我试着用java和androidstudio构建一个简单的android应用程序。添加onclicklistener后，应用程序在模拟器上不断崩溃。

public class MainActivity extends AppCompatActivity {
    EditText first = findViewById(R.id.firstInput);
    EditText second = findViewById(R.id.secondInput);
    EditText result = findViewById(R.id.result);
    private Button add;

    private View.OnClickListener onClickListener = new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            clicked();
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        add = new Button(this);
        add.setOnClickListener(onClickListener);
        };

    void clicked(){
        int i = Integer.parseInt(first.getText().toString());
        int z = Integer.parseInt(second.getText().toString());

        int r = i + z;

        result.setText(r);
    }
};