android Listview模拟聊天界面

package com.example.test;import android.os.Bundle;import android.support.v7.app.AppCompatActivity;import android.view.View;import android.view.ViewGroup;import android.widget.BaseAdapter;import android.widget.EditText;import android.widget.ListView;import android.widget.TextView;import android.widget.Toast;import java.util.ArrayList;import static com.example.test.R.id.tv_receive;import static com.example.test.R.id.tv_send;public class MainActivity extends AppCompatActivity {  private ArrayList<Msg> msgs;  private EditText et_input;  private MyAdapter myAdapter;  private ListView lv;  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    lv = (ListView) findViewById(R.id.lv);    et_input = (EditText) findViewById(R.id.et_input);    findViewById(R.id.bt_send).setOnClickListener(new View.OnClickListener() {      @Override      public void onClick(View v) {        String content = et_input.getText().toString();        if (!content.isEmpty()) {          msgs.add(new Msg(content, Msg.TYPE_SEND));          myAdapter.notifyDataSetChanged();          lv.setSelection(msgs.size() - 1);          et_input.setText("");        } else {          Toast.makeText(MainActivity.this, "请输入内容!", Toast.LENGTH_SHORT).show();        }      }    });    msgs = new ArrayList<>();    msgs.add(new Msg("hello", Msg.TYPE_RECEIVE));    msgs.add(new Msg("who is that?", Msg.TYPE_SEND));    msgs.add(new Msg("this is LiLei,nice to meet you!", Msg.TYPE_RECEIVE));    myAdapter = new MyAdapter();    lv.setAdapter(myAdapter);  }  private class MyAdapter extends BaseAdapter {    @Override    public int getCount() {      return msgs.size();    }    @Override    public Msg getItem(int position) {      return msgs.get(position);    }    @Override    public long getItemId(int position) {      return position;    }    @Override    public View getView(int position, View convertView, ViewGroup parent) {      ViewHolder holder;      if (convertView == null) {        holder = new ViewHolder();        convertView = View.inflate(getApplicationContext(), R.layout.listview_item, null);        holder.tv_receive = (TextView) convertView.findViewById(tv_receive);        holder.tv_send = (TextView) convertView.findViewById(tv_send);        convertView.setTag(holder);      } else {        holder = (ViewHolder) convertView.getTag();      }      Msg msg = getItem(position);      if (msg.type == Msg.TYPE_RECEIVE) {        holder.tv_receive.setVisibility(View.VISIBLE);        holder.tv_send.setVisibility(View.GONE);        holder.tv_receive.setText(msg.content);      } else if (msg.type == Msg.TYPE_SEND) {        holder.tv_send.setVisibility(View.VISIBLE);        holder.tv_receive.setVisibility(View.GONE);        holder.tv_send.setText(msg.content);      }      return convertView;    }  }  private static class ViewHolder {    TextView tv_receive;    TextView tv_send;  }}