As per your code, you wrote button listener code outside the override method. So write your code inside the method as given below :
public class MainActivity extends AppCompatActivity {
TextView textViewSum; EditText editFirst, editSecond; Button buttonSum; double num1,num2,sum;
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);
editFirst = (EditText) findViewById(R.id.edtFirst); editSecond = (EditText) findViewById(R.id.edtSecond); textViewSum = (TextView) findViewById(R.id.textViewSum); buttonSum = (Button) findViewById(R.id.buttonSum);
buttonSum.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) {
num1 = Double.parseDouble(editFirst.getText().toString()); num2 = Double.parseDouble(editSecond.getText().toString()); sum = num1 + num2; textViewSum.setText(Double.toString(sum)); } });
} }

You are calling or setting `setOnClickListner` outside of any method. Put your code inside the method or `onCreate()` method.
Try below code it will solve your problem

EditText firstNumber; EditText secondNumber; TextView addResult; Button btnAdd; double num1, num2, sum; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); firstNumber = (EditText) findViewById(R.id.txtNumber1); secondNumber = (EditText) findViewById(R.id.txtNumber2); addResult = (TextView) findViewById(R.id.txtResult); btnAdd = (Button) findViewById(R.id.btnAdd); final Button mButton = (Button) findViewById(R.id.btnAdd); mButton.setOnClickListener(new View.OnClickListener() { public void onClick(View v) { num1 = Double.parseDouble(firstNumber.getText().toString()); num2 = Double.parseDouble(secondNumber.getText().toString()); sum = num1 + num2; addResult.setText(Double.toString(sum)); } }); }