Java GUI: Key Event Handling and Layout Managers

• The notes cover Key Event Handling, Layout Managers, FlowLayout, BorderLayout, GridLayout, Using Panels for Complex Layouts, and JTextArea.

Key Event Handling

• KeyListener interface is used for handling key events.
• Key events are generated when keys are pressed and released on the keyboard.
• A KeyListener must define three methods: keyPressed, keyReleased, and keyTyped.
• keyPressed is called when any key is pressed.
• keyTyped is called when a key that is not an action key is pressed.
• keyReleased is called when a key is released after a keyPressed or keyTyped event.
• addKeyListener is used to register key event handler methods.
• KeyEvent method getKeyCode gets the virtual key code of the pressed key.
• KeyEvent has virtual key-code constants representing every key.
• getKeyCode return values can be passed to the static KeyEvent method getKeyText to get a string containing the pressed key's name.
• getKeyChar gets the Unicode value of the character typed, returning a char.
• isActionKey determines if the key in the event was an action key.
• getModifiers determines if modifier keys (Shift, Alt, Ctrl) were pressed when the key event occurred.
• Modifier keys can be passed to the static KeyEvent method getKeyModifiersText to get a string containing the names of the pressed modifier keys.
• InputEvent methods (isAltDown, isControlDown, isMetaDown, isShiftDown) return a boolean indicating if the particular key was pressed during the key event.

Introduction to Layout Managers

• Layout managers arrange GUI components in a container.
• Layout managers implement the LayoutManager interface.
• Container's setLayout method takes an object that implements the LayoutManager interface as an argument.
• There are three ways to arrange components in a GUI: absolute positioning, layout managers, and visual programming in an IDE.
• Absolute positioning offers the greatest level of control, but requires setting the container's layout to null and specifying the absolute position and size of each component.
• Layout managers are simpler and faster than absolute positioning, and make GUIs more resizable.
• Visual programming in an IDE involves dragging and dropping GUI components from a tool box onto a design area, and then positioning, sizing, and aligning them as desired.

FlowLayout

• It is the simplest layout manager.
• GUI components are placed from left to right in the order they are added to the container.
• When the edge of the container is reached, components continue on the next line.
• Components can be left-aligned, centered (the default), or right-aligned.
• Default layout for JPanel.
• The setAlignment changes the alignment for the FlowLayout.
  • e.g. FlowLayout.LEFT, FlowLayout.CENTER, FlowLayout.RIGHT
• layoutContainer is an inherited LayoutManager interface method that specifies that a container should be rearranged based on the adjusted layout.

BorderLayout

• It is the default layout manager for a JFrame.
• It arranges components into five regions: NORTH, SOUTH, EAST, WEST, and CENTER.
• Implements the LayoutManager2 interface, adding methods for enhanced layout processing.
• It limits a Container to at most five components, one in each region.
• The component placed in each region can be a container to which other components are attached.
• Constructor arguments specify the number of pixels between components arranged horizontally (horizontal gap space) and vertically (vertical gap space).
• The default gap space is one pixel horizontally and vertically.
• If no region is specified when adding a Component, the layout manager will be added to region BorderLayout.CENTER.

GridLayout

• Divides the container into a grid of rows and columns.
• Implements the LayoutManager interface.
• Every Component has the same width and height.
• Components are added starting at the top-left cell of the grid proceeding left to right until the row is full, then continuing on the next row.
• The validate method recomputes the container's layout based on the current layout manager and GUI components.

Using Panels to Manage More Complex Layouts

• Complex GUIs often require components to be placed in an exact location.
• They often consist of multiple panels, with each panel's components arranged in a specific layout.
• JPanel extends JComponent, and JComponent extends Container, so every JPanel is a Container.
• Every JPanel may have components, including other panels, attached to it.
• JPanel can be used to create a more complex layout where several components are in a specific area of another container.

JTextArea

• Provides an area for manipulating multiple lines of text.
• It is a subclass of JTextComponent.
• JTextComponent declares common methods for JTextFields, JTextAreas, and other text-based GUI components.
• Box is a subclass of Container that uses a BoxLayout to horizontally or vertically arrange GUI components.
• The createHorizontalBox static method creates a Box that arranges components left to right in the order that they are attached.
• getSelectedText returns the selected text from a JTextArea.
• setText changes the text in a JTextArea.
• When text reaches the right edge of a JTextArea, it can wrap to the next line. This is known as line wrapping.
• By default, JTextArea objects do not wrap lines.
• To enable line wrapping functionality, invoke the setLineWrap method with a true argument.