# Class ComponentUI

java.lang.Object
javax.swing.plaf.ComponentUI

Direct Known Subclasses:
  `ButtonUI, ColorChooserUI, ComboBoxUI, DesktopIconUI, DesktopPaneUI, FileChooserUI, InternalFrameUI, LabelUI, LayerUI, ListUI, MenuBarUI, OptionPaneUI, PanelUI, PopupMenuUI, ProgressBarUI, RootPaneUI, ScrollBarUI, ScrollPaneUI, SeparatorUI, SliderUI, SpinnerUI, SplitPaneUI, TabbedPaneUI, TableHeaderUI, TableUI, TextUI, ToolBarUI, ToolTipUI, TreeUI, ViewportUI`

public abstract class ComponentUI extends Object
The base class for all UI delegate objects in the Swing pluggable look and feel architecture. The UI delegate object for a Swing component is responsible for implementing the aspects of the component that depend on the look and feel. The `JComponent` class invokes methods from this class in order to delegate operations (painting, layout calculations, etc.) that may vary depending on the look and feel installed. Client programs should not invoke methods on this class directly.

See Also:
  - `JComponent`
  - `UIManager`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ComponentUI()`
  Sole constructor.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `contains(JComponent c, int x, int y)`
  Returns `true` if the specified x,y location is contained within the look and feel's defined shape of the specified component.
  `static ComponentUI`
  `createUI(JComponent c)`
  Returns an instance of the UI delegate for the specified component.
  `Accessible`
  `getAccessibleChild(JComponent c, int i)`
  Returns the `i`th `Accessible` child of the object.
  `int`
  `getAccessibleChildrenCount(JComponent c)`
  Returns the number of accessible children in the object.
  `int`
  `getBaseline(JComponent c, int width, int height)`
  Returns the baseline.
  `Component.BaselineResizeBehavior`
  `getBaselineResizeBehavior(JComponent c)`
  Returns an enum indicating how the baseline of the component changes as the size changes.
  `Dimension`
  `getMaximumSize(JComponent c)`
  Returns the specified component's maximum size appropriate for the look and feel.
  `Dimension`
  `getMinimumSize(JComponent c)`
  Returns the specified component's minimum size appropriate for the look and feel.
  `Dimension`
  `getPreferredSize(JComponent c)`
  Returns the specified component's preferred size appropriate for the look and feel.
  `void`
  `installUI(JComponent c)`
  Configures the specified component appropriately for the look and feel.
  `void`
  `paint(Graphics g, JComponent c)`
  Paints the specified component appropriately for the look and feel.
  `void`
  `uninstallUI(JComponent c)`
  Reverses configuration which was done on the specified component during `installUI`.
  `void`
  `update(Graphics g, JComponent c)`
  Notifies this UI delegate that it is time to paint the specified component.

  ### Methods declared in class Object

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

- ## Constructor Details

  - ### ComponentUI

    public ComponentUI()
    Sole constructor. (For invocation by subclass constructors, typically implicit.)

- ## Method Details

  - ### installUI

    public void installUI(JComponent c)
    Configures the specified component appropriately for the look and feel. This method is invoked when the `ComponentUI` instance is being installed as the UI delegate on the specified component. This method should completely configure the component for the look and feel, including the following:
    1. Install default property values for color, fonts, borders, icons, opacity, etc. on the component. Whenever possible, property values initialized by the client program should not be overridden.
    2. Install a `LayoutManager` on the component if necessary.
    3. Create/add any required sub-components to the component.
    4. Create/install event listeners on the component.
    5. Create/install a `PropertyChangeListener` on the component in order to detect and respond to component property changes appropriately.
    6. Install keyboard UI (mnemonics, traversal, etc.) on the component.
    7. Initialize any appropriate instance data.
    Parameters:
      `c` - the component where this UI delegate is being installed
    See Also:
      - `uninstallUI(JComponent)`
      - `JComponent.setUI(ComponentUI)`
      - `JComponent.updateUI()`

