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25  
26  package java.awt.event;
27  
28  import java.awt.Component;
29  import sun.awt.AppContext;
30  import sun.awt.SunToolkit;
31  
32  /**
33   * A low-level event which indicates that a Component has gained or lost the
34   * input focus. This low-level event is generated by a Component (such as a
35   * TextField). The event is passed to every <code>FocusListener</code> or
36   * <code>FocusAdapter</code> object which registered to receive such events
37   * using the Component's <code>addFocusListener</code> method. (<code>
38   * FocusAdapter</code> objects implement the <code>FocusListener</code>
39   * interface.) Each such listener object gets this <code>FocusEvent</code> when
40   * the event occurs.
41   * <p>
42   * There are two levels of focus events: permanent and temporary. Permanent
43   * focus change events occur when focus is directly moved from one Component to
44   * another, such as through a call to requestFocus() or as the user uses the
45   * TAB key to traverse Components. Temporary focus change events occur when
46   * focus is temporarily lost for a Component as the indirect result of another
47   * operation, such as Window deactivation or a Scrollbar drag. In this case,
48   * the original focus state will automatically be restored once that operation
49   * is finished, or, for the case of Window deactivation, when the Window is
50   * reactivated. Both permanent and temporary focus events are delivered using
51   * the FOCUS_GAINED and FOCUS_LOST event ids; the level may be distinguished in
52   * the event using the isTemporary() method.
53   * <p>
54   * An unspecified behavior will be caused if the {@code id} parameter
55   * of any particular {@code FocusEvent} instance is not
56   * in the range from {@code FOCUS_FIRST} to {@code FOCUS_LAST}.
57   *
58   * @see FocusAdapter
59   * @see FocusListener
60   * @see <a href="http://docs.oracle.com/javase/tutorial/uiswing/events/focuslistener.html">Tutorial: Writing a Focus Listener</a>
61   *
62   * @author Carl Quinn
63   * @author Amy Fowler
64   * @since 1.1
65   */
66  public class FocusEvent extends ComponentEvent {
67  
68      /**
69       * The first number in the range of ids used for focus events.
70       */
71      public static final int FOCUS_FIRST         = 1004;
72  
73      /**
74       * The last number in the range of ids used for focus events.
75       */
76      public static final int FOCUS_LAST          = 1005;
77  
78      /**
79       * This event indicates that the Component is now the focus owner.
80       */
81      public static final int FOCUS_GAINED = FOCUS_FIRST; //Event.GOT_FOCUS
82  
83      /**
84       * This event indicates that the Component is no longer the focus owner.
85       */
86      public static final int FOCUS_LOST = 1 + FOCUS_FIRST; //Event.LOST_FOCUS
87  
88      /**
89       * A focus event can have two different levels, permanent and temporary.
90       * It will be set to true if some operation takes away the focus
91       * temporarily and intends on getting it back once the event is completed.
92       * Otherwise it will be set to false.
93       *
94       * @serial
95       * @see #isTemporary
96       */
97      boolean temporary;
98  
99      /**
100      * The other Component involved in this focus change. For a FOCUS_GAINED
101      * event, this is the Component that lost focus. For a FOCUS_LOST event,
102      * this is the Component that gained focus. If this focus change occurs
103      * with a native application, a Java application in a different VM, or with
104      * no other Component, then the opposite Component is null.
105      *
106      * @see #getOppositeComponent
107      * @since 1.4
108      */
109     transient Component opposite;
110 
111     /*
112      * JDK 1.1 serialVersionUID
113      */
114     private static final long serialVersionUID = 523753786457416396L;
115 
116     /**
117      * Constructs a <code>FocusEvent</code> object with the
118      * specified temporary state and opposite <code>Component</code>.
119      * The opposite <code>Component</code> is the other
120      * <code>Component</code> involved in this focus change.
121      * For a <code>FOCUS_GAINED</code> event, this is the
122      * <code>Component</code> that lost focus. For a
123      * <code>FOCUS_LOST</code> event, this is the <code>Component</code>
124      * that gained focus. If this focus change occurs with a native
125      * application, with a Java application in a different VM,
126      * or with no other <code>Component</code>, then the opposite
127      * <code>Component</code> is <code>null</code>.
