捕获“返回键”

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub
        if (keyCode == KeyEvent.KEYCODE_BACK) {

        	AlertDialog.Builder builder = new AlertDialog.Builder(this);
        	builder.setTitle("退出：").setMessage("您确定退出？").setCancelable(false).setPositiveButton("退出", new DialogInterface.OnClickListener() {
        	public void onClick(DialogInterface dialog, int id) {
        	SingleDesktopAct.this.finish();
        	}
        	})
        	.setNegativeButton("不退出", new DialogInterface.OnClickListener() {
        	public void onClick(DialogInterface dialog, int id) {
        	dialog.cancel();
        	}
        	});
        	AlertDialog alert = builder.create();
        	alert.show();
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }