亚马逊网络服务-Android抛出java.lang.NullPointerException

Question

我被卡住了。单击按钮时出现错误NullPointerException。我无法确定null在哪里。以下是我的代码供您参考。我已经犯这个错误两周了（

public class DynamoDB extends Activity{
private String tableName = "Tesis_FTSM";
private AmazonDynamoDBClient client;
public void DynamoDB() throws IOException
{
    AWSCredentials credentials = new PropertiesCredentials(DynamoDB.class.getResourceAsStream("Credentials.properties"));
    client = new AmazonDynamoDBClient(credentials);
    client.setRegion(Region.getRegion(Regions.US_WEST_2));
}
public void putItem(String Title, String Author, String Year, String Supervisor, String Program)
{
    Map<String, AttributeValue> item = new HashMap<>();
    item.put("Id", new AttributeValue().withN("1"));
    item.put("Title", new AttributeValue().withS(Title));
    item.put("Author", new AttributeValue().withS(Author));
    item.put("Year", new AttributeValue().withS(Year));
    item.put("Supervisor", new AttributeValue().withS(Supervisor));
    item.put("Program", new AttributeValue().withS(Program));
    PutItemRequest itemRequest = new PutItemRequest().withTableName(
            tableName).withItem(item);
    client.putItem(itemRequest);
    item.clear();
}
}

主要活动.java

public class MainActivity extends Activity
{
    ///////////////////////////////////////////////////////////////////////////////////////////////
    final EditText title = (EditText)findViewById(R.id.title_View);
    final EditText author = (EditText)findViewById(R.id.author_View);
    final EditText year = (EditText)findViewById(R.id.year_View);
    final EditText supervisor = (EditText)findViewById(R.id.super_View);
    final Spinner prog = (Spinner)findViewById(R.id.prog_Spinner);
    final String tajuk = title.getText().toString();
    final String au = author.getText().toString();
    final String tahun = year.getText().toString();
    final String selia = supervisor.getText().toString();
    final String program = prog.getSelectedItem().toString();
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button upload = (Button)findViewById(R.id.upload_Btn);
        upload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    Toast.makeText(getApplicationContext(), "Save to Table", Toast.LENGTH_SHORT).show();
                    Task obj = new Task();
                    obj.execute();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

        });

    }
    public class Task extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... params) {
            Looper.prepare();
            try {
                DynamoDB db = new DynamoDB();
                db.putItem(tajuk,au,tahun,selia,program);

            } catch (Exception e) {
                e.printStackTrace();
            }
            Looper.loop();
            return null;
        }
        protected void onPostExecute(String page) {
            // onPostExecute
        }
    }

}

LOGCAT错误

06-24 12:53:25.321  23051-23079/com.example.user.test2 W/System.err﹕ java.lang.NullPointerException
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at com.example.user.test2.DynamoDB.putItem(DynamoDB.java:61)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at com.example.user.test2.MainActivity$Task.doInBackground(MainActivity.java:66)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at com.example.user.test2.MainActivity$Task.doInBackground(MainActivity.java:44)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at android.os.AsyncTask$2.call(AsyncTask.java:288)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at java.util.concurrent.FutureTask.run(FutureTask.java:237)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
06-24 12:53:25.331  23051-23079/com.example.user.test2 W/System.err﹕ at java.lang.Thread.run(Thread.java:841)

Answer 1

您必须在onCreate方法中的setContentView之后移动这些代码。

final EditText title = (EditText)findViewById(R.id.title_View);
final EditText author = (EditText)findViewById(R.id.author_View);
final EditText year = (EditText)findViewById(R.id.year_View);
final EditText supervisor = (EditText)findViewById(R.id.super_View);
final Spinner prog = (Spinner)findViewById(R.id.prog_Spinner);
final String tajuk = title.getText().toString();
final String au = author.getText().toString();
final String tahun = year.getText().toString();
final String selia = supervisor.getText().toString();
final String program = prog.getSelectedItem().toString();