package br.com.calculadora;

import android.app.Activity;
import android.os.Bundle;
import android.widget.*;
import android.view.*;

public class calcActivity extends Activity {
    /** Called when the activity is first created. */
  
   TextView txtTexto; // <- Remover
   Button btnTroca; // <- Remover
  
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
  txtTexto = (TextView) findViewById(R.teste.txtTexto); // <- Remover daqui
        btnTroca = (Button) findViewById(R.teste.btnTroca);
        
        btnTroca.setOnClickListener(new View.OnClickListener() {
        
         @Override
         public void onClick(View arg0) {
            txtTexto.setText("Você trocou o texto!");
            
         }
      }); <- até aqui
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <TextView android:text="Calculadora 1.0" android:id="@+id/textView1" android:layout_width="wrap_content" android:layout_height="wrap_content"></TextView>
    <TextView android:text="Digite o primeiro número:" android:id="@+id/textView2" android:layout_width="wrap_content" android:layout_height="wrap_content"></TextView>
    <EditText android:layout_height="wrap_content" android:id="@+calc/edtNum1" android:text="" android:layout_width="match_parent"></EditText>
    <TextView android:text="Digite o segundo número:" android:id="@+id/textView3" android:layout_width="wrap_content" android:layout_height="wrap_content"></TextView>
    <EditText android:layout_height="wrap_content" android:id="@+calc/edtNum2" android:text="" android:layout_width="match_parent"></EditText>
    <TextView android:text="Resultado:" android:id="@+id/textView4" android:layout_width="wrap_content" android:layout_height="wrap_content"></TextView>
    <EditText android:layout_height="wrap_content" android:layout_width="match_parent" android:id="@+calc/edtResultado" android:text=""></EditText>
    <LinearLayout android:layout_height="wrap_content" android:layout_width="match_parent" android:id="@+id/linearLayout1">
        <Button android:text="Somar" android:id="@+calc/btnSomar" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
    </LinearLayout>
</LinearLayout>