Reading and Writing the Clipboard

Now that you know about the different java.awt.datatransfer classes required to use the clipboard, let's put them all together in an example. Example 16.1 creates a TextField for input (copying), a read-only TextArea for output (pasting), and a couple of buttons to control its operation. Figure 16.1 shows the program's user interface. When the user clicks on the Copy button or presses Return in the TextField, the text in the TextField is copied to the Clipboard. When the user clicks on the Paste button, the contents of the clipboard are drawn in the TextArea. Since the clipboard is not private, you can copy or paste from anywhere on your desktop, not just this program.

Example 16.1: Using the System Clipboard

// Java 1.1 only import java.io.*; import java.awt.*; import java.awt.datatransfer.*;
public class ClipMe extends Frame {
 TextField tf; TextArea ta; Button copy, paste; Clipboard clipboard = null; ClipMe() {
 super ("Clipping Example"); add (tf = new TextField("Welcome"), "North"); add (ta = new TextArea(), "Center"); ta.setEditable(false); Panel p = new Panel(); p.add (copy = new Button ("Copy")); p.add (paste = new Button ("Paste")); add (p, "South"); setSize (250, 250);
}
public static void main (String args[]) {
 new ClipMe().show();
}
public boolean handleEvent (Event e) {
 if (e.id == Event.WINDOW_DESTROY) {
 System.exit(0);
return true; // never gets here
}
return super.handleEvent (e);
}
public boolean action (Event e, Object o) {
 if (clipboard == null) clipboard = getToolkit().getSystemClipboard(); if ((e.target == tf) || (e.target == copy)) {
 StringSelection data; data = new StringSelection (tf.getText()); clipboard.setContents (data, data);
}
else if (e.target == paste) {
 Transferable clipData = clipboard.getContents(this); String s; try {
 s = (String)(clipData.getTransferData( DataFlavor.stringFlavor));
}
catch (Exception ee) {
 s = ee.toString();
}
ta.setText(s);
}
return true;
}
} 

Figure 16.1: Using the system clipboard

We won't say anything about how the display is set up; that should be familiar. All the interesting stuff happens in the action method, which is called in response to a button click. We check which button the user clicked; if the user clicked the Copy button, we read the text field tf and use it to create a new StringSelection named data. If the user clicked the Paste button, we retrieve the data from the clipboard by calling getContents(). This gives us an object about which (strictly speaking) we know nothing, except that it implements Transferable. In this case, we're pretty sure that we're getting text from the clipboard, so we call getTransferData() and ask for the data in the stringFlavor form. We catch the exception that might occur if we're wrong about the data flavor. This program has no way of placing anything but text on the clipboard, but there's no guarantee that the user didn't cut some other kind of object from a native application.

Once we have our String, we call the setText() method of the TextArea to tell it about the new string, and we are finished.