怎么在listview中清除原数据

之前也看到了用list.clean()方法，但是不知道为什么一直不成功，希望大家帮忙
package com.example.lin.yy1702a_30;import android.content.DialogInterface;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;public class ListViewDemo extends AppCompatActivity implements View.OnClickListener{
    private ListView listView;
    private SimpleAdapter simpleAdapter;
    public List<Map<String,Object>> list=new ArrayList<Map<String,Object>>();
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_interface);
        /*
        处理三步走
         */
        //第一步得到对象
        listView=(ListView)findViewById(R.id.list_item);
        //默认数据
        ShuJu();
        //第二步实例化SimpleAdapter
        simpleAdapter=new SimpleAdapter(
                this,
                list,
                R.layout.listview,
                new String[]{"class_name","name","text_geren"},
                new int[]{R.id.class_name,R.id.name,R.id.text_geren}
        );
        //第三步设置适配器
        listView.setAdapter(simpleAdapter);
    }    //数据源--------是我要清除的
    protected void ShuJu(){
        Map map=new HashMap();
        map.put("class_name","计算机1702");
        map.put("name","小白");
        map.put("text_geren","无");
        Map map1=new HashMap();
        map1.put("class_name","计算机1702");
        map1.put("name","小民");
        map1.put("text_geren","无");
        Map map2=new HashMap();
        map2.put("class_name","计算机1702");
        map2.put("name","小天");
        map2.put("text_geren","无");
        list.add(map);
        list.add(map1);
        list.add(map2);
    }    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.return_button:
                finish();
                break;
            case R.id.add:
                final LinearLayout linearLayout=(LinearLayout)getLayoutInflater().inflate(R.layout.load_layout,null);
                new AlertDialog.Builder(this)
                        .setView(linearLayout)
                        //点击按钮时前面的内容删除，后面的内容添加，怎么做。
                        .setPositiveButton("确定", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                               //是我要添加的
                                EditText text_class=(EditText)linearLayout.findViewById(R.id.text_class);
                                EditText text_name=(EditText)linearLayout.findViewById(R.id.text_name);
                                EditText text_geren=(EditText)linearLayout.findViewById(R.id.text_geren);
                                TextView class_name=(TextView) findViewById(R.id.class_name);
                                class_name.setText(text_class.getText().toString());
                                TextView name=(TextView) findViewById(R.id.name);
                                name.setText(text_name.getText().toString());
                                TextView text_geren1=(TextView) findViewById(R.id.text_geren1);
                                text_geren1.setText(text_geren.getText().toString());
                            }
                        }).create().show();
        }
    }
}