java - 通过改造调用 api 时，尝试在空对象引用错误上调用虚拟方法 'int java.util.ArrayList.size()'

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

这是我的 api 调用，发生错误

public void loadData(){
    pd=new ProgressDialog(this);
    pd.setMessage("loading");
    pd.show();
    mApi= new RetrofitHelper<AuthApi>().getApi(AuthApi.class);
    mCall=mApi.studentlist(pageNo);
    mCall.enqueue(new Callback<ResultObject<ArrayList<Person>>>() {
        @Override
        public void onResponse(Call<ResultObject<ArrayList<Person>>> call, Response<ResultObject<ArrayList<Person>>> response) {
            for(int i=0;i<response.body().getData().size();i++){
                mData.add(response.body().getData().get(i));
            }
            count=response.body().getCount();
            mAdapter=new PersonAdapter(Members.this,R.layout.item_person,mData);
            list.setAdapter(mAdapter);
            pd.hide();
        }

        @Override
        public void onFailure(Call<ResultObject<ArrayList<Person>>> call, Throwable t) {
            Toast.makeText(Members.this,"failed",Toast.LENGTH_SHORT).show();
            pd.hide();
        }
    });


 }

这是我在调用 api 时的 logcat，你可以看到我正确地得到了响应，并且我在后端跟踪球，一切都很好，没有错误

    07-01 12:23:56.180 12981-13143/com.example.kethan.project D/OkHttp: --> GET http://192.168.0.107:8000/students/?page=1 http/1.1
07-01 12:23:56.180 12981-13143/com.example.kethan.project D/OkHttp: --> END GET
07-01 12:23:56.183 12981-12981/com.example.kethan.project D/ActivityThreadInjector: clearCachedDrawables.
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: <-- 200 OK http://192.168.0.107:8000/students/?page=1 (80ms)
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: Date: Sat, 01 Jul 2017 06:53:53 GMT
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: Server: WSGIServer/0.2 CPython/3.5.2
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: Content-Length: 849
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: Vary: Accept, Cookie
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: Content-Type: application/json
07-01 12:23:56.261 12981-13143/com.example.kethan.project D/OkHttp: X-Frame-Options: SAMEORIGIN
07-01 12:23:56.262 12981-13143/com.example.kethan.project D/OkHttp: Allow: GET, POST, HEAD, OPTIONS
07-01 12:23:56.263 12981-13143/com.example.kethan.project D/OkHttp: {"count":37,"next":"http://192.168.0.107:8000/students/?page=2","previous":null,"results":[{"id":35,"name":"kethan","mobile":"8142142827","time":"10:5:0","late":"late by: 5mins 0secs","date":"2017/6/16"},{"id":36,"name":"kethan","mobile":"8142142827","time":"23:28:0","late":"late by: 13hrs 28mins 0secs","date":"2017/06/29"},{"id":37,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"},{"id":38,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"},{"id":39,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"},{"id":40,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"},{"id":41,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"},{"id":42,"name":"kethan","mobile":"8142142827","time":"134","late":"123","date":"123"}]}
07-01 12:23:56.263 12981-13143/com.example.kethan.project D/OkHttp: <-- END HTTP (849-byte body)
07-01 12:23:56.292 12981-12981/com.example.kethan.project D/AndroidRuntime: Shutting down VM


                                                                            --------- beginning of crash
07-01 12:23:56.292 12981-12981/com.example.kethan.project E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            Process: com.example.kethan.project, PID: 12981
                                                                            java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
                                                                                at com.example.kethan.project.Members$2.onResponse(Members.java:108)
                                                                                at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
                                                                                at android.os.Handler.handleCallback(Handler.java:739)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                at android.os.Looper.loop(Looper.java:148)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5441)
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)

这是结果对象

public class ResultObject<T> {

private int count;

private T data;

public int getCount() {
    return count;
}

public T getData() {
    return data;
}

public void setCount(int count) {
    this.count = count;
}

public void setData(T data) {
    this.data = data;
}
}

这是 Person pojo 类

public class Person {

private int id;

private String time;

private String late;

private String date;

private String name;

private String mobile;

public String getDate() {
    return date;
}

public int getId() {
    return id;
}

public String getTime() {
    return time;
}

public void setTime(String time) {
    this.time = time;
}

public String getLate() {
    return late;
}

public void setLate(String late) {
    this.late = late;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getMobile() {
    return mobile;
}

public void setMobile(String mobile) {
    this.mobile = mobile;
}

public void setDate(String date) {
    this.date = date;
}

public void setId(int id) {
    this.id = id;
}
}

Answer 1

这意味着 response.body().getData() 返回 null。由于无法获取 null 的大小，因此会引发异常。

为了使问题更清楚，让我们将 for 循环分成多行。 (这也是一个好主意，这样您的代码将会更加高效。您总共只调用 size() 一次，而不是每次执行循环时调用一次。

ArrayList<Person> listOfPersons = response.body().getData(); //This will be null, I suspect.
int size = listOfPersons.size(); //Exception will happen on this line, since listOfPersons is null
for(int i = 0; i < size; ++i)
{
    mData.add(listOfPersons.get(i));
}

您需要执行安全检查以验证 ResultObject 包含您要查找的信息。命中断点并查看响应正文的内容。