在doInBackground方法中发生的异步任务＃1

Question

我认为没有办法确保所有信息都被给出，然后列出我的所有三个班级。 但是，在所有这些之前，我将包括我的logcat错误以及我的应用应该做的事情：

Logcat错误

03-27 17:10:45.368 2373-2665/edu.ggc.amauldin.currencyappaustin E/AndroidRuntime: 
 FATAL EXCEPTION: AsyncTask #1
Process: DOMAIN, PID: 2373
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:304)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
at java.util.concurrent.FutureTask.run(FutureTask.java:242)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method
 'void java.util.Scanner.close()' on a null object reference
at DOMAIN.FindRate.doInBackground(FindRate.java:59)
at DOMAIN.FindRate.doInBackground(FindRate.java:25)
at android.os.AsyncTask$2.call(AsyncTask.java:292)

应用程序被认为是：

用户在EditText字段中输入一个值，然后选择一个基础货币和一个转换为2个微调器的货币，然后按下转换按钮并输出新货币。 我正在尝试使用异步任务和fixer.io来实现这一点 ，希望学习如何更好地理解和使用它们。

类别：

我将列出我的课程，看看是否能更好地阐明我的情况。 如果需要，我会省略我的进口等，以减少这篇文章的一面，让我知道。

MainActivity.java

public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener{
public static double sum;
private static String urlIO = "http://api.fixer.io/latest?base=";
Spinner spinner, spinner2;
Button convBtn, aboutBtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    final EditText enteredAmt = (EditText)findViewById(R.id.editText);
    final TextView result = (TextView)findViewById(R.id.resultTv);
    spinner = (Spinner) findViewById(R.id.spinner);
    spinner2 = (Spinner) findViewById(R.id.spinner2);
    convBtn = (Button) findViewById(R.id.button);
    aboutBtn = (Button) findViewById(R.id.button2);

    ArrayAdapter adapter = ArrayAdapter.createFromResource(this, R.array.currencyTypes, android.R.layout.simple_spinner_item);
    spinner.setAdapter(adapter);
    spinner2.setAdapter(adapter);

    spinner.setOnItemSelectedListener(this);
    spinner2.setOnItemSelectedListener(this);

    convBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            FindRate covertMe = new FindRate(result);
            sum = Double.parseDouble(enteredAmt.getText().toString());

            try{
                if(spinner.getSelectedItem().toString().equals("USD") && spinner2.getSelectedItem().toString().equals("EUR")){
                    covertMe.execute(new URL(urlIO + "USD&symbols=GBP"));
                }
            } catch (MalformedURLException e){
                e.printStackTrace();
            }
        }
    });
    aboutBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getBaseContext(), "By: Austin Mauldin \n ITEC 4550", Toast.LENGTH_SHORT).show();
        }
    });

}

FindRate.java

public class FindRate extends AsyncTask<URL, Void, Double> {
private static final String rightForm = "###.00";
private static final String errorCode = "error code: it messed up";
private static final String checkRSP = "Checking";

private TextView tV;
private Gson gSON;

FindRate(TextView inp){
    tV = inp;
    gSON = new GsonBuilder().create();

}

@Override
protected Double doInBackground(URL... params) {

    HttpURLConnection connectStat = null;
    Scanner scanner = null;
    StringBuilder myJsonString = new StringBuilder();

    try {
        if (params.length != 1)
            throw new IllegalArgumentException(this.getClass().getName() + errorCode);
        connectStat = (HttpURLConnection) params[0].openConnection();
        InputStream inputer = new BufferedInputStream(connectStat.getInputStream());
        scanner = new Scanner(inputer);
        while (scanner.hasNext()) myJsonString.append(scanner.nextLine());
        Log.v(checkRSP, "Response(" + connectStat.getResponseCode() + "):" +
                connectStat.getResponseMessage());
    } catch (IOException e){
        Log.e(checkRSP, e.getStackTrace().toString());
        return Double.valueOf(-1D);
    } finally {
        scanner.close();
        connectStat.disconnect();
    }
    String jSon = myJsonString.toString();
    LookupRate cuRate = gSON.fromJson(jSon, LookupRate.class);
    Log.v(checkRSP, jSon);
    Log.v(checkRSP, cuRate.toString());
    return Double.valueOf(cuRate.findRate());

}

@Override
protected  void onPostExecute(Double result){
    DecimalFormat format = new DecimalFormat(rightForm);
    String str = format.format(result);
    tV.setText(format.format(MainActivity.sum * Double.parseDouble(str))+ "");
}
 }

LookupRate.java

这是另一个我不喜欢的领域我想从fixer.io输出中获取rate（s）值，所以如果它有帮助这就是fixer.io输出看起来像{"base":"USD","date":"2016-03-24","rates":{"GBP":0.70771}}

public class LookupRate {
private String to;
private Double rate;
private String from;

public Double findRate() {
    return rate;
}

@Override
public  String toString(){
    String str1 = "Checking" +
            " [rates=" + rate + "]";
    return str1;
}
 }

澄清

应用程序在点击转换按钮时崩溃只是为了确保完全清楚。 我在这做错了什么？

Answer 1

更改

finally {
    scanner.close();
    connectStat.disconnect();
}

至

finally {
 if(null != scanner){   scanner.close();}
  if(null != connectStat){  connectStat.disconnect();}
 }

我猜测在初始化扫描程序之前会抛出异常，因此会出现空指针异常。 黄金法则，总是检查null