https://openjdk.java.net/ 
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package java.awt.event;

import java.awt.Component;
import java.io.ObjectStreamException;

import sun.awt.AWTAccessor;
import sun.awt.AppContext;
import sun.awt.SunToolkit;


A low-level event which indicates that a Component has gained or lost the input focus. This low-level event is generated by a Component (such as a TextField). The event is passed to every FocusListener or FocusAdapter object which registered to receive such events using the Component's addFocusListener method. (FocusAdapter objects implement the FocusListener interface.) Each such listener object gets this FocusEvent when the event occurs. 

There are two levels of focus events: permanent and temporary. Permanent
focus change events occur when focus is directly moved from one Component to
another, such as through a call to requestFocus() or as the user uses the
TAB key to traverse Components. Temporary focus change events occur when
focus is temporarily lost for a Component as the indirect result of another
operation, such as Window deactivation or a Scrollbar drag. In this case,
the original focus state will automatically be restored once that operation
is finished, or, for the case of Window deactivation, when the Window is
reactivated. Both permanent and temporary focus events are delivered using
the FOCUS_GAINED and FOCUS_LOST event ids; the level may be distinguished in
the event using the isTemporary() method.

 Every FocusEvent records its cause - the reason why this event was generated. The cause is assigned during the focus event creation and may be retrieved by calling getCause. 
 An unspecified behavior will be caused if the id parameter of any particular FocusEvent instance is not in the range from FOCUS_FIRST to FOCUS_LAST. 
Author:Carl Quinn, Amy Fowler
See Also:
Since:1.1
/**
 * A low-level event which indicates that a Component has gained or lost the
 * input focus. This low-level event is generated by a Component (such as a
 * TextField). The event is passed to every {@code FocusListener} or
 * {@code FocusAdapter} object which registered to receive such events
 * using the Component's {@code addFocusListener} method.
 * ({@code FocusAdapter} objects implement the {@code FocusListener}
 * interface.) Each such listener object gets this {@code FocusEvent} when
 * the event occurs.
 * <p>
 * There are two levels of focus events: permanent and temporary. Permanent
 * focus change events occur when focus is directly moved from one Component to
 * another, such as through a call to requestFocus() or as the user uses the
 * TAB key to traverse Components. Temporary focus change events occur when
 * focus is temporarily lost for a Component as the indirect result of another
 * operation, such as Window deactivation or a Scrollbar drag. In this case,
 * the original focus state will automatically be restored once that operation
 * is finished, or, for the case of Window deactivation, when the Window is
 * reactivated. Both permanent and temporary focus events are delivered using
 * the FOCUS_GAINED and FOCUS_LOST event ids; the level may be distinguished in
 * the event using the isTemporary() method.
 * <p>
 * Every {@code FocusEvent} records its cause - the reason why this event was
 * generated. The cause is assigned during the focus event creation and may be
 * retrieved by calling {@link #getCause}.
 * <p>
 * An unspecified behavior will be caused if the {@code id} parameter
 * of any particular {@code FocusEvent} instance is not
 * in the range from {@code FOCUS_FIRST} to {@code FOCUS_LAST}.
 *
 * @see FocusAdapter
 * @see FocusListener
 * @see <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/focuslistener.html">Tutorial: Writing a Focus Listener</a>
 *
 * @author Carl Quinn
 * @author Amy Fowler
 * @since 1.1
 */
public class FocusEvent extends ComponentEvent {

    
This enum represents the cause of a FocusEvent- the reason why it occurred. Possible reasons include mouse events, keyboard focus traversal, window activation. If no cause is provided then the reason is UNKNOWN. 
Since:9
/**
     * This enum represents the cause of a {@code FocusEvent}- the reason why it
     * occurred. Possible reasons include mouse events, keyboard focus
     * traversal, window activation.
     * If no cause is provided then the reason is {@code UNKNOWN}.
     *
     * @since 9
     */
    public enum Cause {
        
The default value.

/**
         * The default value.
         */
        UNKNOWN,
        
An activating mouse event.

/**
         * An activating mouse event.
         */
        MOUSE_EVENT,
        
A focus traversal action with unspecified direction.

/**
         * A focus traversal action with unspecified direction.
         */
        TRAVERSAL,
        
An up-cycle focus traversal action.

/**
         * An up-cycle focus traversal action.
         */
        TRAVERSAL_UP,
        
A down-cycle focus traversal action.

/**
         * A down-cycle focus traversal action.
         */
        TRAVERSAL_DOWN,
        
A forward focus traversal action.

