The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
It is often useful for a background task to provide interim results while it is still working. The task can do this by invoking
SwingWorker.publish
. This method accepts a variable number of arguments. Each argument must be of the type specified by SwingWorker
's second type parameter.
To collect results provided by publish
, override
SwingWorker.process
This method will be invoked from the event dispatch thread. Results from multiple invocations of publish
are often accumulated for a single invocation of process
.
Let's look at the way the
example uses Flipper.java
publish
to provide interim results. Click the Launch button to run Flipper
using
Java™ Web Start (download JDK 7 or later). Or, to compile and run the example yourself, consult the example index.
This program tests the fairness of
java.util.Random
by generating a series of random boolean
values in a background task. This is equivalent to flipping a coin; hence the name Flipper
. To report its results, the background task uses an object of type FlipPair
private static class FlipPair { private final long heads, total; FlipPair(long heads, long total) { this.heads = heads; this.total = total; } }
The heads
field is the number of times the random value has been true
; the total
field is the total number of random values.
The background task is represented by an instance of FlipTask
:
private class FlipTask extends SwingWorker<Void, FlipPair> {
Since the task does not return a final result, it does not matter what the first type parameter is; Void
is used as a placeholder. The task invokes publish
after each "coin flip":
@Override protected Void doInBackground() { long heads = 0; long total = 0; Random random = new Random(); while (!isCancelled()) { total++; if (random.nextBoolean()) { heads++; } publish(new FlipPair(heads, total)); } return null; }
(The isCancelled
method is discussed in the next section.) Because publish
is invoked very frequently, a lot of FlipPair
values will probably be accumulated before process
is invoked in the event dispatch thread; process
is only interested in the last value reported each time, using it to update the GUI:
protected void process(List<FlipPair> pairs) { FlipPair pair = pairs.get(pairs.size() - 1); headsText.setText(String.format("%d", pair.heads)); totalText.setText(String.format("%d", pair.total)); devText.setText(String.format("%.10g", ((double) pair.heads)/((double) pair.total) - 0.5)); }
If Random
is fair, the value displayed in devText
should get closer and closer to 0 as Flipper
runs.
setText
method used in Flipper
is actually "thread safe" as defined in its
specification. That means that we could dispense with publish
and process
and set the text fields directly from the worker thread. We've chosen to ignore this fact in order to provide a simple demonstration of SwingWorker
interim results.