Question

我在下面粘贴了一些代码。我正在尝试使用此按钮激活下面的计算并将结果放在EditText视图中。我发现程序中没有列出任何错误，但是当我按下按钮（calcbutton）时，它在我的Android手机上崩溃了。

我想知道我的BigDecimal计算是否有问题，但我不知道为什么没有报告错误。

public String massVal;
public String volVal;
public String volunitdes;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_sfp);

//declare ListView
    final ListView mass;

//get ListView object from xml
    mass = (ListView) findViewById(R.id.mass);

//defined array values to show in ListView
    String[] masslv = new String[]{"g", "mg", "µg", "mol", "mmol", "µmol"};

//define new Adapter
    ArrayAdapter<String> massAdapter = new ArrayAdapter<String>
            (this, android.R.layout.simple_list_item_1, android.R.id.text1, masslv);

//assign Adapter to ListView
    mass.setAdapter(massAdapter);

//ListView Item Click Listener
    mass.setOnItemClickListener(new OnItemClickListener(){
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        //ListView Clicked item index
        int itemPosition = position;
        //ListView Clicked item value
        String massVal = (String) mass.getItemAtPosition(position);

        //declare EditText for mass and place clicked value (String massVal) into box
            EditText masstext;
            masstext = (EditText) findViewById(R.id.masstext);
            masstext.setText(massVal);
        }});



//button for final calculation
Button calcbutton;
calcbutton = (Button) findViewById(R.id.calcbutton);

calcbutton.setOnClickListener(new View.OnClickListener(){
  public void onClick(View v) {
  //declare all manual number entries as EditText, then String, then BigDecimal
  EditText CONCdes = (EditText) findViewById(R.id.CONCdes);
  EditText voldes = (EditText) findViewById(R.id.voldes);
  EditText MWbox = (EditText) findViewById(R.id.MWbox);

  String CONCVAL = CONCdes.getText().toString();
  String VOLDES = voldes.getText().toString();
  String MW = MWbox.getText().toString();

  BigDecimal CONCVALbd = new BigDecimal(CONCVAL).setScale(2, RoundingMode.HALF_UP);
  BigDecimal VOLDESbd = new BigDecimal(VOLDES).setScale(2, RoundingMode.HALF_UP);
  BigDecimal MWbd = new BigDecimal(MW).setScale(2, RoundingMode.HALF_UP);

  //default calculation assumes ug/ml, ml vol units to measure, and mg units to weigh

  BigDecimal FINPOWDMEAS;
  FINPOWDMEAS = VOLDESbd.multiply(CONCVALbd).divide(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);

  if (massVal.equals("g"))
    {FINPOWDMEAS = FINPOWDMEAS.multiply(BigDecimal.valueOf(1000000)).setScale(2, RoundingMode.HALF_UP);}

  if (massVal.equals("mg"))
    {FINPOWDMEAS = FINPOWDMEAS.multiply(BigDecimal.valueOf(1000));}

  if (massVal.equals("mol"))
    {FINPOWDMEAS = VOLDESbd.multiply(CONCVALbd).multiply(MWbd).multiply(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);}

  if (massVal.equals("mmol"))
    {FINPOWDMEAS = (VOLDESbd.multiply(CONCVALbd).divide(BigDecimal.valueOf(1000))).multiply(MWbd).multiply(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);

  if (massVal.equals("µmol"))
    {FINPOWDMEAS = (VOLDESbd.multiply(CONCVALbd).divide(BigDecimal.valueOf(1000000))).multiply(MWbd).multiply(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);}

  if (volVal.equals("L"))
    {FINPOWDMEAS = FINPOWDMEAS.divide(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);}

  if (volVal.equals("dL"))
    {FINPOWDMEAS = FINPOWDMEAS.divide(BigDecimal.valueOf(100)).setScale(2, RoundingMode.HALF_UP);}

  if (volVal.equals("µL")){FINPOWDMEAS = FINPOWDMEAS.multiply(BigDecimal.valueOf(1000)).setScale(2, RoundingMode.HALF_UP);}

  //final calculation value to string
  EditText FINPOWDMEASURE = (EditText) findViewById(R.id.FINPOWDMEASURE);
  String FINALPOWDERMEASURE = FINPOWDMEAS.toString();
  FINPOWDMEASURE.setText(FINALPOWDERMEASURE);}});

例外：

Process: vanwert.sdb, PID: 907
java.lang.NullPointerException
        at vanwert.sdb.SFP$4.onClick(SFP.java:134)
        at android.view.View.performClick(View.java:4438)
        at android.view.View$PerformClick.run(View.java:18422)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

该行

String massVal = (String) mass.getItemAtPosition(position);

让您将值分配给名为massVal的新变量，其范围与先前声明的massVal不同。
切换到：

massVal = (String) mass.getItemAtPosition(position);

Answer 2

有一些问题。

在这里宣布一个班级成员massVal很好。

public String massVal;

然后重新声明并在匿名OnItemClickListener中分配 massVal。

String massVal = (String) mass.getItemAtPosition(position);

在提供此问题的匿名OnClickListener中，与其上方的匿名OnItemClickListener的范围完全不同，massVal永远不会被分配，因此仍为null。

通常更好的设计是这样的：

public class SomeActivity extends Activity {

    private Button myButton;
    private EditText myEditText;
    private String myString;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main.xml);

        myEditText = (EditText) findViewById(R.id.my_edit_text);
        myButton = (Button) findViewById(R.id.my_button);

        myButton.setOnClickListener(new View.OnClickListener() {
            String text = myEditText.getText()
            if (text != null) {
                myString = text.toString();
                // ... continue processing your string
            }
        });
    }
}

Android程序崩溃但Android Studio中没有明显错误

2 个答案: