Question

我知道有很多关于NullPointerException的问题。我一直在四处寻找答案，但我找不到合适的答案。当我尝试打开energy.xml时它会崩溃，这就是java的问题。谁知道为什么？从Logcat我可以看到问题出在OnCreate - btnNrj.setOnClickListener(this); 我试过清理和重建。我只能假设我之后会遇到更多问题，但现在最重要的是让它不会崩溃。

public class ActivityEnergy extends Activity implements OnClickListener {
    EditText etNum10;
    EditText etNum20;
    EditText etNum30;
    int nrjspinner = 0;
    Object value;

    Button btnNrj;

    String oper = "";

    TextView tvResult;

 ArrayList<String> nrjmethod = new ArrayList<String>();

    @Override
    public void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.energy);

     // find the elements
    etNum10 = (EditText) findViewById(R.id.etNum10);
    etNum20 = (EditText) findViewById(R.id.etNum20);
    etNum30 = (EditText) findViewById(R.id.etNum30);

        btnNrj = (Button) findViewById(R.id.btnNrj);

        tvResult = (TextView) findViewById(R.id.tvResult);

        // set a listener
        btnNrj.setOnClickListener(this);

  Spinner spinner = (Spinner) findViewById(R.id.spinner); 

  // Create the ArrayAdapter
  ArrayAdapter <CharSequence> adapter = ArrayAdapter.createFromResource( this,
          R.array.nrjmethod, android.R.layout.simple_spinner_item);

                 // Set the Adapter
  adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
  spinner.setAdapter(adapter);


  // Set the ClickListener for Spinner
  spinner.setOnItemSelectedListener(new OnItemSelectedListener() {

      @Override
      public void onItemSelected(AdapterView<?> parent, View view,
              int position, long id) {

          value = parent.getItemAtPosition(position);
          switch (position) {
          case 0:
              nrjspinner = 1;
              break;

          case 1:
              nrjspinner = 0.8;
              break;

          case 2:
              nrjspinner = 0.8;
              break;

          case 3:
              nrjspinner = 0.8;
              break;

          case 4:
              nrjspinner = 0.8;
              break;

          case 5:
              nrjspinner = 0.8;
              break;

          case 6:
              nrjspinner = 0.8;
              break;

          case 7:
              nrjspinner = 0.8;
              break;

          case 8:
              nrjspinner = 0.8;
              break;

          case 9:
              nrjspinner = 0.8;
              break;

          case 10:
              nrjspinner = 0.6;
              break;

          case 11:
              nrjspinner = 0.6;
              break;
          }

      }


      @Override
      public void onNothingSelected(AdapterView<?> arg0) {



    } 

        });


 }

    @Override
         public void onClick(View v) {
                // TODO Auto-generated method stub
                float num10 = 0;
                float num20 = 0;
                float num30 = 0;
                float result = 0;

                // check if the fields are empty
                if (TextUtils.isEmpty(etNum10.getText().toString())
                    || TextUtils.isEmpty(etNum20.getText().toString())
                    || TextUtils.isEmpty(etNum30.getText().toString())) {
                  return;
                }

                // read EditText and fill variables with numbers
                num10 = Float.parseFloat(etNum10.getText().toString());
                num20 = Float.parseFloat(etNum20.getText().toString());
                num30 = Float.parseFloat(etNum30.getText().toString());

             // defines the button that has been clicked and performs the corresponding operation
               switch (v.getId()) {
                case R.id.btnNrj:
                  oper = "";
                  result = (float) ((nrjspinner * ((num10 * num20) / num30))*0.001);
                  break;
                default:
                  break;

                }

             // form the output line
                tvResult.setText("Your result = " + result);

    }


}

这是logcat

10-27 17:40:58.832: E/AndroidRuntime(1497): FATAL EXCEPTION: main

10-27 17:40:58.832: E/AndroidRuntime(1497): java.lang.RuntimeException: Unable to start activity ComponentInfo{se.develope.axson/se.develope.axson.ActivityEnergy}: java.lang.NullPointerException

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread.access$600(ActivityThread.java:141)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.os.Handler.dispatchMessage(Handler.java:99)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.os.Looper.loop(Looper.java:137)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread.main(ActivityThread.java:5103)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at java.lang.reflect.Method.invokeNative(Native Method)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at java.lang.reflect.Method.invoke(Method.java:525)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at dalvik.system.NativeStart.main(Native Method)

10-27 17:40:58.832: E/AndroidRuntime(1497): Caused by: java.lang.NullPointerException

10-27 17:40:58.832: E/AndroidRuntime(1497):     at se.develope.axson.ActivityEnergy.onCreate(ActivityEnergy.java:52)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.Activity.performCreate(Activity.java:5133)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)

10-27 17:40:58.832: E/AndroidRuntime(1497):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)

10-27 17:40:58.832: E/AndroidRuntime(1497):     ... 11 more

Answer 1

我放弃了为NullPointerException找到解决方案。相反，我找到了另一种方法来解决没有听众的问题。是@Chronos在另一个线程中提供了解决方案。

在定义按钮广告的XML中

android:onClick="ButtonOnClick"

并在代码中定义“ButtonOnClick”。就我而言，它看起来像这样：

public void ButtonOnClick(View v){
// TODO Auto-generated method stub
        float num10 = 0;
        float num20 = 0;
        float num30 = 0;
        float result = 0;

        // check if the fields are empty
        if (TextUtils.isEmpty(etNum10.getText().toString())
            || TextUtils.isEmpty(etNum20.getText().toString())
            || TextUtils.isEmpty(etNum30.getText().toString())) {
          return;
        }

        // read EditText and fill variables with numbers
        num10 = Float.parseFloat(etNum10.getText().toString());
        num20 = Float.parseFloat(etNum20.getText().toString());
        num30 = Float.parseFloat(etNum30.getText().toString());

     // defines the button that has been clicked and performs the corresponding operation
        switch (v.getId()) {
        case R.id.btnNrj:
          oper = "";
          result = (float) ((nrjspinner * ((num10 * num20) / num30))*0.001);
          break;
        default:
          break;

你可以在这个主题中找到Chronos答案： Android OnClickListener - identify a button

NullPointerException - 打开特定活动时应用程序崩溃

1 个答案: