android按2次返回鍵退出程序的代碼

在作安卓應用是咱們常常要判斷用戶對返回鍵的操做，通常爲了防止誤操做都是在用戶連續按下兩次返回鍵的時候提示用戶是否退出應用程序。方法不少種，這裏經過記錄按鍵時間計算時間差實現：

import android.app.Activity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends Activity {

    private long exitTime = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            exit();
            return false;
        }
        return super.onKeyDown(keyCode, event);
    }

    public void exit() {
        if ((System.currentTimeMillis() - exitTime) > 2000) {
            Toast.makeText(getApplicationContext(), "再按一次退出程序",
                    Toast.LENGTH_SHORT).show();
            exitTime = System.currentTimeMillis();
        } else {
            finish();
            System.exit(0);
        }
    }

}