/**
         * A forward focus traversal action.
         */
        TRAVERSAL_FORWARD,
        
A backward focus traversal action.

/**
         * A backward focus traversal action.
         */
        TRAVERSAL_BACKWARD,
        
Restoring focus after a focus request has been rejected.

/**
         * Restoring focus after a focus request has been rejected.
         */
        ROLLBACK,
        
A system action causing an unexpected focus change.

/**
         * A system action causing an unexpected focus change.
         */
        UNEXPECTED,
        
An activation of a toplevel window.

/**
         * An activation of a toplevel window.
         */
        ACTIVATION,
        
Clearing global focus owner.

/**
         * Clearing global focus owner.
         */
        CLEAR_GLOBAL_FOCUS_OWNER
    }

    
The first number in the range of ids used for focus events.

/**
     * The first number in the range of ids used for focus events.
     */
    public static final int FOCUS_FIRST         = 1004;

    
The last number in the range of ids used for focus events.

/**
     * The last number in the range of ids used for focus events.
     */
    public static final int FOCUS_LAST          = 1005;

    
This event indicates that the Component is now the focus owner.

/**
     * This event indicates that the Component is now the focus owner.
     */
    public static final int FOCUS_GAINED = FOCUS_FIRST; //Event.GOT_FOCUS

    
This event indicates that the Component is no longer the focus owner.

/**
     * This event indicates that the Component is no longer the focus owner.
     */
    public static final int FOCUS_LOST = 1 + FOCUS_FIRST; //Event.LOST_FOCUS

    
A focus event has the reason why this event was generated.
The cause is set during the focus event creation.

See Also:
@serial
Since:9
/**
     * A focus event has the reason why this event was generated.
     * The cause is set during the focus event creation.
     *
     * @serial
     * @see #getCause()
     * @since 9
     */
    private final Cause cause;

    
A focus event can have two different levels, permanent and temporary.
It will be set to true if some operation takes away the focus
temporarily and intends on getting it back once the event is completed.
Otherwise it will be set to false.

See Also:
@serial
/**
     * A focus event can have two different levels, permanent and temporary.
     * It will be set to true if some operation takes away the focus
     * temporarily and intends on getting it back once the event is completed.
     * Otherwise it will be set to false.
     *
     * @serial
     * @see #isTemporary
     */
    boolean temporary;

    
The other Component involved in this focus change. For a FOCUS_GAINED
event, this is the Component that lost focus. For a FOCUS_LOST event,
this is the Component that gained focus. If this focus change occurs
with a native application, a Java application in a different VM, or with
no other Component, then the opposite Component is null.

See Also:
  • getOppositeComponent
Since:1.4
/**
     * The other Component involved in this focus change. For a FOCUS_GAINED
     * event, this is the Component that lost focus. For a FOCUS_LOST event,
     * this is the Component that gained focus. If this focus change occurs
     * with a native application, a Java application in a different VM, or with
     * no other Component, then the opposite Component is null.
     *
     * @see #getOppositeComponent
     * @since 1.4
     */
    transient Component opposite;

    /*
     * JDK 1.1 serialVersionUID
     */
    private static final long serialVersionUID = 523753786457416396L;

    
Constructs a FocusEvent object with the specified temporary state, opposite Component and the Cause.UNKNOWN cause. The opposite Component is the other Component involved in this focus change. For a FOCUS_GAINED event, this is the Component that lost focus. For a FOCUS_LOST event, this is the Component that gained focus. If this focus change occurs with a native application, with a Java application in a different VM, or with no other Component, then the opposite Component is null. 
 This method throws an IllegalArgumentException if source is null. 
Params:
  • source –  The Component that originated the event
  • id –  An integer indicating the type of event. For information on allowable values, see the class description for FocusEvent
  • temporary –  Equals true if the focus change is temporary; false otherwise
  • opposite –  The other Component involved in the focus change, or null
Throws:
See Also:
Since:1.4
/**
     * Constructs a {@code FocusEvent} object with the
     * specified temporary state, opposite {@code Component} and the
     * {@code Cause.UNKNOWN} cause.
     * The opposite {@code Component} is the other
     * {@code Component} involved in this focus change.
     * For a {@code FOCUS_GAINED} event, this is the
     * {@code Component} that lost focus. For a
     * {@code FOCUS_LOST} event, this is the {@code Component}
     * that gained focus. If this focus change occurs with a native
     * application, with a Java application in a different VM,
     * or with no other {@code Component}, then the opposite
     * {@code Component} is {@code null}.
     * <p> This method throws an
     * {@code IllegalArgumentException} if {@code source}
     * is {@code null}.
     *
     * @param source     The {@code Component} that originated the event
     * @param id         An integer indicating the type of event.
     *                     For information on allowable values, see
     *                     the class description for {@link FocusEvent}
     * @param temporary  Equals {@code true} if the focus change is temporary;
     *                   {@code false} otherwise
     * @param opposite   The other Component involved in the focus change,
     *                   or {@code null}
     * @throws IllegalArgumentException if {@code source} equals {@code null}
     * @see #getSource()
     * @see #getID()
     * @see #isTemporary()
     * @see #getOppositeComponent()
     * @see Cause#UNKNOWN
     * @since 1.4
     */
    public FocusEvent(Component source, int id, boolean temporary,
                      Component opposite) {
        this(source, id, temporary, opposite, Cause.UNKNOWN);
    }

