当我完成活动时，我正在为Listview上显示的数组添加一个值。
在第一次意图后我刷新游戏（回到main_activity）并开始游戏agian。
当我完成那个游戏时，我在数组中添加了另一个值，但它只显示输入的当前（lawst）值的新值。
我想在数组中显示最后十个值。现在我看到的另一个问题是，当我打开游戏并且arraylist是空的时我得到了一个错误：
java.lang.NullPointerException: Attempt to invoke virtual method ' java.lang.String java.lang.String.toString()' on a null object reference
人员类（在比赛之后，它将数据发送到前十名）：
public class Person extends Activity{
EditText name;//define
Button ok;
TextView enter;
Integer Score;
Integer score;//c'tor
String Name;
String pname;//c'tor
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_person);
Intent intent = getIntent();//getting extra from intent
Score = intent.getIntExtra("Score", 0);//defualt value if thers no score
name =(EditText)findViewById(R.id.namep);
Name = name.getText().toString();//getting the name that was input to the EditText box
Log.d("name", Name);
ok =(Button)findViewById(R.id.send);
enter =(TextView)findViewById(R.id.entername);
Toast toast = Toast.makeText(getApplicationContext(), "Lets see if you are can be on TopTen", Toast.LENGTH_SHORT);//Toast Message
toast.show();
ok.setOnClickListener(SendActionListener);//set OnClicklistener
}
public Person(){
this(null,0);
}
public Person(String Tname,int Tscore){
this.score = Tscore;//score getting Score from last activity
this.pname = Tname;//pname getting the Name from the editBox
}
public String toString(){
return "Name :"+this.pname.toString() +" Score : " +this.score.toString();
}
private OnClickListener SendActionListener = new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Integer s;
String n;
s = Score;//getting score
n = name.getText().toString();
Log.d("Name", n);
//details=name+" "+score;//making one string that include the name and the score for the array
Log.d("ScoreIs", String.valueOf(s));
Log.d("Name is",n);
Intent person = new Intent(Person.this,TopTen.class);
person.putExtra("Score", s);//send extra with the intent
person.putExtra("Name", n);
startActivity(person);//start activity
Log.d("inside", "activity Person");
};
};
public Integer getScore() {
return score;
}
public String getName(){
return pname;
}
/*class ScoreComparator implements Comparator<Person>{
@Override
public int compare(Person person1, Person person2) {
// TODO Auto-generated method stub
return person1.getScore().compareTo(person2.getScore());
}
}*/
}//end of Person
TopTen类（带有te数组，列表视图和排序）：
public class TopTen extends ListActivity{
ArrayList<Person>TopTens = new ArrayList<Person>(10);
String Name;
Integer Score;
ListView list;
Person newScore;
Person temp;
int i=0;
int j=0;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_topten);
Intent intent = getIntent();//getting Extra from last Intent
Name = intent.getStringExtra("Name");//getting name
Score = intent.getIntExtra("Score", 0);//getting score
newScore = new Person(Name,Score);//creating new person
ActionBar actionBar = getActionBar();//ActionBar Define
actionBar.setDisplayHomeAsUpEnabled(true);
list = (ListView)findViewById(android.R.id.list);//find list by id
list.setOnItemClickListener(new OnItemClickListener() {//if clicked on item in the list view
@Override
public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
long arg3) {
// TODO Auto-generated method stub
Intent i = new Intent(TopTen.this, MainActivity.class);//creat intent
startActivity(i);//start activity go back to main
}
});
int size = TopTens.size();//last value in arry
if(size<10)
TopTens.add(newScore);
if(!TopTens.isEmpty()){//while not empty there is a value
if(TopTens.get(size).getScore()<newScore.getScore()){//check last value score is lower then newscore score
Toast toast = Toast.makeText(getApplicationContext(), "You hit the Top Ten",Toast.LENGTH_SHORT);//if yes you in top ten
toast.show();
TopTens.add(newScore);
for(i=0;i<TopTens.size();i++){//running all over the array
for(j=0;j<TopTens.size();j++){
