Android的listview怎样动态改变显示的数量

我做了一个简单的小程序，在一个界面上有一个按钮，然后点击按钮就增加一行listview，但是我
我的程序点击却没有反应，应该怎样解决，这是源代码
package com.example.dianjishiyan;
import java.util.ArrayList;
import java.util.List;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener{

ListView lv;
Button bt;
myad adaptper;
List<crime> a=new ArrayList<crime>();
int b=a.size()+1; 
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    lv=(ListView) findViewById(R.id.lv);
    bt=(Button) findViewById(R.id.bt);
    adaptper=new myad();
    lv.setAdapter(adaptper);

    }
class myad extends BaseAdapter{

    TextView tv;        
    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return b;
    }

    @Override
    public String getItem(int position) {
        return null;
        //return a[position];
    }

    @Override
    public long getItemId(int position) {
            return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        LinearLayout ll=null;
        if(convertView!=null){
            ll=(LinearLayout) convertView;
        }else{
            ll=(LinearLayout) LayoutInflater.from(MainActivity.this).inflate(R.layout.textview, null);
        }
        //String b=getItem(position);
        tv=(TextView) ll.findViewById(R.id.textView1);
        tv.setText("textview");
        return ll;
}

}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub        
    crime c=new crime();
    a.add(c);
}

}

<LinearLayout 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:orientation="vertical"
tools:context="com.example.dianjishiyan.MainActivity" >

<ListView
    android:id="@+id/lv"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1" />


<Button 
    android:id="@+id/bt"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="add"
    />

<?xml version="1.0" encoding="utf-8"?>
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:layout_margin="5dp"
    android:gravity="left|center"
    android:textSize="30sp"/>

package com.example.dianjishiyan;

public class crime {

}