Android 如何监听返回键

public boolean onKeyDown(int keyCode, KeyEvent event) {

if (keyCode == KeyEvent.KEYCODE_BACK) {
这里是按了返回键之后所要做的事情
}
return super.onKeyDown(keyCode, event);
}

private long mExitTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
if ((System.currentTimeMillis() - mExitTime) > 2000) {
Toast.makeText(this, "再按一次退出", Toast.LENGTH_SHORT).show();
mExitTime = System.currentTimeMillis();
} else {
finish();
}
return true;
}
return super.onKeyDown(keyCode, event);
}