Class FocusTraversalPolicy

public abstract class FocusTraversalPolicy
extends Object

A FocusTraversalPolicy defines the order in which Components with a particular focus cycle root are traversed. Instances can apply the policy to arbitrary focus cycle roots, allowing themselves to be shared across Containers. They do not need to be reinitialized when the focus cycle roots of a Component hierarchy change.

The core responsibility of a FocusTraversalPolicy is to provide algorithms determining the next and previous Components to focus when traversing forward or backward in a UI. Each FocusTraversalPolicy must also provide algorithms for determining the first, last, and default Components in a traversal cycle. First and last Components are used when normal forward and backward traversal, respectively, wraps. The default Component is the first to receive focus when traversing down into a new focus traversal cycle. A FocusTraversalPolicy can optionally provide an algorithm for determining a Window's initial Component. The initial Component is the first to receive focus when a Window is first made visible.

FocusTraversalPolicy takes into account focus traversal policy providers. When searching for first/last/next/previous Component, if a focus traversal policy provider is encountered, its focus traversal policy is used to perform the search operation.

Please see How to Use the Focus Subsystem, a section in The Java Tutorial, and the Focus Specification for more information.

Constructors

FocusTraversalPolicy

public FocusTraversalPolicy()

Methods

getComponentAfter

public abstract Component getComponentAfter(Container aContainer,
                                            Component aComponent)

Returns the Component that should receive the focus after aComponent. aContainer must be a focus cycle root of aComponent or a focus traversal policy provider.

Parameters:

aContainer - a focus cycle root of aComponent or focus traversal policy provider

aComponent - a (possibly indirect) child of aContainer, or aContainer itself

Returns:

the Component that should receive the focus after aComponent, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if aContainer is not a focus cycle root of aComponent or a focus traversal policy provider, or if either aContainer or aComponent is null

getComponentBefore

public abstract Component getComponentBefore(Container aContainer,
                                             Component aComponent)

Returns the Component that should receive the focus before aComponent. aContainer must be a focus cycle root of aComponent or a focus traversal policy provider.

Parameters:

aContainer - a focus cycle root of aComponent or focus traversal policy provider

aComponent - a (possibly indirect) child of aContainer, or aContainer itself

Returns:

the Component that should receive the focus before aComponent, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if aContainer is not a focus cycle root of aComponent or a focus traversal policy provider, or if either aContainer or aComponent is null

getFirstComponent

public abstract Component getFirstComponent(Container aContainer)

Returns the first Component in the traversal cycle. This method is used to determine the next Component to focus when traversal wraps in the forward direction.

Parameters:

aContainer - the focus cycle root or focus traversal policy provider whose first Component is to be returned

Returns:

the first Component in the traversal cycle of aContainer, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if aContainer is null

getLastComponent

public abstract Component getLastComponent(Container aContainer)

Returns the last Component in the traversal cycle. This method is used to determine the next Component to focus when traversal wraps in the reverse direction.

Parameters:

aContainer - the focus cycle root or focus traversal policy provider whose last Component is to be returned

Returns:

the last Component in the traversal cycle of aContainer, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if aContainer is null

getDefaultComponent

public abstract Component getDefaultComponent(Container aContainer)

Returns the default Component to focus. This Component will be the first to receive focus when traversing down into a new focus traversal cycle rooted at aContainer.

Parameters:

aContainer - the focus cycle root or focus traversal policy provider whose default Component is to be returned

Returns:

the default Component in the traversal cycle of aContainer, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if aContainer is null

getInitialComponent

public Component getInitialComponent(Window window)

Returns the Component that should receive the focus when a Window is made visible for the first time. Once the Window has been made visible by a call to show() or setVisible(true), the initial Component will not be used again. Instead, if the Window loses and subsequently regains focus, or is made invisible or undisplayable and subsequently made visible and displayable, the Window's most recently focused Component will become the focus owner. The default implementation of this method returns the default Component.

Parameters:

window - the Window whose initial Component is to be returned

Returns:

the Component that should receive the focus when window is made visible for the first time, or null if no suitable Component can be found

Throws:

IllegalArgumentException - if window is null

See Also:

getDefaultComponent(java.awt.Container), Window.getMostRecentFocusOwner()