android,刚点debug就崩溃了

Question

MainActivity.java

 package com.emple.divide;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.GridView;
import android.widget.TextView;

public class MainActivity extends Activity {
    private String[] str=new String[]{"one","two","three","four"};
    private GridView gv;
    private TextView tv;
    private Button bt;
    ArrayAdapter<String> arrayAdapter=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,str);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        gv=(GridView)findViewById(R.id.gv);
        tv=(TextView)findViewById(R.id.tv);
        bt=(Button)findViewById(R.id.bt);
        bt.setOnClickListener(new bt_click());
        gv.setOnItemClickListener(new gv_click());
    }

    class bt_click implements OnClickListener
    {
        public void onClick(View v)
        {
            gv.setNumColumns(2);
            gv.setAdapter(arrayAdapter);
        }
    }

    class gv_click implements OnItemClickListener
    {
        public void onItemClick(AdapterView<?> parent,View v,int position,long arg3)
        {
            switch (arrayAdapter.getCount())
            {
            case 4:
                tv.setText(str[position]);
                break;
            default:break;
            }
        }
    }

    @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;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

arrayadapter初始化过早，arrayadapter在activity类实例化的话的时候就初始化了，arrayadapter初始化的时候传入的this是一个context，这个时候context还是无效的，需要在oncreate调用之后才对arrayadapter进行初始化，放在supo.oncreate之后，或者放在onresume里面都可以