Exemplo Soma

final EditText num1Ed = (EditText) this.findViewById(R.id.num1ed);

final EditText num2Ed = (EditText) this.findViewById(R.id.num2ed);

final EditText resEd = (EditText) this.findViewById(R.id.res);

Button soma = (Button) this.findViewById(R.id.btSoma);

somaButton.setOnClickListener(new Button.OnClickListener(){

@Override 

public void OnClick(View v){

double num1 = Double.parseDouble(num1Ed.getEditableText().toString());

double num2 = Double.parseDouble(num2Ed.getEditableText().toString());

double res = num1 + num2;



resEd.setText(Double.toString(res));
}});