Working with Gridbag Layout in Java
Transitioning from console apps to GUI apps is a very challenging goal for up and coming developers. The challenge comes when you discover that GUI is only fun when you know how, when, and what to use. <!--more-->
Table of contents
Introduction
Knowing how to position your GUI components on the screen and the right layout to use is a tedious task and an essential aspect of programming GUI in Java. In this article, we will be diving deep into implementing GridBag layout in Java and how you can use it to achieve that desired User Interface (UI).
Prerequisites
To follow along the reader will need:
- A working knowledge of basic programming in Java.
- Basic understanding of OOP principles in Java.
- Have Java installed in your machine and a suitable Integrated Development Environment (IDE). I recommend NetBeans because we will be making use of it in this tutorial.
Goal
By the end of this tutorial, the reader will:
- Get familiar with the different types of layout managers in Java.
- Have in-depth knowledge of Gridbag layout in Java.
- Know when, why, and how to use Gridbag layout in Java.
Gridbag layout
Arranging components in a container
Java provides different options to position components in the container, which includes:
- Absolute positioning: This gives you absolute control over the size and position of components on the panel. It allows you to specify the size and position of each component explicitly.
You can achieve absolute positioning by setting the container’s layout manager to null. i.e. setLayout(null);. And also setting the containers setSize and setBounds method of each of the components any desired value.
- Visual programming: This is a GUI design tool that allows you to drag and drop components from a set of components into the design area of the IDE. These components are usually located at the sidebar of the IDE. You can find them in the component toolbox of the IDE.
We can modify them to fit design needs. You can also add event handling codes to these components after the design. The event handling codes add functionalities to the components. The IDE creates the code for the GUI during all the design processes so you can later change it to fit any needs.
- Layout managers: This involves using a defined or customized layout to position components on the panel or frame. Layout managers give more flexibility in determining components size. But give less control over the absolute position of each component.
What are layout managers
Layout managers are responsible for positioning each GUI component in their frames or panel. Layout managers allow you to align components in desired positions. Instead of bothering about their exact position, we rather lay them out on the panel with some specific constraints, and the layout manager takes care of the positioning.
Types of layout managers
Below are some of the layout managers Java provides:
- Flow layout
- Border layout
- Box layout
- Grid layout
- Group layout
- GridBag layout
In this article, we will be considering the Gridbag layout and building an amazing user interface with it.
What is GridBag layout manager
The Gridbag layout manager places components in rows and columns similar to the grid layout. But with Gridbag layout, you can specify the size of each row or column. You can also decide whether a component spans many rows or columns.
Gridbag layout is one of the most flexible but complex layout managers among all that Java provides. It is used to build complicated UIs, and it is one of the most used layout managers. When working with components of different sizes, then the Gridbag layout is an ideal fit.
Gridbag layout represents the position of each component in the form of a table containing rows and columns. The component's positions are outlined using gridlines like in the image below.
It is always advisable to draft out the components in a panel using gridlines before coding them out with Gridbag layout. This will give you a visual insight into the location of each component on the panel. As seen from the image above, some components occupy only one grid, while some occupy more than one.
This implies that the components don’t need to be of the same size. You can specify how many grids a component spans using the gridweight and gridheight methods of the GridBagConstraints object we will consider later in this article.
Working with GridBag layout
Now, let’s analyze the image above, considering each component, x and y position, and the span.
| Component | x | y | span |
|---|---|---|---|
| ComboBox “Easy” | 0 | 0 | 1 |
| ComboBox “Multiplication” | 2 | 0 | 1 |
| TextField1 | 0 | 1 | 3 |
| TextField2 | 1 | 2 | 1 |
| Button “Start” | 0 | 3 | 1 |
| Button “Enter” | 1 | 3 | 1 |
| Button “Stop” | 2 | 3 | 1 |
The first element at the top left corner of the panel is located at position (0, 0). This is the starting point of the layout, and every other component follows. From the example above, the “Easy” checkbox takes this first position. It only spans 1 column, which is specified in the table.
The span specifies how many cells a component occupies. The default is 1.
For the sake of this tutorial, we will explicitly add the 1 in our definition for clarity. The next component of the grid is the “Multiplication” checkbox located at position (2, 0). You can find it in column 2, and Like the “Easy” checkbox, this also spans 1 column.
