为什么点击按钮不能改变textview的值

XML 代码:

<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: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" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:gravity="center"
    android:text="Your total is 0"
    android:textSize="20dp" />

<Button
    android:id="@+id/button2"
    android:layout_width="250dp"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/button1"
    android:layout_centerVertical="true"
    android:text="Subtract one"
    android:textSize="20dp" />

<Button
    android:id="@+id/button1"
    android:layout_width="250dp"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="62dp"
    android:text="Add one"
    android:textSize="20dp" />

Java 代码：

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 {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
int counter;
Button add, sub;
TextView display;
@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);
    counter = 0;
    add = (Button) findViewById(R.id.button1);
    sub = (Button) findViewById(R.id.button2);
    display = (TextView) findViewById(R.id.textView1);
    add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            counter++;
            display.setText("Your total is " + counter);
        }
    });
    sub.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            counter--;
            display.setText("Your total is " + counter);
        }
    });
    return true;
}
}

当我点击按钮时， textview的 text一点也没变化。我想根据按钮的点击增加或减少，来显示计算器的值。如何实现？