为计算器android创建添加功能

Question

我需要在Android中创建一个计算器。我需要创建一个plus函数来添加到数字（整数）。这是我的代码。我还需要创建其他函数，比如减法。

public class MainActivity extends AppCompatActivity {

    Button one,two,three,four,five,six,seven,eight,nine,plus,minus,mul,divide,equal;
    TextView textView;
    String currentDisplayedInput="";
    private String inputToBeParsed = "";

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

        one = (Button) findViewById(R.id.one);
        two = (Button) findViewById(R.id.two);
        three = (Button) findViewById(R.id.three);
        four = (Button) findViewById(R.id.four);
        five = (Button) findViewById(R.id.five);
        six = (Button) findViewById(R.id.six);
        seven = (Button) findViewById(R.id.seven);
        eight = (Button) findViewById(R.id.eight);
        nine = (Button) findViewById(R.id.nine);
        plus = (Button) findViewById(R.id.plus);
        minus = (Button) findViewById(R.id.minus);
        mul = (Button) findViewById(R.id.multiply);
        divide = (Button) findViewById(R.id.divide);
        equal=(Button)findViewById(R.id.equal);
        textView = (TextView) findViewById(R.id.textView);

        one.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                currentDisplayedInput += "1";
                //inputToBeParsed += "1";
                textView.setText(currentDisplayedInput);

            }
        });

        two.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                currentDisplayedInput += "2";
                //inputToBeParsed += "1";
                textView.setText(currentDisplayedInput);

            }
        });

        equal.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });
    }

}

我需要做的就是当我点击1 + 1时，TextView应该显示结果2。