我是 Android 的新手，正在尝试学习如何使用 ProgressBar 和 Handler。需要注意的是，我的目标 API 版本是 11。
以下是我的代码和其中出现的错误：
public class HandlerDemo extends Activity {
ProgressBar bar;
Handler handler = new Handler() {
@Override
public void handleMessage() { // Shows "method must override or implement supertype method" on this line.
bar.incrementProgressBy(5);
}
@Override
public void close() {
}
@Override
public void flush() {
}
@Override
public void publish(LogRecord arg0) {
}
};
AtomicBoolean isRunning = new AtomicBoolean(false);
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler_demo);
bar = (ProgressBar)findViewById(android.R.id.progress);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_handler_demo, menu);
return true;
}
public void onStart() {
super.onStart();
bar.setProgress(0);
Thread background = new Thread(new Runnable() {
@Override
public void run() {
try {
for(int i = 0; i < 20 && isRunning.get(); i++) {
Thread.sleep(1000);
handler.sendMessage(handler.obtainMessage()); // Shows "Method is undefined for type Handler" on this line.
}
} catch(Throwable t) { }
}
});
isRunning.set(true);
background.start();
}
public void onStop() {
super.onStop();
isRunning.set(false);
}
}
我为什么会出现这些错误？