executes after the "if" condition gets the String chosen from the java.awt.List in the ChooseFavoriteColorDialog class. The "setText" function of java.awt.Label is called to display text on the screen. The "append" of java.awt.TextArea is called to append text to the current text displayed in the TextArea. Scrolling a little farther down, you can see the class declaration for ChooseFavoriteColorDialog which extends java.awt.Dialog and implements both the ActionListener and ItemListener interfaces. The ActionListener is for the button "btnCancel" and the ItemListener is for the list "lstColorChoices". Once again we must write an "actionPerformed" function for the class implementing the ActionListener. And for this class we must also write a function called "itemStateChanged(ItemEvent ie)" which must be coded for implementors of ItemListener. Whenever either of these events occur, the last line of code in the event handling functions is the "dispose" function which makes the dialog dissapear from the screen and returns execution back to the function that originally called the "show" function of the java.awt.Dialog class. All constructors of java.awt.Dialog require that a java.awt.Frame parent be passed into the constructor to instantiate a new java.awt.Dialog. Therefore, whenever you write your own java.awt.Dialog subclass make sure that one of the parameters to the constructor is a java.awt.Frame subclass. Another new statement that we haven't discussed yet appears in the code "for(int i=0;i<strItems.length;i++)". This instruction starts a loop(somewhat similar to a while loop) which declares and initializes a variable "i" for use within the loop block of code. The loop continues to repeat itself for as long as the boolean expression represented by "i<strItems.length" evaluates to true. At the end of each of the loops cycles, the variable "i" is incremented(increased) by the value 1 and the loop repeats. So, on the first cycle i=0, on the second cycle i=1, on the third cycle i=2, etc. This continues until i=strItems.length which is the number of elements that are in the array. If we didn't do the check of i<strItems.length then eventually we would generate an ArrayIndexOutOfBoundsException which happens when trying to access an element of the array that doesn't exist. Our second example will implement TextField, TextArea, Choice, Checkbox, and CheckboxGroup. Here is the example:
import java.awt.*; import java.awt.event.*; import java.awt.image.BufferedImage; class AWTSurvey extends Frame implements ActionListener { TextField txtName=new TextField(); CheckboxGroup cbgGender=new CheckboxGroup(); Checkbox cbGenderMale=new Checkbox("Male", cbgGender, true); Checkbox cbGenderFemale=new Checkbox("Female", cbgGender, false); Choice chcMonth=new Choice(); Checkbox cbLikeEnglish=new Checkbox("I like English", false); Checkbox cbLikeMath=new Checkbox("I like Math", false); Checkbox cbLikeScience=new Checkbox("I like Science", false); Button btnShowSurvey=new Button("Show Survey"); TextArea txtArea=new TextArea(); public static void main(String args[]) { try { AWTSurvey aFrame=new AWTSurvey(); Dimension dimScreen=Toolkit.getDefaultToolkit().getScreenSize(); aFrame.setSize(dimScreen.width, dimScreen.height-40); aFrame.setVisible(true); } catch(Exception ex) { } } AWTSurvey() { super("AWT Survey"); setIconImage(new BufferedImage(1, 1, BufferedImage.TYPE_INT_RGB)); addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent we) { System.exit(0); } }); chcMonth.add("January"); chcMonth.add("February"); chcMonth.add("March"); chcMonth.add("April"); chcMonth.add("May"); chcMonth.add("June"); chcMonth.add("July"); chcMonth.add("August"); chcMonth.add("September"); chcMonth.add("October"); chcMonth.add("November"); chcMonth.add("December"); Panel tempPan=new Panel(); tempPan.setLayout(new GridLayout(6, 1)); Panel tempPanA=new Panel(); tempPanA.setLayout(new BorderLayout()); tempPanA.add("West", new Label("Name: ")); tempPanA.add("Center", txtName); tempPan.add(tempPanA); Panel tempPanB=new Panel(); tempPanB.setLayout(new