But, notice the space between the two checkboxes, which is one of the powerful features of the Gridbag layout. The elements are evenly spaced, and each element sits as desired in its respective position. The next component is Textfield1, which is located at (0, 1).
You can find this component in columns 0 and row 1. It also spans three columns. The fact that it spans through columns 2 and 3 does not matter in the definition. One important thing to consider when laying out a component is the start position.
We can set components to span as many rows and columns as required. Depending on the direction it spans towards, it won’t affect other cells located next to the current cell. Other components follow suit in the same order and span one cell each. Textfield2 located at (1, 2), button “Start” at (0, 3), button “Enter” at (1, 3), button “Stop” at (2, 3).
Setting the panel’s layout
We set the Gridbag layout as the layout of our panel like other layout managers. This is done by passing an object of GridBagLayout into the panel setLayout method as shown below:
panel1 = new JPanel();
panel1.setLayout(new GridBagLayout());
Another method that you can use to set the panel layout is passing the layout object as an argument in the panel instance object constructor, as shown below:
panel1 = new JPanel(new GridBagLayout());
The two methods work fine, but it is advisable to use the first method for the sake of clarity.
GridBag constraints
We determine the size and position of each component in the panel using the Gridbag constraints. Therefore, each element in the panel has some characteristics which are defined by the Gridbag constraints. Some commonly used methods in the GridBagConstraints class are listed below:
| Field | Functionality |
|---|---|
| int gridx | The column of the component |
| int gridy | The row of the component |
| int gridwidth | The number of columns a component span |
| int gridheight | The number of rows a component span |
| int fill | A constant that determines how a component handles remaining space in a cell |
| int anchor | A constant that determines where the component is placed if it does not fill the cell |
| Insets ins | Determines the padding around each component |
Methods of adding constraints to component
The first approach we will consider is to create a GridBagConstraints object. Then add the constraint object along with the component into the panel. You can use this constraint object to add constraints to other components, but the object's fields are changed when necessary.
Below is an example:
panel1 = new JPanel();
panel1.setLayout(new GridBagLayout());
textField1 = new JTextField(33);
textField2 = new JTextField(3);
GridBagConstraints constr = new GridBagConstraints();
constr.gridx = 0;
constr.gridy = 1;
constr.gridheight = 1;
constr.gridwidth = 3;
constr.anchor = GridBagConstraint.CENTER;
panel1.add(textField1, constr);
constr.gridx = 1;
constr.gridy = 2;
constr.gridwidth = 1;
panel1.add(textField2, constr);
The snippet above creates a single constraint object and uses it to add two components to the panel. Notice that all the necessary fields are filled before adding the constraint to the textField1. The constraints fields are then modified before adding to textField2. Using this method, change only the necessary fields to avoid repetition.
The second approach is to create a helper method to call and pass values for each field of the constraint. For example, the snippet below uses the add method to add constraints to a component and then to the panel.
public static void add(JPanel panel, JComponent comp, int x, int y, int width, int height) {
GridBagConstraints constr = new GridBagConstraints();
constr.gridx = x;
constr.gridy = y;
constr.gridheight = height;
constr.gridwidth = width;
constr.insets = new Insets(2, 2, 2, 2);
constr.anchor = GridBagConstraints.CENTER;
constr.fill = GridBagConstraints.BOTH;
panel.add(comp, constr);
}
This method is then called whenever you need to add a component to the panel. You must pass in all the parameters to add the component successfully.
For example:
add(panel1, textField1, 0, 1, 3, 1);
In a case where you need to add more constraints, then specify these constraints in the method’s parameter.
The CAL user interface
The user interface (UI) above is a good example demonstrating the capabilities of Gridbag layout. Let’s build the UI using Gridbag layout.
