FlowLayout

FlowLayout is the default LayoutManager for a Panel. A FlowLayout adds components to the container in rows, working from left to right. When it can't fit any more components in a row, it starts a new row--not unlike a word processor with word wrap enabled. When the container gets resized, the components within it get repositioned based on the container's new size. If sufficient space is available, components within FlowLayout containers are given their preferred size. If there is insufficient space, you do not see the components in their entirety.

FlowLayout Methods

FlowLayout defines three constants, all of which are used to specify alignment. The alignment tells FlowLayout where to start positioning the components on each row. Each component is still added from left to right, no matter what the alignment setting is.

• public final static int LEFT
• LEFT is the constant for left alignment.
• public final static int CENTER
• CENTER is the constant for center alignment and is the default.
• public final static int RIGHT
• RIGHT is the constant for right alignment.

• public FlowLayout ()
• This constructor creates a FlowLayout using default settings: center alignment with a horizontal and vertical gap of five pixels. The gap is the space between the different components in the different directions. By default, there will be five pixels between components. The constructor is usually called within a call to setLayout(): setLayout (new FlowLayout()). Figure 7.1 shows how the default FlowLayout behaves with different screen sizes. As the screen C shows, if the screen is too small, the components will not be shrunk so that they can fit better.

Figure 7.1: FlowLayout with six buttons and three different screen sizes

• public FlowLayout (int alignment)
• This version of the constructor creates a FlowLayout using the specified alignment and a horizontal and vertical gap of five pixels. Valid alignments are the FlowLayout constants, although there is no verification. Figure 7.2 shows the effect of different alignments: FlowLayout.LEFT (screen A), FlowLayout.CENTER (B), and FlowLayout.RIGHT (C).

Figure 7.2: FlowLayout with three different alignments

• public FlowLayout (int alignment, int hgap, int vgap)
• The final version of the constructor is called by the other two. It requires you to explicitly specify the alignment, horizontal gap (hgap), and vertical gap (vgap). This creates a FlowLayout with an alignment of alignment, horizontal gap of hgap, and vertical gap of vgap. The units for gaps are pixels. It is possible to have negative gaps if you want components to be placed on top of one another. Figure 7.3 shows the effect of changing the gap sizes.

Figure 7.3: FlowLayout with hgap of 0 and vgap of 20

Informational methods

• public int getAlignment ()
• The getAlignment() method retrieves the current alignment of the FlowLayout. The return value should equal one of the class constants LEFT, CENTER, or RIGHT.
• public void setAlignment (int alignment)
• The setAlignment() method changes the FlowLayout alignment to alignment. The alignment value should equal one of the class constants LEFT, CENTER, or RIGHT, but this method does not check. After changing the alignment, you must validate() the Container.
• public int getHgap ()
• The getHgap() method retrieves the current horizontal gap setting.
• public void setHgap (int hgap)
• The setHgap() method changes the current horizontal gap setting to hgap. After changing the gaps, you must validate() the Container.
• public int getVgap ()
• The getVgap() method retrieves the current vertical gap setting.
• public void setVgap (int hgap)
• The setVgap() method changes the current vertical gap setting to vgap. After changing the gaps, you must validate() the Container.

• public void addLayoutComponent (String name, Component component)
• The addLayoutComponent() method of FlowLayout does nothing.
• public void removeLayoutComponent (Component component)
• The removeLayoutComponent() method of FlowLayout does nothing.
• public Dimension preferredLayoutSize (Container target)
• The preferredLayoutSize() method of FlowLayout calculates the preferred dimensions for the target container. The FlowLayout computes the preferred size by placing all the components in one row and adding their individual preferred sizes along with gaps and insets.
• public Dimension minimumLayoutSize (Container target)
• The minimumLayoutSize() method of FlowLayout calculates the minimum dimensions for the container by adding up the sizes of the components. The FlowLayout computes the minimum size by placing all the components in one row and adding their individual minimum sizes along with gaps and insets.
• public void layoutContainer (Container target)
• The layoutContainer() method draws target's components on the screen, starting with the first row of the display, going left to right across the screen, based on the current alignment setting. When it reaches the right margin of the container, it skips down to the next row, and continues drawing additional components.

• public String toString ()
• The toString() method of FlowLayout returns the current horizontal and vertical gap settings along with the alignment (left, center, right). For a FlowLayout that uses all the defaults, toString() produces:

java.awt.FlowLayout[hgap=5,vgap=5,align=center]