GridBagLayout in Java: Creating Flexible GUIs with GridBagConstraints - Prof. Douglas J. W, Quizzes of Computer Science

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CS251 Intermediate Programming

Lecture 13

GUIs

• Midterm next Wednesday.• Homework 3 due Monday.• Quiz 3 today.

GridBagLayout

java.awt.GridBagLayout

is the most powerful and

fl exible layout manager that comes with Java (but there are

GridBagLayout many third-party layout managers available.)

gets its name from its use of a rectangular

GridBagLayout multiple grid squares.not have to be the same size, and the cells can take upgrid of cells. The “bag” part means that the grid squares do

uses a helper class

java.awt.GridBagConstraints

which holds the 11 (!)

parameters that control how

GridBagLayout

sets up a

component.

The 11 parameters are:

gridx

gridy

gridwidth

gridheight

weightx

weighty

anchor

fill

insets

ipadx

, and

ipady

The rest of the lecture will explain what these mean.

manager isrows. If we set up the window with a content panel whose layoutIt has a natural resemblance to a table with 2 columns and 4

GridBagLayout

, we can use these row and column

They are thenumbers to cause it to arrange the components like the above.

gridx

and

gridy

parameters of

GridBagConstraints

Rectangle Info

Height:

Y:^ X:

Width:

// ...p.add(yTextField, gbc);gbc.gridx = 2;JTextField yTextField = new JTextField(10);p.add(yCaption, gbc);gbc.gridy = 2;gbc.gridx = 1;JLabel yCaption = new JLabel("Y:");p.add(xTextField, gbc); // Reusing gbc ok because fields copied by layoutgbc.gridx = 2; // Keeping gridy from aboveJTextField xTextField = new JTextField(10);p.add(xCaption, gbc);gbc.gridy = 1;gbc.gridx = 1;JLabel xCaption = new JLabel("X:");JPanel p = new JPanel(gbl);GridBagConstraints gbc = new GridBagConstraints();GridBagLayout gbl = new GridBagLayout();

To

fi nd the grid, we look for the boundary lines bet ween

components. Each column and each row in

GridBagLayout

boundary lines cut through other components. extends the entire height or width of the panel. Some of the

Rectangle Info

Y:

Height:

X:

Width:

quite what we wanted. By putting the widthWhen we display it on the screen, it looks like this, which isn't

JTextField

in cell

(2,2)

, we left cells

(3,2)

and

(4,2)

open. Fortunately,

GridBagLayout

allows a component to span multiple cells using

the

gridwidth

and

gridheight

parameters.

Rectangle Info

Y:

Height:

X:

Width:

Now we get something like this. What's happening?

Rectangle Info

Y:

Height:

X:

Width:

size. Each component has a minimum, preferred, and maximumHow Components Help the Layout Managers

actually use.The maximum size is the most space the component canlayout.manager will usually use the preferred size to do thewhen it isn't being stretched or compressed. A layoutThe preferred size is the “normal” size for the componentcan be compressed and still function properly.The minimum size indicates the smallest the component

JComponent

has methods for getting/setting these:

Preferred Sizes

being used and the conpreferred, and maximum sizes based on the look-and-feel Most components will supply their own values for minimum,

fi guration of the component.

JTextField

uses the number of columns parameter you

passed to the constructor to determine its preferred width.

fi Because we used a gridwidth of 3, the cell containing the text elds spans 3 columns, but the text

fi eld inside it doesn't

fi ll the

there is a way to getentire cell because the cell is larger than its preferred width. But

GridBagLayout

to override the preferred

size and stretch the component to

fi t the grid cell.

Rectangle Info

Y:

Height:

X:

Width:

// ...p.add(widthTextField, gbc);gbc.fill = GridBagConstraints.HORIZONTAL;gbc.gridwidth = 3; // Take up (2,2), (3,2), and (4,2)gbc.gridx = 2;JTextField widthTextField = new JTextField(10);p.add(widthCaption, gbc);gbc.gridy = 2;gbc.gridx = 1;JLabel widthCaption = new JLabel("Width:");// ...

lined up properly next to the textNow it looks like this. It would be nice if the captions were all

fi elds rather than centered over

each other. The

JLabel

s don't

fi ll up their entire cells, but if we

stretch the component to

fi ll the grid cell, the component will add

change the alignment of the component within the cell.its own empty space around the text. What we want to do is

Rectangle Info

Y:

Height:

X:

Width:

// ...p.add(widthTextField, gbc);gbc.fill = GridBagConstraints.HORIZONTAL;gbc.gridwidth = 3; // Take up (2,2), (3,2), and (4,2)gbc.gridx = 2;JTextField widthTextField = new JTextField(10);p.add(widthCaption, gbc);gbc.anchor = GridBagConstraints.EAST;gbc.gridy = 2;gbc.gridx = 1;JLabel widthCaption = new JLabel("Width:");// ...

Now the captions are aligned.

Rectangle Info

Y:

Height:

X:

Width: