Class FormLayout

Object
  FormLayout

All Implemented Interfaces:

LayoutManager

public class FormLayout
extends Object
implements LayoutManager

FormLayout provides a simple means of controlling layout which is sufficiently powerful to enable many layouts to be achieved with few programming statements. Four types of constraint are provided:

• placement at a fixed distance from an edge of the container;
• placement at a fixed distance in relation to a neighbouring component;
• placement with one edge aligned with the same edge of another component;
• placement at a fractional position within the available width or height of the container.

Unlike other layout managers, the FormLayout constructor requires a parameter, the container it is responsible for. With this layout manager, components should not be added to the container by the programmer; instead, calls should be made to the layout manager indicating how the children are to be constrained, and the layout manager will itself tell the container of the children it has gained. The FormLayout object needs to be told of these constraints with methods:

The inspiration for this design came from the form widget in X Windows. There is also a FormLayout in SWT developed by IBM and maintained now by Eclipse, which is more sophisticated and flexible, but requires more lines of code to insert the components. It is also similar to Sun's SpringLayout, but with fewer features and requiring rather less code.

The size of the container is determined from the components placed with respect to the edges or the other components in the container. Components placed at a fraction of the width or height of the container are not considered in determining size; they rely on space being present as a result of the size of the other components.

Relative placement can be made with respect to more than one neighbour on the left or top side. FormLayout will determine which takes the greater amount of space and set this position for the component.

Example

      Container cpane = getContentPane();
      FormLayout form = new FormLayout(cpane);

      JLabel lab1 = new JLabel("Name: ") ;
      form.setTopAnchor( lab1, 3 );
      form.setLeftAnchor( lab1, 4 );
      JLabel lab2 = new JLabel("Address: ");

      JTextField name = new JTextField(" ",50);
      form.setLeftRelative( name, lab1, 3 );
      form.setLeftRelative( name, lab2, 3 );
      form.setTopAnchor( name, 3 );

      form.setLeftAnchor( lab2, 4 );
      form.setTopRelative( lab2, name, 3 );
      JTextArea addr = new JTextArea(" ",3,50 );
      JScrollPane jsp = new JScrollPane( addr, 
               JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED, 
               JScrollPane.HORIZONTAL_SCROLLBAR_NEVER );
      form.setLeftAlign( jsp, name );
      form.setTopRelative( jsp, name, 3 );
      form.setRightAnchor( jsp, 4 );

      JLabel lab4 = new JLabel("Adult ");
      form.setLeftAlign( lab4, name );
      form.setTopRelative( lab4, jsp, 3 );

      JTextField adults = new JTextField(" ",3);
      form.setLeftRelative( adults, lab4, 3 );
      form.setTopRelative( adults, jsp, 3 );

      JLabel Ch = new JLabel("Child");
      form.setLeftRelative( Ch, adults, 16 );
      form.setTopRelative( Ch, jsp, 3 );

      JTextField children = new JTextField(" ",3);
      form.setLeftRelative( children, Ch, 3 );
      form.setTopRelative( children, jsp, 3 );

      JLabel Con = new JLabel("Concession");
      form.setLeftRelative( Con, children, 16 );
      form.setTopRelative( Con, jsp, 3 );

      JTextField concess = new JTextField(" ",3);
      form.setLeftRelative( concess, Con, 3 );
      form.setTopRelative( concess, jsp, 3 );

      JButton accept = new JButton("Accept");
      form.setHorizontalAnchor( accept, 0.2 );
      form.setTopRelative( accept, adults, 3 );
      JButton reject = new JButton("Reject");
      form.setHorizontalAnchor( reject, 0.5 );
      form.setTopAlign( reject, accept );
      accept.addActionListener( this );
      reject.addActionListener( this );

      JButton quit = new JButton("Exit");
      form.setHorizontalAnchor( quit, 0.8 );
      form.setTopAlign( quit, accept );
      form.setBottomAnchor( quit, 5 );
      quit.addActionListener( this );
      pack();
      

FormLayout does not have any concept of filling available space, so there are situations where it will not give as neat appearance as GridBag, such as with JScrollPanes. There are also occasions after pack() has been called when FormLayout requests the preferred size of the components within it, and resizes the container accordingly. If the size reported by a component is incorrect, FormLayout will have no way of detecting this. It may be necessary to call pack() a second time in these situations.

EOL

public static String EOL

End-of-line character(s) on the current machine.

FormLayout

public FormLayout(Container c)

Construct a FormLayout object, telling the layout manager which container it is responsible for.

addLayoutComponent

public void addLayoutComponent(String name,
                               Component comp)

If the layout manager uses a per-component string, adds the component "comp" to the layout, associating it with the string specified by "name".

Actually, this does nothing. Components must be added by setting constraints. We will tell the container about the additions, so there is no need for the container to tell us when something is added.

Specified by:

addLayoutComponent in interface LayoutManager

fetchPlacements

public String fetchPlacements()

For debugging purposes, it can be useful to fetch the size and placement of each component controlled by this FormLayout object.

layoutContainer

public void layoutContainer(Container parent)

Lays out the specified container. Normally called by the container when it needs to draw itself.

Specified by:

layoutContainer in interface LayoutManager

Throws:

RuntimeException - Is thrown if the parameter does not match the container being laid out by this manager.

minimumLayoutSize

public Dimension minimumLayoutSize(Container parent)

Calculates the minimum size dimensions for the specified container, given the components it contains.

Specified by:

minimumLayoutSize in interface LayoutManager

Returns:

the size of container required.

Throws:

RuntimeException - can occur because the parent parameter is not the container which the layout manager is responsible for, or because the constraints are not consistent, eg. because they are circular or because a component given as a neighbour has not been added to the layout manager.

preferredLayoutSize

public Dimension preferredLayoutSize(Container parent)

Calculates the preferred size dimensions for the specified container, given the components it contains.

Specified by:

preferredLayoutSize in interface LayoutManager

Returns:

the size of container required.

Throws:

RuntimeException - can occur because the parent parameter is not the container which the layout manager is responsible for, or because the constraints are not consistent, eg. because they are circular or because a component given as a neighbour has not been added to the layout manager.

removeLayoutComponent

public void removeLayoutComponent(Component comp)

Removes the specified component from the layout. Actually, we will not allow this. It may cause a dependency to be removed. Simply return.

Specified by:

removeLayoutComponent in interface LayoutManager

setBottomAlign

public void setBottomAlign(Component thisobj,
                           Component botnbr)

Sets the component to which thisobj will be aligned on the bottom side.

setBottomAnchor

public void setBottomAnchor(Component thisobj,
                            int dist)

Sets the distance in pixels between the bottom edge of the component and the bottom container margin.

setBottomRelative

public void setBottomRelative(Component thisobj,
                              Component botnbr,
                              int gap)

Sets the distance the component will be from a neighbour on its bottom side.

setHorizontalAnchor

public void setHorizontalAnchor(Component thisobj,
                                double dist)

Sets the horizontal position of the centre of the component.

Parameters:

dist - is a fraction of the container dimension.

setLeftAlign

public void setLeftAlign(Component thisobj,
                         Component leftnbr)

Sets the component to which thisobj will be aligned on the left side.

setLeftAnchor

public void setLeftAnchor(Component thisobj,
                          int dist)

Sets the distance in pixels between the left edge of the component and the left container margin.

setLeftRelative

public void setLeftRelative(Component thisobj,
                            Component leftnbr,
                            int gap)

Sets the distance the component will be from a neighbour on its left side.

setRightAlign

public void setRightAlign(Component thisobj,
                          Component rightnbr)

Sets the component to which thisobj will be aligned on the right side.

setRightAnchor

public void setRightAnchor(Component thisobj,
                           int dist)

Sets the distance in pixels between the right edge of the component and the right container margin.

setRightRelative

public void setRightRelative(Component thisobj,
                             Component rightnbr,
                             int gap)

Sets the distance the component will be from a neighbour on its right side.

setTopAlign

public void setTopAlign(Component thisobj,
                        Component topnbr)

Sets the component to which thisobj will be aligned on the top side.

setTopAnchor

public void setTopAnchor(Component thisobj,
                         int dist)

Sets the distance in pixels between the top edge of the component and the top container margin.

setTopRelative

public void setTopRelative(Component thisobj,
                           Component topnbr,
                           int gap)

Sets the distance the component will be from a neighbour on its top side.

setVerticalAnchor

public void setVerticalAnchor(Component thisobj,
                              double dist)

Sets the vertical position of the centre of the component.

Parameters:

dist - is a fraction of the container dimension.