AWT CONTROLS

  The Awt supports the following types of controls:

o   Labels

o   Push buttons

o   Check boxes

o   Choice list

o   Lists

o   Scroll bar

o   Text editing

  Labels: A label is an object of type Label, and it contains a string, which it displays. Labels are passive controls that do not support any interaction with the user. Label define the following constructors:

            Label () throw HeadlessException

            Label (String str) throws Headlessexception

            Label (string str,int how)throw HeadlessException

The first version creates a blank label. The second version creates a label that contains the string specified by str. This string is left- justified. The third version creates a label that contains the string specified by str using alignment specified by how. The value of how must be one three constants: Label.LEFT, Label.RIGHT, or Label.CENTER.

We can set or change the text in a label by using the setText() method, We can  obtain the current label by calling getText(). The methods are:

                        void setText(string str)

                        String getText()

  Button: The most widely used control is the push button. A push button is a component that contains a label and that generates an event when it is pressed. Push buttons are objects of type Button.

            Button() throw HeadlessException

            Button(String str)throws HeadlessException

The first version creates a empty button. The second version creates a button that contains str as a label.

After a button has been created, you can set its label, by calling setLabel(). We can retrieve its label by calling getLabel(). The methods are:

                        void setLabel(String str)

                        String getLabel()

            Here, str the new label for the button.

Handling button: Each time a button is pressed, an action event is generated. This is sent to any listeners that previously registered an interest in receiving action event notification from that component. Each listener implements the ActionListener interface. This interface defines the actionPerformed() methods, which is called when an event occurs. An ActionEvent object is supplied as the argument to this method.  getSource() methods to the button objects that we added to the window.

  Check Boxes: A check box is a control that is used to turn an option on or off. It consists of a small box that can either contain a check mark or not. Check boxes are object objects of the Checkbox class. Checkbox support this constructor:

                        Checkbox()throw HeadlessException

                        Checkbox(String str)throw HeadlessException

                        Checkbox(String str,boolen on)throw HeadlessException

Checkbox(String str,boolen on,CheckboxGroup cbGroup) throw HeadlessException

Checkbox(String str,CheckboxGroup cbGroup,boolen on) throw HeadlessException

The first form creates a check box whose label is initially blank. The state of this checked box is unchecked. The second form creates a check box whose label is specified by str. The state of this checked box is unchecked. The third form allows you to set the initial state of the check box. If on is true, the check box is initially checked; otherwise it is cleared. The fourth and fifth forms creates a check box whose label specify by the str and whose group is specified by cbGroup. 

To retrieve the current state of a check box, call getState(). To set its state, call setState().We can obtain the current label associated with a check box by calling getLabel().To set the label, call setLabel().These methods are

                                    boolean getState()

                                    void setState(Boolean on)

                                    String getLabel()

                                    void setLabel(String str)

Handling Checkbox: Each time a check box is selected or deselected, an item event is generated. This is sent to any listeners that previously registered an interest in receiving action event notification from that component. Each listener implements the ItemListener interface.

  Choice list: The choice class is used to create a pop-up list of items from which the user may choose. To add a selection to the list, call add().It has this general form:

                        void  add(String name)

Here, name is the name of the item being added. Items are added to the list in the order in which call to add() occur. To determine which item is currently selected, we may call either getSelectItem() or getSelectIndex(). These methods are:

                        String getSelectItem()

                        int getSelectIndex()

            The getSelectItem() methods return a string containing the name of the item.

            The getSelectIndex() methods return the index of the item. The first item is at index 0.

            By default, the first item added to the list is selected.

To obtain the number of items in the list, call getItemCount(). We can set the currently selected item using the select() methods with either a zero-based integer index or a string that will match a name in the list.

                        int getItemCount()

                        void select(int index)

                        void select(String name)

We can obtain the name associated with the item at that index by calling getItem(), which has this general form :

                        String getItem(int index)

            Here, index specifies the index of the desired element.

Handling Choice List: Each item a choices is selected, an item event is generated. This is sent to any listeners that previously registered an interest in receiving action event notification from that component. Each listener implements the ItemListener interface. That interface defines the itemStateChange() methods. An ItemEvent object is supplied as the argument to this method.

  Lists: The List class provides a compact multiple choices, scrolling selection list. The choice object, which shows only the single selected item in the menu. List provides these constructors:

                        List() throw headlessException

                        List(int numRows) throw headlessException

                        List(int numRows,boolean multipleSelect ) throw headlessException

The first version creates a list control that allows only one item to be selected at any one  item. In second version, the value of numRows specifies the number of entries in the list that will always be visible. In the third form, if multipleSelect is true, then the user may select one or more item at a time.

            To add a selection to the list , call  add().

                        void add(String name)

                        void add(String name,int index)

            Here name is the name of the item added to the list. The first form adds item to the           end of the list. The second form adds the item in the index specified by the index.