import java.awt.*;
import javax.swing.*;
public class CAL extends JFrame {
static JButton start, stop, enter;
static JComboBox<String> level, type;
static final String[] levelList = {"Easy", "Medium", "Hard"};
static final String[] typeList = {"Multiply", "Add", "Subtract", "Divide", "Random"};
static JTextField question, answer;
static JPanel panel;
// helper method to add component to the panel
public static void addComponent(JPanel p, JComponent comp, int x, int y, int width, int height) {
GridBagConstraints constr = new GridBagConstraints();
constr.gridx = x;
constr.gridy = y;
constr.gridheight = height;
constr.gridwidth = width;
constr.weightx = 2.0;
constr.weighty = 1.0;
constr.insets = new Insets(1, 1, 1, 1);
constr.anchor = GridBagConstraints.CENTER;
constr.fill = GridBagConstraints.BOTH;
p.add(comp, constr);
}
public CAL() {
super("Computer Assisted Learning"); // title of GUI
panel = new JPanel();
panel.setLayout(new GridBagLayout()); // set panel's layout
// initialize components
start = new JButton("Start");
stop = new JButton("Stop");
enter = new JButton("Enter");
answer = new JTextField(3);
question = new JTextField("Press start to begin", 33);
level = new JComboBox<>(levelList);
type = new JComboBox<>(typeList);
// disabled textField
question.setEnabled(false);
// style components
start.setFont(new Font("Monospaced", Font.BOLD, 16));
stop.setFont(new Font("Monospaced", Font.BOLD, 16));
enter.setFont(new Font("Monospaced", Font.BOLD, 18));
level.setFont(new Font("Monospaced", Font.PLAIN, 16));
type.setFont(new Font("Monospaced", Font.PLAIN, 16));
question.setFont(new Font("Monospaced", Font.BOLD, 19));
answer.setFont(new Font("Monospaced", Font.BOLD, 21));
answer.setHorizontalAlignment(JTextField.CENTER);
// adds components to panel using helper method
addComponent(panel, level, 0, 0, 1, 1);
addComponent(panel, type, 2, 0, 1, 1);
addComponent(panel, question, 0, 1, 3, 1);
addComponent(panel, start, 0, 3, 1, 1);
addComponent(panel, answer, 1, 2, 1, 1);
addComponent(panel, stop, 2, 3, 1, 1);
addComponent(panel, enter, 1, 3, 1, 1);
// adds panel to frame
add(panel);
}
// main method to invoke the class
public static void main(String[] args) {
CAL cal = new CAL();
cal.setVisible(true);
cal.pack();
cal.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
The program above starts with declaring all the components and fields that we will use in the layout. It then proceeds to create the addComponent helper method that adds components with constraints to the panel.
This method's insets, anchor, and fill remain unchanged throughout the program. This is because these values are the same among all components. If these values are different, create another method and pass these constraints to the method.
It is possible to have different addComponent methods but with different signatures. Only do this when the components have varying constraints. The constructor starts by initializing the superclass constructor (JFrame).
super("Computer Assisted Learning");
The string passed in is the title of the application.
It then creates a new panel object and sets the panel’s layout to “GridBagLayout”. This is the point where we set the layout of our panel. The layouts can be any of the layouts listed above, but for the sake of this tutorial, we will use “GridBagLayout”.
panel = new JPanel();
panel.setLayout(new GridBagLayout()); // set panel's layout
Now that the next series of statements initialized the components declared and then we added some styling to these components. We then proceeded to add each component to the panel using the addComponent method defined earlier. It takes in as an argument the panel to add the component to, the component, and the constraints.
add(panel, level, 0, 0, 1, 1);
add(panel, type, 2, 0, 1, 1);
add(panel, question, 0, 1, 3, 1);
add(panel, start, 0, 3, 1, 1);
add(panel, answer, 1, 2, 1, 1);
add(panel, stop, 2, 3, 1, 1);
add(panel, enter, 1, 3, 1, 1);
Then we use the add method inherited from JFrame to add our panel to the frame.
add(panel);
We need to add all the panels defined to the frame in order to to view the components.
// main method to invoke the class
public static void main(String[] args) {
CAL cal = new CAL();
cal.setVisible(true);
cal.pack();
cal.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
The main method of the program firsts creates an object of the class and then invokes some of the methods. The setVisible sets the visibility of the app’s window. pack adjusts the window to accommodate the size of its subcomponents.
The setDefaultCloseOperation takes in an operation that determines what will happen when the user clicks on the close icon on the window. In this case, JFrame.EXIT_ON_CLOSE will stop the app.
Conclusion
Building a GUI app in Java is an interesting adventure for developers as users can visually interact with your app. What’s more important is to know how to build the app using the right tools.
Using CAL to explain the concept of Gridbag layout was to show the implementation and importance of this concept. While this article covers the main part of working with Gridbag layout, check out the official Java documentation for more on Gridbag layout.
Happy coding!
Peer Review Contributions by: Odhiambo Paul
