Java Android应用程序崩溃，原因是迭代器

我有一个名为listOfDrills的列表，它有迭代器drillIt，这就是迭代器没有

next（）

值的原因（我很遗憾，该列表不是空的，因为它可以与

listOfDrills.get（0）

）配合使用）。代码如下：

package com.simplemathgame;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Random;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class GamePlayActivity extends Activity {
    int addDrills;
    int subDrills;
    int mulDrills;
    int divDrills;
    int minBound;
    int maxBound;

    TextView drillTextPlace;
    Button nextButton;
    Button backButton;

    List<Drill> listOfDrills = new ArrayList<Drill>();
    Iterator<Drill> drillIt = listOfDrills.iterator();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_game_play);

        //get values from other activity
        Bundle extras = getIntent().getExtras();
        addDrills = extras.getInt("addDrills");
        subDrills = extras.getInt("subDrils");
        mulDrills = extras.getInt("mulDrills");
        divDrills = extras.getInt("divDrills");
        minBound = extras.getInt("minBound");
        maxBound = extras.getInt("maxBound");

        for(int i = 0; i < addDrills; i++){
            AddDrill tmpDrill = new AddDrill(minBound,maxBound);
            listOfDrills.add(tmpDrill);
        }

        for(int i = 0; i < subDrills; i++){
            SubDrill tmpDrill = new SubDrill(minBound,maxBound);
            listOfDrills.add(tmpDrill);
        }

        for(int i = 0; i < mulDrills; i++){
            MulDrill tmpDrill = new MulDrill(minBound,maxBound);
            listOfDrills.add(tmpDrill);
        }

        for(int i = 0; i < divDrills; i++){
            DivDrill tmpDrill = new DivDrill(minBound,maxBound);
            listOfDrills.add(tmpDrill);
        }

        long seed = System.nanoTime();
        Collections.shuffle(listOfDrills, new Random(seed));

        drillTextPlace = (TextView) findViewById(R.id.drill_text);
        nextButton = (Button) findViewById(R.id.next_button);
        backButton = (Button) findViewById(R.id.back_button);

        if(drillIt.hasNext()){
            drillTextPlace.setText((drillIt.next()).printDrill());
        }

        nextButton.setOnClickListener(new OnClickListener(){

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if(drillIt.hasNext()){
                    drillTextPlace.setText(drillIt.next().printDrill());
                }
            }
        });
    }

}

如果我删除

if

语句，只留下

drillTextPlace.setText（（drillIt.next（））.printDrill（））
应用程序崩溃了

为什么会这样？我做错了什么？谢谢
 您应该在填写完列表后创建迭代器。如文件所示：

此类的迭代器和listIterator返回的迭代器
方法是快速失败的：如果列表在任何时候进行了结构修改
创建迭代器后的时间，以任何方式，除了通过
迭代器自己的remove或add方法，迭代器将抛出
ConcurrentModificationException


移动

您创建迭代器太早，需要在使用迭代器之前创建迭代器。当它为空时，您正在创建它。即使这样，每次你打next时，你都会向前一步，永远不会后退…
你会收到什么错误消息？显示完整的堆栈跟踪。
if(drillIt.hasNext()){
            drillTextPlace.setText((drillIt.next()).printDrill());
        }

Iterator<Drill> drillIt = listOfDrills.iterator();

if (drillIt.hasNext()) {
    drillTextPlace.setText((drillIt.next()).printDrill());
}