Android-Button&setting up values

Layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="top"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >
    <Button
        android:id="@+id/bSub"
        android:layout_width="250sp"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/tvDisplay"
        android:layout_below="@+id/tvDisplay"
        android:layout_marginTop="51dp"
        android:text="@string/Subtractone"
        android:textSize="20sp" />
    <Button
        android:id="@+id/bAdd"
        android:layout_width="250sp"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/tvDisplay"
        android:layout_below="@+id/tvDisplay"
        android:text="@string/Addone"
        android:textSize="20sp" />
    <TextView
        android:id="@+id/tvDisplay"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="You total is 0"
        android:textSize="45sp" />
</RelativeLayout>




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Main Activity:

package com.nanolaser;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends Activity {
    int counter;
    Button add, sub;
    TextView display;
   
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
     counter=0;
     add=(Button)findViewById(R.id.bAdd);
     sub=(Button)findViewById(R.id.bSub);
     display=(TextView)findViewById(R.id.tvDisplay);
     add.setOnClickListener(new View.OnClickListener() {

   @Override
   public void onClick(View v) {
    // TODO Auto-generated method stub
    counter++;
    display.setText("You total is "+counter);
   }
  });
     sub.setOnClickListener(new View.OnClickListener() {

   @Override
   public void onClick(View v) {
    // TODO Auto-generated method stub
     counter--;
     display.setText("You total is "+counter);
   }
  });
 }
 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  // Inflate the menu; this adds items to the action bar if it is present.
  getMenuInflater().inflate(R.menu.main, menu);
  return true;
 }
}


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