import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    //记录第一次按返回键的时间
    private long mExitTime = 0;

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

    //5秒内按两次返回键退出应用
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            if ((System.currentTimeMillis() - mExitTime) > 5000) {
                Toast.makeText(this, "再按一次退出应用", Toast.LENGTH_SHORT).show();
                mExitTime = System.currentTimeMillis();
            } else {
                finish();
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }
}