    
Constructs a FocusEvent object with the specified temporary state, opposite Component and the cause. The opposite Component is the other Component involved in this focus change. For a FOCUS_GAINED event, this is the Component that lost focus. For a FOCUS_LOST event, this is the Component that gained focus. If this focus change occurs with a native application, with a Java application in a different VM, or with no other Component, then the opposite Component is null. 
 This method throws an IllegalArgumentException if source or cause is null. 
Params:
  • source –  The Component that originated the event
  • id –  An integer indicating the type of event. For information on allowable values, see the class description for FocusEvent
  • temporary – Equals true if the focus change is temporary; false otherwise
  • opposite –  The other Component involved in the focus change, or null
  • cause –     The focus event cause.
Throws:
See Also:
Since:9
/**
     * Constructs a {@code FocusEvent} object with the
     * specified temporary state, opposite {@code Component} and the cause.
     * The opposite {@code Component} is the other
     * {@code Component} involved in this focus change.
     * For a {@code FOCUS_GAINED} event, this is the
     * {@code Component} that lost focus. For a
     * {@code FOCUS_LOST} event, this is the {@code Component}
     * that gained focus. If this focus change occurs with a native
     * application, with a Java application in a different VM,
     * or with no other {@code Component}, then the opposite
     * {@code Component} is {@code null}.
     * <p> This method throws an
     * {@code IllegalArgumentException} if {@code source} or {@code cause}
     * is {@code null}.
     *
     * @param source    The {@code Component} that originated the event
     * @param id        An integer indicating the type of event.
     *                  For information on allowable values, see
     *                  the class description for {@link FocusEvent}
     * @param temporary Equals {@code true} if the focus change is temporary;
     *                  {@code false} otherwise
     * @param opposite  The other Component involved in the focus change,
     *                  or {@code null}
     * @param cause     The focus event cause.
     * @throws IllegalArgumentException if {@code source} equals {@code null}
     *                                  or if {@code cause} equals {@code null}
     * @see #getSource()
     * @see #getID()
     * @see #isTemporary()
     * @see #getOppositeComponent()
     * @see Cause
     * @since 9
     */
    public FocusEvent(Component source, int id, boolean temporary,
                      Component opposite, Cause cause) {
        super(source, id);
        if (cause == null) {
            throw new IllegalArgumentException("null cause");
        }
        this.temporary = temporary;
        this.opposite = opposite;
        this.cause = cause;
    }

    
Constructs a FocusEvent object and identifies whether or not the change is temporary. 
 This method throws an IllegalArgumentException if source is null. 
Params:
  • source –  The Component that originated the event
  • id –  An integer indicating the type of event. For information on allowable values, see the class description for FocusEvent
  • temporary – Equals true if the focus change is temporary; false otherwise
Throws:
See Also:
/**
     * Constructs a {@code FocusEvent} object and identifies
     * whether or not the change is temporary.
     * <p> This method throws an
     * {@code IllegalArgumentException} if {@code source}
     * is {@code null}.
     *
     * @param source    The {@code Component} that originated the event
     * @param id        An integer indicating the type of event.
     *                     For information on allowable values, see
     *                     the class description for {@link FocusEvent}
     * @param temporary Equals {@code true} if the focus change is temporary;
     *                  {@code false} otherwise
     * @throws IllegalArgumentException if {@code source} equals {@code null}
     * @see #getSource()
     * @see #getID()
     * @see #isTemporary()
     */
    public FocusEvent(Component source, int id, boolean temporary) {
        this(source, id, temporary, null);
    }

    
Constructs a FocusEvent object and identifies it as a permanent change in focus. 
 This method throws an IllegalArgumentException if source is null. 
Params:
  • source –  The Component that originated the event
  • id –  An integer indicating the type of event. For information on allowable values, see the class description for FocusEvent
Throws:
See Also:
/**
     * Constructs a {@code FocusEvent} object and identifies it
     * as a permanent change in focus.
     * <p> This method throws an
     * {@code IllegalArgumentException} if {@code source}
     * is {@code null}.
     *
     * @param source    The {@code Component} that originated the event
     * @param id        An integer indicating the type of event.
     *                     For information on allowable values, see
     *                     the class description for {@link FocusEvent}
     * @throws IllegalArgumentException if {@code source} equals {@code null}
     * @see #getSource()
     * @see #getID()
     */
    public FocusEvent(Component source, int id) {
        this(source, id, false);
    }

    
Identifies the focus change event as temporary or permanent.

Returns:true if the focus change is temporary; false otherwise
/**
     * Identifies the focus change event as temporary or permanent.
     *
     * @return {@code true} if the focus change is temporary;
     *         {@code false} otherwise
     */
    public boolean isTemporary() {
        return temporary;
    }

    
Returns the other Component involved in this focus change. For a
FOCUS_GAINED event, this is the Component that lost focus. For a
FOCUS_LOST event, this is the Component that gained focus. If this
focus change occurs with a native application, with a Java application
in a different VM or context, or with no other Component, then null is
returned.

Returns:the other Component involved in the focus change, or null
Since:1.4
/**
     * Returns the other Component involved in this focus change. For a
     * FOCUS_GAINED event, this is the Component that lost focus. For a
     * FOCUS_LOST event, this is the Component that gained focus. If this
     * focus change occurs with a native application, with a Java application
     * in a different VM or context, or with no other Component, then null is
     * returned.
     *
     * @return the other Component involved in the focus change, or null
     * @since 1.4
     */
    public Component getOppositeComponent() {
        if (opposite == null) {
            return null;
        }

        return (SunToolkit.targetToAppContext(opposite) ==
                AppContext.getAppContext())
                ? opposite
                : null;
    }

    
Returns a parameter string identifying this event.
This method is useful for event-logging and for debugging.

Returns:a string identifying the event and its attributes
/**
     * Returns a parameter string identifying this event.
     * This method is useful for event-logging and for debugging.
     *
     * @return a string identifying the event and its attributes
     */
    public String paramString() {
        String typeStr;
        switch(id) {
            case FOCUS_GAINED:
                typeStr = "FOCUS_GAINED";
                break;
            case FOCUS_LOST:
                typeStr = "FOCUS_LOST";
                break;
            default:
                typeStr = "unknown type";
        }
        return typeStr + (temporary ? ",temporary" : ",permanent") +
                ",opposite=" + getOppositeComponent() + ",cause=" + getCause();
    }

    
Returns the event cause.

Returns:one of Cause values
Since:9
/**
     * Returns the event cause.
     *
     * @return one of {@link Cause} values
     * @since 9
     */
    public final Cause getCause() {
        return cause;
    }

    
Checks if this deserialized FocusEvent instance is compatible with the current specification which implies that focus event has non-null cause value. If the check fails a new FocusEvent instance is returned which cause field equals to Cause.UNKNOWN and its other fields have the same values as in this FocusEvent instance. 
Throws:
See Also:
Returns:a newly created object from deserialized data
@serial
Since:9
/**
     * Checks if this deserialized {@code FocusEvent} instance is compatible
     * with the current specification which implies that focus event has
     * non-null {@code cause} value. If the check fails a new {@code FocusEvent}
     * instance is returned which {@code cause} field equals to
     * {@link Cause#UNKNOWN} and its other fields have the same values as in
     * this {@code FocusEvent} instance.
     *
     * @return a newly created object from deserialized data
     * @throws ObjectStreamException if a new object replacing this object could
     *         not be created
     * @serial
     * @see #cause
     * @since 9
     */
    @SuppressWarnings("serial")
    Object readResolve() throws ObjectStreamException {
        if (cause != null) {
            return this;
        }
        FocusEvent focusEvent = new FocusEvent(new Component(){}, getID(),
                isTemporary(), getOppositeComponent());
        focusEvent.setSource(null);
        focusEvent.consumed = consumed;

        AWTAccessor.AWTEventAccessor accessor =
                AWTAccessor.getAWTEventAccessor();
        accessor.setBData(focusEvent, accessor.getBData(this));
        return focusEvent;
    }


}