  - ### uninstallUI

    public void uninstallUI(JComponent c)
    Reverses configuration which was done on the specified component during `installUI`. This method is invoked when this `UIComponent` instance is being removed as the UI delegate for the specified component. This method should undo the configuration performed in `installUI`, being careful to leave the `JComponent` instance in a clean state (no extraneous listeners, look-and-feel-specific property objects, etc.). This should include the following:
    1. Remove any UI-set borders from the component.
    2. Remove any UI-set layout managers on the component.
    3. Remove any UI-added sub-components from the component.
    4. Remove any UI-added event/property listeners from the component.
    5. Remove any UI-installed keyboard UI from the component.
    6. Nullify any allocated instance data objects to allow for GC.
    Parameters:
      `c` - the component from which this UI delegate is being removed; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    See Also:
      - `installUI(JComponent)`
      - `JComponent.updateUI()`

  - ### paint

    public void paint(Graphics g, JComponent c)
    Paints the specified component appropriately for the look and feel. This method is invoked from the `ComponentUI.update` method when the specified component is being painted. Subclasses should override this method and use the specified `Graphics` object to render the content of the component.
    Parameters:
      `g` - the `Graphics` context in which to paint
      `c` - the component being painted; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    See Also:
      - `update(Graphics, JComponent)`

  - ### update

    public void update(Graphics g, JComponent c)
    Notifies this UI delegate that it is time to paint the specified component. This method is invoked by `JComponent` when the specified component is being painted.

    By default this method fills the specified component with its background color if its `opaque` property is `true`, and then immediately calls `paint`. In general this method need not be overridden by subclasses; all look-and-feel rendering code should reside in the `paint` method.

    Parameters:
      `g` - the `Graphics` context in which to paint
      `c` - the component being painted; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    See Also:
      - `paint(Graphics, JComponent)`
      - `JComponent.paintComponent(Graphics)`

  - ### getPreferredSize

    public Dimension getPreferredSize(JComponent c)
    Returns the specified component's preferred size appropriate for the look and feel. If `null` is returned, the preferred size will be calculated by the component's layout manager instead (this is the preferred approach for any component with a specific layout manager installed). The default implementation of this method returns `null`.
    Parameters:
      `c` - the component whose preferred size is being queried; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    Returns:
      a `Dimension` object containing given component's preferred size appropriate for the look and feel
    See Also:
      - `JComponent.getPreferredSize()`
      - `LayoutManager.preferredLayoutSize(Container)`

  - ### getMinimumSize

    public Dimension getMinimumSize(JComponent c)
    Returns the specified component's minimum size appropriate for the look and feel. If `null` is returned, the minimum size will be calculated by the component's layout manager instead (this is the preferred approach for any component with a specific layout manager installed). The default implementation of this method invokes `getPreferredSize` and returns that value.
    Parameters:
      `c` - the component whose minimum size is being queried; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    Returns:
      a `Dimension` object or `null`
    See Also:
      - `JComponent.getMinimumSize()`
      - `LayoutManager.minimumLayoutSize(Container)`
      - `getPreferredSize(JComponent)`

  - ### getMaximumSize

    public Dimension getMaximumSize(JComponent c)
    Returns the specified component's maximum size appropriate for the look and feel. If `null` is returned, the maximum size will be calculated by the component's layout manager instead (this is the preferred approach for any component with a specific layout manager installed). The default implementation of this method invokes `getPreferredSize` and returns that value.
    Parameters:
      `c` - the component whose maximum size is being queried; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
    Returns:
      a `Dimension` object or `null`
    See Also:
      - `JComponent.getMaximumSize()`
      - `LayoutManager2.maximumLayoutSize(Container)`