GridLayout(1, 3)); tempPanB.add(new Label("Gender:")); tempPanB.add(cbGenderMale); tempPanB.add(cbGenderFemale); tempPan.add(tempPanB); Panel tempPanC=new Panel(); tempPanC.setLayout(new BorderLayout()); tempPanC.add("West", new Label("Favorite Month:")); tempPanC.add("Center", chcMonth); tempPan.add(tempPanC); Panel tempPanD=new Panel(); tempPanD.setLayout(new GridLayout(1, 3)); tempPanD.add(cbLikeEnglish); tempPanD.add(cbLikeMath); tempPanD.add(cbLikeScience); tempPan.add(tempPanD); Panel tempPanE=new Panel(); tempPanE.add(btnShowSurvey); btnShowSurvey.addActionListener(this); tempPan.add(tempPanE); tempPan.add(new Label("Survey Output:")); add("North", tempPan); add("Center", txtArea); } public void actionPerformed(ActionEvent ae) { Object evSource=ae.getSource(); if(evSource==btnShowSurvey) { String strName=txtName.getText(); boolean blnGenderMale=true; Checkbox cbSelected=cbgGender.getSelectedCheckbox(); if(cbSelected==cbGenderFemale) blnGenderMale=false; String strMonth=chcMonth.getSelectedItem(); boolean blnLikeEnglish=cbLikeEnglish.getState(); boolean blnLikeMath=cbLikeMath.getState(); boolean blnLikeScience=cbLikeScience.getState(); StringBuffer strBuffer=new StringBuffer(); strBuffer.append("Name: "); strBuffer.append(strName); strBuffer.append("\n"); strBuffer.append("Gender: "); if(blnGenderMale) { strBuffer.append("Male"); } else { strBuffer.append("Female"); } strBuffer.append("\n"); strBuffer.append("Favorite Month: "); strBuffer.append(strMonth); strBuffer.append("\n"); if(blnLikeEnglish) { strBuffer.append("Likes English"); } else { strBuffer.append("Doesn't like English"); } strBuffer.append("\n"); if(blnLikeMath) { strBuffer.append("Likes Math"); } else { strBuffer.append("Doesn't like Math"); } strBuffer.append("\n"); if(blnLikeScience) { strBuffer.append("Likes Science"); } else { strBuffer.append("Doesn't like Science"); } strBuffer.append("\n"); txtArea.setText(strBuffer.toString()); } } }
Scroll down to the declaration of "cbGenderMale", the constructor has three parameters. The first parameter is the label to display with the Checkbox. The second parameter is the CheckboxGroup to group this Checkbox with. The third parameter is the initial state of the Checkbox(whether it is currently checked). Next, you should notice the declaration of a Choice box called "cbMonth". Notice that it initially it doesn't contain any items. We need to add the items at some point before we expect the user to make a selection. In this example, we will add items in the constructor of our AWTSurvey Frame. Three Checkboxs are declared next and their labels are set in the constructor. Notice that only two parameters are used in these Checkboxs' constructors because they aren't grouped together with a CheckboxGroup. The second parameter is whether or not the Checkbox is initially checked or not. Scroll down to "chcMonth.add("January"))", this is where we place selectable items in the Choice box. Next, scroll down to "actionPerformed". First, we obtain the name of the person from "txtName" then we declare a boolean(true or false) with the value set to true. We then obtain the selected Checkbox by using CheckboxGroup's function "getSelectedCheckbox". If the "cbSelected" is the female gender Checkbox then we set "blnGenderMale" to false. After that, we declare a String "strMonth" to store the selected item of the Choice box "chcMonth". For our last input gathering we handle the individual Checkboxs for liking subjects. Each one stores a boolean value that is obtained using "getState". If the Checkbox has a check mark in it then the "getState" function returns true otherwise it returns false. Lastly, we create a StringBuffer called "strBuffer" for building a String that we will use to set the text of "txtArea" by using the TextArea's "setText" function. In this lesson you learned about some of the most common awt Object types. There are many more that can be found using javadocs that can be downloaded from Oracle's web site. The more you practice with the awt libraries the more proficient you will become.