                        String getSelectedItem()

                        int getSelectedIndex()  

The getSelectedItem() methods return a string containing the name of the item. If more than one item is selected, or if no selected, or if no selection has been made, null is returned. getSelectIndex() methods return the index of  the item.

Handling List: We will need to implement the ActionListener interface. Each time a List item is double clicked, an ActionEvent object is generated. Its getActionCommand() methods can be used to retrieve  the name of the newly selected item.each item an item is selected or deselected with a single click, an ItemEvent object is generated .getStateChange() can be used to determine whether a selection or deselection  triggered this event.  

  Scroll Bars: Scroll Bars are used to select continuous values between a specified minimum and maximum Scroll bars may be oriented horizontally or vertically. Scroll bar are encapsulated by the Scrollbar class.

            Scrollbar define the following constructor :

                        Scrollbar()throws HeadlessException

                        Scrollbar(int style)throws HeadlessException

Scrollbar(int style,int initialValue,int thumbSize,int min,int max)throws HeadlessException

The first form creates a vertical scroll bar. The Second and third forms allow we to specify orientation of the scroll bar. If style is scrollbar.VERTICAL, a vertical scroll bar created. If style is scrollbar.HORIZONTAL, a horizontal scroll bar created. The third form of the constructor, the initial value of the scroll bar is passed in initialValue.

The number of units represented by the height of the thumb is passed in thumbsize. The minimum and maximum values for the scroll bar are specified by min and max.     

                        void setValues(int initialvalue,int thumbsize,int min,int max)

            The same meaning as they have in the third constructor just described:

To obtain current value of the scroll bar call getValue(). It returns the current string. To set the current value, call setValue(). These methods are:

                        int getValue()

                        void setValue(int newValue)

Here, new value means the new value of the scroll bar. We can also retrieve the minimum and maximum values via getMinimum() and getMaximum() as shown here:

                        int getMinimum()

                        int getMaximum()

Handling Scroll bar: We need to implement the AdjuestmentListener interface. Each time a user interface with a scroll bar, an AdjustmentEvent object is generated. Its getAdjustmentType() method can be used to determine the type of the adjustment. 

  TextArea:  Some time a single line of text input is not enough for a given task. To handle these situations the AWT includes a simple multiline editor called TextArea. The following constructors are used for TextArea :

                         TextArea()throws HeadlessException

                         TextArea(int numList,int numChars) throws HeadlessException

                         TextArea(String str) throws HeadlessException

                         TextArea(String str,int numLines,int numChars) throws HeadlessException

 TextArea(String str,int numLines,int numChars,int sBars) throws HeadlessException

Here, numLines specifies the height, in lines of the text area, and numChars specifies its Width in character. Initial text can be specified by str. In the fifth form, you can specifies the scroll bars that you want the control to have sBars must be one of this values.

TextArea is a subclass of TextComponent. Therefore, it supports the getText(), setText(), getSelectedText(), select(), isEditable(), setEditable() methods.

                        void append(string str)

                        void insert(String str,int index)

                        void replaceRange(String str,int Index,int endIndex)

The append() methods append the string specified by str to the end of the current text.insert() insert the starting passed in str at the specified index. To replace text, call replaceRange(). It replace the character from startIndex to the endIndex-1, with the replacement text passed in str.

  TextField: The TextField class implements a single-line text-entry area, usually called an edit control. Text fields allow the user to enter the strings and to edit the text using the arrow keys. cut and paste keys, and mouse selections. TextFields is a subclass of TextComponent.

TextField()throws HeadlessException

TextField(int numChars)throws HeadlessException

TextField(String str) throws HeadlessException

TextField(String str,int numChars) throws HeadlessException

The first version creates a default text field. The second version creates a text fields that is numChars characters wide. The third version initialize the text field call with the string contain in str. The fourth form initializes a text field and sets its width.

TextField (and its superclass TextComponent) provides several methods that allow us to utilize a text field. To obtain the string currently contained in the text field, call getText(). To set the text call the setText().

String getText()

void setText(String str)

Here, str in the new string. 

The user can select a portion of the text in a text filed. Also we can select a portion of text under program control by using select(). We can obtain the currently selected text by calling getSelectText(). These methods are:

                        String getSelectText()

                        void select(int startIndex,int endIndex);

getSelectText() returns the selected text. The select() method select the characters beginning at startIndex and ending at endIndex -1.

We can modify the text in the text field by calling the setEditable(). We can determine it by calling isEditable(). These methods are:

                        boolen isEditable()

                        void setEditable(Boolean canEdit)

isEditable() returns true if the text may be changed and false if not. In setEditable(), if canEdit is true, the text may be changed. If it is false the text cannot be altered.

void setEchoChar(Char ch)

boolen echoCharIsSet()

char getEchoChar()

Here, ch specifies the character to be echoed.

Therefore we can display the echoing of the character as they are typed by calling setEchoChar(). We can check a text field to see if it is in this mode with the echoCharIsSet() method. We can retrieve the echo character by calling getEchoChar() method.