  - ### contains

    public boolean contains(JComponent c, int x, int y)
    Returns `true` if the specified x,y location is contained within the look and feel's defined shape of the specified component. `x` and `y` are defined to be relative to the coordinate system of the specified component. Although a component's `bounds` is constrained to a rectangle, this method provides the means for defining a non-rectangular shape within those bounds for the purpose of hit detection.
    Parameters:
      `c` - the component where the x,y location is being queried; this argument is often ignored, but might be used if the UI object is stateless and shared by multiple components
      `x` - the x coordinate of the point
      `y` - the y coordinate of the point
    Returns:
      `true` if the specified `x,y` location is contained within the look and feel's defined shape for the given component
    See Also:
      - `JComponent.contains(int, int)`
      - `Component.contains(int, int)`

  - ### createUI

    public static ComponentUI createUI(JComponent c)
    Returns an instance of the UI delegate for the specified component. Each subclass must provide its own static `createUI` method that returns an instance of that UI delegate subclass. If the UI delegate subclass is stateless, it may return an instance that is shared by multiple components. If the UI delegate is stateful, then it should return a new instance per component. The default implementation of this method throws an error, as it should never be invoked.
    Parameters:
      `c` - a `JComponent` for which to create a UI delegate
    Returns:
      a `ComponentUI` object for `c`

  - ### getBaseline

    public int getBaseline(JComponent c, int width, int height)
    Returns the baseline. The baseline is measured from the top of the component. This method is primarily meant for `LayoutManager`s to align components along their baseline. A return value less than 0 indicates this component does not have a reasonable baseline and that `LayoutManager`s should not align this component on its baseline.

    This method returns -1. Subclasses that have a meaningful baseline should override appropriately.

    Parameters:
      `c` - `JComponent` baseline is being requested for
      `width` - the width to get the baseline for
      `height` - the height to get the baseline for
    Returns:
      baseline or a value < 0 indicating there is no reasonable baseline
    Throws:
      `NullPointerException` - if `c` is `null`
      `IllegalArgumentException` - if width or height is < 0
    Since:
      1.6
    See Also:
      - `JComponent.getBaseline(int,int)`

  - ### getBaselineResizeBehavior

    public Component.BaselineResizeBehavior getBaselineResizeBehavior(JComponent c)
    Returns an enum indicating how the baseline of the component changes as the size changes. This method is primarily meant for layout managers and GUI builders.

    This method returns `BaselineResizeBehavior.OTHER`. Subclasses that support a baseline should override appropriately.

    Parameters:
      `c` - `JComponent` to return baseline resize behavior for
    Returns:
      an enum indicating how the baseline changes as the component size changes
    Throws:
      `NullPointerException` - if `c` is `null`
    Since:
      1.6
    See Also:
      - `JComponent.getBaseline(int, int)`

  - ### getAccessibleChildrenCount

    public int getAccessibleChildrenCount(JComponent c)
    Returns the number of accessible children in the object. If all of the children of this object implement `Accessible`, this method should return the number of children of this object. UIs might wish to override this if they present areas on the screen that can be viewed as components, but actual components are not used for presenting those areas. Note: As of v1.3, it is recommended that developers call `Component.AccessibleAWTComponent.getAccessibleChildrenCount()` instead of this method.
    Parameters:
      `c` - `JComponent` for which to get count of accessible children
    Returns:
      the number of accessible children in the object
    See Also:
      - `getAccessibleChild(JComponent, int)`

  - ### getAccessibleChild

    public Accessible getAccessibleChild(JComponent c, int i)
    Returns the `i`th `Accessible` child of the object. UIs might need to override this if they present areas on the screen that can be viewed as components, but actual components are not used for presenting those areas.

    Note: As of v1.3, it is recommended that developers call `Component.AccessibleAWTComponent.getAccessibleChild()` instead of this method.

    Parameters:
      `c` - a `JComponent` for which to get a child object
      `i` - zero-based index of child
    Returns:
      the `i`th `Accessible` child of the object
    See Also:
      - `getAccessibleChildrenCount(JComponent)`