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Posted
on March 1st, 2001
| Bruce Eckel's Thinking in Java | Contents | Prev | Next |
A
Label
does exactly what it sounds like it should: places a label on the form. This is
particularly important for text fields and text areas that don’t have
labels of their own, and can also be useful if you simply want to place textual
information on a form. You can, as shown in the first example in this chapter,
use
drawString( )
inside
paint( )
to place text in an exact location. When you use a
Label
it allows you to (approximately) associate the text with some other component
via the layout manager (which will be discussed later in this chapter).
With
the constructor you can create a blank label, a label with initial text in it
(which is what you’ll typically do), and a label with an alignment of
CENTER,
LEFT,
or
RIGHT
(
static
final int
s
defined in class
Label).
You can also change the label and its alignment with setText( )
and setAlignment( ),
and if you’ve forgotten what you’ve set these to you can read the
values with getText( )
and getAlignment( ).
This example shows what you can do with labels:
//: Label1.java // Using labels import java.awt.*; import java.applet.*; public class Label1 extends Applet { TextField t1 = new TextField("t1", 10); Label labl1 = new Label("TextField t1"); Label labl2 = new Label(" "); Label labl3 = new Label(" ", Label.RIGHT); Button b1 = new Button("Test 1"); Button b2 = new Button("Test 2"); public void init() { add(labl1); add(t1); add(b1); add(labl2); add(b2); add(labl3); } public boolean action (Event evt, Object arg) { if(evt.target.equals(b1)) labl2.setText("Text set into Label"); else if(evt.target.equals(b2)) { if(labl3.getText().trim().length() == 0) labl3.setText("labl3"); if(labl3.getAlignment() == Label.LEFT) labl3.setAlignment(Label.CENTER); else if(labl3.getAlignment()==Label.CENTER) labl3.setAlignment(Label.RIGHT); else if(labl3.getAlignment() == Label.RIGHT) labl3.setAlignment(Label.LEFT); } else return super.action(evt, arg); return true; } } ///:~
The
first use of the label is the most typical: labeling a
TextField
or
TextArea.
In the second part of the example, a bunch of empty spaces are reserved and
when you press the “Test 1” button
setText( )
is used to insert text into the field. Because a number of blank spaces do not
equal the same number of characters (in a proportionally-spaced
font) you’ll see that the text gets truncated when inserted into the label.
The
third part of the example reserves empty space, then the first time you press
the “Test 2” button it sees that there are no characters in the
label (since trim( )
removes all of the blank spaces at each end of a
String)
and inserts a short label, which is initially left-aligned. The rest of the
times you press the button it changes the alignment so you can see the effect.
You
might think that you could create an empty label and then later put text in it
with
setText( ).
However, you cannot put text into an empty label – presumably because it
has zero width – so creating a label with no text seems to be a useless
thing to do. In the example above, the “blank” label is filled with
empty spaces so it has enough width to hold text that’s placed inside
later.
Similarly,
setAlignment( )
has
no effect on a label that you’d typically create with text in the
constructor. The label width is the width of the text, so changing the
alignment doesn’t do anything. However, if you start with a long label
and then change it to a shorter one you can see the effect of the alignment.
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