Text Input Core Java

We are finally ready to start introducing the Swing user interface components. We start with components that let a user input and edit text. You can use the JTextField and JText- Area components for gathering text input. A text field can accept only one line of text; a text area can accept multiple lines of text. A JPasswordField accepts one line of text without showing the contents.

All three of these classes inherit from a class called JTextComponent. You will not be able to constructa JTextComponent yourself because it is an abstract class. On the other hand, as is so often the case in Java, when you go searching through the API documentation, you may find that the methods you are looking for are actually in the parent class JTextComponent rather than in the derived class. For example, the methods that get or set the text in a text field or text area are actually methods in JTextComponent.

Text Fields

The usual way to add a text field to a window is to add it to a panel or other container— just as you would a button:

This code adds a text field and initializes the text field by placing the string "Default input" inside it. The second parameter of this constructor sets the width. In this case, the width is 20 “columns.” Unfortunately, a column is a rather imprecise measurement. One column is the expected width of one character in the font you are using for the text. The idea is that if you expect the inputs to be n characters or less, you are supposed to specify n as the column width. In practice, this measurement doesn’t work out too well, and you should add 1 or 2 to the maximum input length to be on the safe side. Also, keep in mind that the number of columns is only a hint to the AWT that gives the preferred size. If the layout manager needs to grow or shrink the text field, it can adjust its size. The column width that you set in the JTextField constructor is not an upper limit on the number of characters the user can enter. The user can still type in longer strings, but the input scrolls when the text exceeds the length of the field. Users tend to find scrolling text fields irritating, so you should size the fields generously. If you need to reset the number of columns at runtime, you can do that with the setColumns method.

In general, you want to let the user add text (or edit the existing text) in a text field.Quite often these text fields start out blank. To make a blank text field, just leave out the string as a parameter for the JTextField constructor:

You can change the content of the text field at any time by using the setText method from the JText Component parent class mentioned in the previous section. For example:

And, as was also mentioned in the previous section, you can find out what the user typed by calling the getText method. This method returns the exact text that the user typed. To trim any extraneous leading and trailing spaces from the data in a text field, apply the trim method to the return value of getText:

To change the font in which the user text appears, use the setFont method.

  • JTextField(int cols)
    constructs an empty JTextField with a specified number of columns.
  • JTextField(String text, int cols)
    constructs a new JTextField with an initial string and the specified number of columns.
  • int getColumns()
  • void setColumns(int cols)
    gets or sets the number of columns that this text field should use.
  • void revalidate()
    causes the position and size of a component to be recomputed.
  • void setFont(Font f)
    sets the font of this component.
  • void validate()
    recomputes the position and size of a component. If the component is a container, the positions and sizes of its components are recomputed.
  • Font getFont()
    gets the font of this component.

Labels and Labeling Components

Labels are components that hold text. They have no decorations (for example, no boundaries). They also do not react to user input. You can use a label to identify components.

For example, unlike buttons, text fields have no label to identify them. To label a component that does not itself come with an identifier:

  1. Construct a JLabel component with the correct text.
  2. Place it close enough to the component you want to identify so that the user can see that the label identifies the correct component.

The constructor for a JLabel lets you specify the initial text or icon, and optionally, the alignment of the content. You use constants from the SwingConstants interface to specify alignment. That interface defines a number of useful constants such as LEFT, RIGHT, CENTER, NORTH, EAST, and so on. The JLabel class is one of several Swing classes that implement this interface. Therefore, you can specify a right-aligned label either as

or

The setText and setIcon methods let you set the text and icon of the label at runtime.

Labels can be positioned inside a container like any other component. This means you can use the techniques you have seen before to place labels where you need them.

  • JLabel(String text)
  • JLabel(Icon icon)
  • JLabel(String text, int align)
  • JLabel(String text, Icon icon, int align)
    constructs a label.
  • String getText()
  • void setText(String text)
    gets or sets the text of this label.
  • Icon getIcon()
  • void setIcon(Icon icon)
    gets or sets the icon of this label.

Password Fields

Password fields are a special kind of text field. To avoid nosy bystanders being able to glance at a password, the characters that the user entered are not actually displayed. Instead, each typed character is represented by an echo character, typically an asterisk (*). Swing supplies a JPasswordField class that implements such a text field. The password field is another example of the power of the model -view -controller architecture pattern. The password field uses the same model to store the data as a regular text field, but its view has been changed to display all characters as echo characters. • JPassword Field(String text, int columns) constructs a new password field.

  • void setEchoChar(char echo)
    sets the echo character for this password field. This is advisory; a particular look and feel may insist on its own choice of echo character. A value of 0 resets the echo character to the default.
  • char[] getPassword()
    returns the text contained in this password field. For stronger security, you should overwrite the content of the returned array after use. (The password is not returned as a String because a string would stay in the virtual machine until it is garbage-collected.)

Text Areas

Sometimes, you need to collect user input that is more than one line long. As mentioned earlier, you use the JTextArea component for this collection. When you place a text area component in your program, a user can enter any number of lines of text, using the ENTER key to separate them. Each line ends with a 'n'. a text area at work.

Text components

Text components

In the constructor for the JTextArea component, you specify the number of rows and columns for the text area. For example,

where the columns parameter works as before—and you still need to add a few more columns for safety’s sake. Also, as before, the user is not restricted to the number of rows and columns; the text simply scrolls when the user inputs too much. You can also use the setColumns method to change the number of columns, and the setRows method to change the number of rows. These numbers only indicate the preferred size—the layout manager can still grow or shrink the text area.

If there is more text than the text area can display, then the remaining text is simply clipped. You can avoid clipping long lines by turning on line wrapping:

This wrapping is a visual effect only; the text in the document is not changed—no 'n' characters are inserted into the text.

Scroll Panes

In Swing, a text area does not have scrollbars. If you want scrollbars, you have to insert the text area inside a scroll pane.

The scroll pane now manages the view of the text area. Scrollbars automatically appear if there is more text than the text area can display, and they vanish again if text is deleted and the remaining text fits inside the area. The scrolling is handled internally in the scroll pane— your program does not need to process scroll events. This is a general mechanism that works for any component, not just text areas. To add scrollbars to a component, put them inside a scroll pane.

javax.swing.JTextArea 1.2

  • JTextArea()
  • JTextArea(int rows, int cols)
  • JTextArea(String text, int rows, int cols)
    constructs a new text area.
  • void setColumns(int cols)
    tells the text area the preferred number of columns it should use.
  • void setRows(int rows)
    tells the text area the preferred number of rows it should use.
  • void append(String newText)
    appends the given text to the end of the text already in the text area.
  • void setLineWrap(boolean wrap)
    turns line wrapping on or off.
  • void setWrapStyleWord(boolean word)
    If word is true, then long lines are wrapped at word boundaries. If it is false, then long lines are broken without taking word boundaries into account.
  • void setTabSize(int c)
    sets tab stops every c columns. Note that the tabs aren’t converted to spaces but cause alignment with the next tab stop.
  • JScrollPane(Component c)
    creates a scroll pane that displays the content of the specified component.

Scrollbars are supplied when the component is larger than the view.



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