Module java.desktop
Package java.awt

# Class FocusTraversalPolicy

java.lang.Object
java.awt.FocusTraversalPolicy

Direct Known Subclasses:
  `ContainerOrderFocusTraversalPolicy`, `InternalFrameFocusTraversalPolicy`

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.

Since:
  1.4
See Also:
  - `Container.setFocusTraversalPolicy(java.awt.FocusTraversalPolicy)`
  - `Container.getFocusTraversalPolicy()`
  - `Container.setFocusCycleRoot(boolean)`
  - `Container.isFocusCycleRoot(java.awt.Container)`
  - `Container.setFocusTraversalPolicyProvider(boolean)`
  - `Container.isFocusTraversalPolicyProvider()`
  - `KeyboardFocusManager.setDefaultFocusTraversalPolicy(java.awt.FocusTraversalPolicy)`
  - `KeyboardFocusManager.getDefaultFocusTraversalPolicy()`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `FocusTraversalPolicy()`
  Constructs a `FocusTraversalPolicy`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `abstract Component`
  `getComponentAfter(Container aContainer, Component aComponent)`
  Returns the Component that should receive the focus after aComponent.
  `abstract Component`
  `getComponentBefore(Container aContainer, Component aComponent)`
  Returns the Component that should receive the focus before aComponent.
  `abstract Component`
  `getDefaultComponent(Container aContainer)`
  Returns the default Component to focus.
  `abstract Component`
  `getFirstComponent(Container aContainer)`
  Returns the first Component in the traversal cycle.
  `Component`
  `getInitialComponent(Window window)`
  Returns the Component that should receive the focus when a Window is made visible for the first time.
  `abstract Component`
  `getLastComponent(Container aContainer)`
  Returns the last Component in the traversal cycle.

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### FocusTraversalPolicy

    protected FocusTraversalPolicy()
    Constructs a `FocusTraversalPolicy`.

- ## Method Details

  - ### 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()`