[Chapter 7] 7.2 FlowLayout

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| Chapter 7 Layouts |

## 7.2 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

Constants

`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.

Constructors

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`.

LayoutManager methods

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.

Miscellaneous methods

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]

The LayoutManager Interface |  | BorderLayout