128      * <p> This method throws an
129      * <code>IllegalArgumentException</code> if <code>source</code>
130      * is <code>null</code>.
131      *
132      * @param source     The <code>Component</code> that originated the event
133      * @param id         An integer indicating the type of event.
134      *                     For information on allowable values, see
135      *                     the class description for {@link FocusEvent}
136      * @param temporary  Equals <code>true</code> if the focus change is temporary;
137      *                   <code>false</code> otherwise
138      * @param opposite   The other Component involved in the focus change,
139      *                   or <code>null</code>
140      * @throws IllegalArgumentException if <code>source</code> equals {@code null}
141      * @see #getSource()
142      * @see #getID()
143      * @see #isTemporary()
144      * @see #getOppositeComponent()
145      * @since 1.4
146      */
147     public FocusEvent(Component source, int id, boolean temporary,
148                       Component opposite) {
149         super(source, id);
150         this.temporary = temporary;
151         this.opposite = opposite;
152     }
153 
154     /**
155      * Constructs a <code>FocusEvent</code> object and identifies
156      * whether or not the change is temporary.
157      * <p> This method throws an
158      * <code>IllegalArgumentException</code> if <code>source</code>
159      * is <code>null</code>.
160      *
161      * @param source    The <code>Component</code> that originated the event
162      * @param id        An integer indicating the type of event.
163      *                     For information on allowable values, see
164      *                     the class description for {@link FocusEvent}
165      * @param temporary Equals <code>true</code> if the focus change is temporary;
166      *                  <code>false</code> otherwise
167      * @throws IllegalArgumentException if <code>source</code> equals {@code null}
168      * @see #getSource()
169      * @see #getID()
170      * @see #isTemporary()
171      */
172     public FocusEvent(Component source, int id, boolean temporary) {
173         this(source, id, temporary, null);
174     }
175 
176     /**
177      * Constructs a <code>FocusEvent</code> object and identifies it
178      * as a permanent change in focus.
179      * <p> This method throws an
180      * <code>IllegalArgumentException</code> if <code>source</code>
181      * is <code>null</code>.
182      *
183      * @param source    The <code>Component</code> that originated the event
184      * @param id        An integer indicating the type of event.
185      *                     For information on allowable values, see
186      *                     the class description for {@link FocusEvent}
187      * @throws IllegalArgumentException if <code>source</code> equals {@code null}
188      * @see #getSource()
189      * @see #getID()
190      */
191     public FocusEvent(Component source, int id) {
192         this(source, id, false);
193     }
194 
195     /**
196      * Identifies the focus change event as temporary or permanent.
197      *
198      * @return <code>true</code> if the focus change is temporary;
199      *         <code>false</code> otherwise
200      */
201     public boolean isTemporary() {
202         return temporary;
203     }
204 
205     /**
206      * Returns the other Component involved in this focus change. For a
207      * FOCUS_GAINED event, this is the Component that lost focus. For a
208      * FOCUS_LOST event, this is the Component that gained focus. If this
209      * focus change occurs with a native application, with a Java application
210      * in a different VM or context, or with no other Component, then null is
211      * returned.
212      *
213      * @return the other Component involved in the focus change, or null
214      * @since 1.4
215      */
216     public Component getOppositeComponent() {
217         if (opposite == null) {
218             return null;
219         }
220 
221         return (SunToolkit.targetToAppContext(opposite) ==
222                 AppContext.getAppContext())
223             ? opposite
224             : null;
225     }
226 
227     /**
228      * Returns a parameter string identifying this event.
229      * This method is useful for event-logging and for debugging.
230      *
231      * @return a string identifying the event and its attributes
232      */
233     public String paramString() {
234         String typeStr;
235         switch(id) {
236           case FOCUS_GAINED:
237               typeStr = "FOCUS_GAINED";
238               break;
239           case FOCUS_LOST:
240               typeStr = "FOCUS_LOST";
241               break;
242           default:
243               typeStr = "unknown type";
244         }
245         return typeStr + (temporary ? ",temporary" : ",permanent") +
246             ",opposite=" + getOppositeComponent();
247     }
248 
249 }