获取空指针异常

Question

当我尝试访问在视图类中定义的文本视图时，出现NullPointerException异常。 我正在从设置类访问它。 我的代码的一小部分是：

查看类

public class view1 extends menu {
    public static TextView text1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.view);
        text1=(TextView)findViewById(R.id.textfile1);
        text1.setText("product");
    }

    public void small(String mytext) { // this is my method which I want to access
        text1.setText(mytext);
    }
}

设置类

public class Setting extends Activity {


    private Spinner spinner1;
      private Button apply;
      TextView small1;
      private view1 view11;




      @Override


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.setting);
        //setContentView(R.layout.view);

        addItemsOnSpinner1();
        addListenerOnSpinnerItemSelection();



}

    public void addItemsOnSpinner1() {

        spinner1 = (Spinner) findViewById(R.id.spinner1);
        List<String> list = new ArrayList<String>();
        list.add("Small");
        list.add("Medium");
        list.add("Large");
        ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, list);
        dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner1.setAdapter(dataAdapter);
      }

    public void addListenerOnSpinnerItemSelection() {
        spinner1 = (Spinner) findViewById(R.id.spinner1);
        apply = (Button) findViewById(R.id.apply);
        spinner1.setOnItemSelectedListener(new CustomOnItemSelectedListener());


        apply.setOnClickListener(new OnClickListener() {

              @Override
              public void onClick(View v) {

                  String mytext = "Something else";
                  view11.small(mytext);





                  }

堆栈跟踪

01-17 00:11:02.064: E/AndroidRuntime(4191): FATAL EXCEPTION: main
01-17 00:11:02.064: E/AndroidRuntime(4191): java.lang.NullPointerException
01-17 00:11:02.064: E/AndroidRuntime(4191):     at com.ramanrayat.notelet.Setting$1.onClick(Setting.java:106)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.view.View.performClick(View.java:4240)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.view.View$PerformClick.run(View.java:17721)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.os.Handler.handleCallback(Handler.java:730)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.os.Handler.dispatchMessage(Handler.java:92)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.os.Looper.loop(Looper.java:137)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at android.app.ActivityThread.main(ActivityThread.java:5103)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at java.lang.reflect.Method.invokeNative(Native Method)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at java.lang.reflect.Method.invoke(Method.java:525)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
01-17 00:11:02.064: E/AndroidRuntime(4191):     at dalvik.system.NativeStart.main(Native Method)

setting.xml代码

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/RelativeLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center_horizontal"
    android:orientation="horizontal" >

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="38dp"
        android:text="Setting"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:textSize="40dp" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView2"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="57dp"
        android:gravity="center"
        android:text="Font Size"
        android:textSize="30dp" />

    <Spinner
        android:id="@+id/spinner1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="30dp" />

    <Button
        android:id="@+id/apply"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_alignRight="@+id/textView2"
        android:layout_below="@+id/spinner1"
        android:layout_marginTop="58dp"
        android:text="Apply" />

</RelativeLayout>

Answer 1

Chnage

 if(String.valueOf(spinner1.getSelectedItem())=="Small")

至

if(String.valueOf(spinner1.getSelectedItem()).equals("Small"))

使用.equals或.equalsIgnoreCase比较字符串

与其将textview设为静态，不如使用意图在活动之间传递值。

Intent intent = new Intent(ActivityName.this,Settings.class);
intent.putExtra("key",text1.getText().toString());
startActivity(intent);

然后

String value = getIntent().getStringExtra("key");

更改

  public static TextView text1; 

至

   public TextView text1; 

还要遵循Java命名约定

Answer 2

更换

public static TextView text1;

同

TextView text1;

text1不能为静态。

下次，请单击错误日志中的文件名，并指出代码清单中的行号是错误实际指向的行。

学会用大写字母开头所有的班级名称（不过，这不是引起问题的原因）。 而且在我使用它时，请停止在类名称和变量中使用数字，尤其是数字1，在某些字体中数字可能会模糊地显示为“ l”。

Answer 3

查找您的LogCat，它将告诉您NPE在哪里。
另一个问题是：不要使用等于来比较String而不是==

==是比较两个对象的地址，而equals是比较它们的值。

 if(String.valueOf(spinner1.getSelectedItem()).equals("Small")) {
     String mytext = "Something else ";
     view11.small(mytext); // view11 is clas view1 reference
 }
 else if(String.valueOf(spinner1.getSelectedItem()).equals("Medium")) {
     finish();
 }