if(TopTens.get(i).getScore()>TopTens.get(j).getScore()){//if the plce iam in which is i
//change i and j lets say i is one and j is two change them
temp = TopTens.get(i);
TopTens.set(i, TopTens.get(j));
TopTens.set(j, temp);
}
}
}
}
}//else//else if TopTens is empty there is no values add the new value
populateListView();//list func
}
public boolean onOptionsItemSelected(MenuItem item){//actionbar activity
Intent myIntent = new Intent(getApplicationContext(), MainActivity.class);
startActivityForResult(myIntent, 0);
return true;
}
private void populateListView(){
System.out.println(TopTens);
Log.d("inside", "populateListView");
ArrayAdapter<Person> adapter = new ArrayAdapter<Person>(this, R.layout.activity_topten,R.id.text1,TopTens);
//set and arrayadapter to thow array in the list
Log.d("Inside", "LIST:");
list.setAdapter(adapter);//set/use the adapter
Log.d("Inside", "Adapter:");
}
}
当我刚启动应用程序时打开Listview时，我得到null的Logcat：
05-05 17:42:23.981: I/Timeline(13730): Timeline: Activity_launch_request id:com.example.flagsgame time:70151464
05-05 17:42:24.044: D/AndroidRuntime(13730): Shutting down VM
05-05 17:42:24.044: E/AndroidRuntime(13730): FATAL EXCEPTION: main
05-05 17:42:24.044: E/AndroidRuntime(13730): Process: com.example.flagsgame, PID: 13730
05-05 17:42:24.044: E/AndroidRuntime(13730): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.flagsgame/com.example.flagsgame.TopTen}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toString()' on a null object reference
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2329)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2389)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread.access$900(ActivityThread.java:147)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1296)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.os.Handler.dispatchMessage(Handler.java:102)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.os.Looper.loop(Looper.java:135)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread.main(ActivityThread.java:5254)
05-05 17:42:24.044: E/AndroidRuntime(13730): at java.lang.reflect.Method.invoke(Native Method)
05-05 17:42:24.044: E/AndroidRuntime(13730): at java.lang.reflect.Method.invoke(Method.java:372)
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
05-05 17:42:24.044: E/AndroidRuntime(13730): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toString()' on a null object reference
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.example.flagsgame.Person.toString(Person.java:64)
05-05 17:42:24.044: E/AndroidRuntime(13730): at java.lang.StringBuilder.append(StringBuilder.java:202)
05-05 17:42:24.044: E/AndroidRuntime(13730): at java.util.AbstractCollection.toString(AbstractCollection.java:374)
05-05 17:42:24.044: E/AndroidRuntime(13730): at java.lang.StringBuilder.append(StringBuilder.java:202)
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.android.internal.os.LoggingPrintStream.println(LoggingPrintStream.java:290)
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.example.flagsgame.TopTen.populateListView(TopTen.java:82)
05-05 17:42:24.044: E/AndroidRuntime(13730): at com.example.flagsgame.TopTen.onCreate(TopTen.java:71)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.Activity.performCreate(Activity.java:5933)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
05-05 17:42:24.044: E/AndroidRuntime(13730): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2282)
05-05 17:42:24.044: E/AndroidRuntime(13730): ... 10 more
答案 0 :(得分：1)
您需要检查pname和分数是否为NULL。
public String toString(){
return "Name :"+this.pname.toString() +" Score : " +this.score.toString();
}
编辑：
尝试从
更改此设置ArrayList<Person>TopTens = new ArrayList<Person>(10);
到
ArrayList<Person>TopTens = new ArrayList<Person>();
或以这种方式打印TopTen ArrayList
for (Person p : TopTens)
if (p != null)
System.out.println(p);
而不是
System.out.println(TopTens);