在 android 应用程序中出现错误：尝试在空对象引用上调用接口方法

Question

我在单选组中有三个单选按钮，当用户选中任何单选按钮时，它必须保持选中状态，直到用户选中任何其他按钮（无论用户是否关闭应用程序），为此我使用了 sharedPreferences 来存储此选定的复选框值并在加载 activity 时检索，但是当我加载 activity 以选择复选框时出现错误。下面是我的代码。

Activity.java:

public static SharedPreferences pref;
public static String lang;
public static String chk;
public static RadioButton gujarati;
public static RadioButton hindi;
public static RadioButton english;
public static EditText txtV;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.select_language);
//  
//  String chk = pref.getString("key_name", null);
//  if(chk=="Gujarati"){
//      gujarati.setChecked(true);
//  }
//  else if(chk=="Hindi"){
//      hindi.setChecked(true);
//  }
//  else{
    //  english.setChecked(true);
    //}
    //editor.remove("key_name3"); // will delete key key_name3
    //editor.remove("key_name4"); // will delete key key_name4

    // Save the changes in SharedPreferences
    //editor.commit(); // commit changes
    //editor.clear();
     //editor.commit(); 

    RadioGroup group = (RadioGroup) findViewById(R.id.radioGroup1);
    gujarati = (RadioButton) findViewById(R.id.radio0);
    hindi = (RadioButton) findViewById(R.id.radio1);
    english = (RadioButton) findViewById(R.id.radio2);
    txtV = (EditText) findViewById(R.id.editText1);

    //int radio_selected = getSharedPreferences("your_prefs", Activity.MODE_PRIVATE).getInt("my_selected_radio",0);
    //int radio_selected = getSharedPreferences("your_prefs", Activity.MODE_PRIVATE).edit().putInt("my_selected_radio",selectedValue).commit();
    chk = pref.getString("key_name", null);

    if(chk == "Gujarati"){
        gujarati.setChecked(true);
    }
    if(chk == "Hindi"){
        hindi.setChecked(true);
    }
    if(chk == "English"){
        english.setChecked(true);
    }

    group.setOnCheckedChangeListener(new OnCheckedChangeListener(){

        @Override
        public void onCheckedChanged(RadioGroup group, int checkedId) {
            // TODO Auto-generated method stub
            if(checkedId == R.id.radio0) {
                lang = "Gujarati";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0); 
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                chk = pref.getString("key_name", null);
                txtV.setText(chk);

            } else if(checkedId == R.id.radio1) {
                lang = "Hindi";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0); 
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                chk = pref.getString("key_name", null);
                txtV.setText(chk);
            } else {
                lang = "English";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0); 
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                chk = pref.getString("key_name", null);
                txtV.setText(chk);

            }

        }

    });

activity.xml:

<RadioGroup
    android:id="@+id/radioGroup1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="25dp" >

    <RadioButton
        android:id="@+id/radio0"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="RadioButton" />

    <RadioButton
        android:id="@+id/radio1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="RadioButton" />

    <RadioButton
        android:id="@+id/radio2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="RadioButton" />
</RadioGroup>


<EditText
    android:id="@+id/editText1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/radioGroup1"
    android:layout_below="@+id/radioGroup1"
    android:layout_marginLeft="15dp"
    android:ems="10" >

    <requestFocus />
</EditText>

日志猫：

01-05 04:21:35.055: E/AndroidRuntime(2665): FATAL EXCEPTION: main
01-05 04:21:35.055: E/AndroidRuntime(2665): Process: com.finaldbparse.finaldbparse, PID: 2665
01-05 04:21:35.055: E/AndroidRuntime(2665): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.finaldbparse.finaldbparse/com.finaldbparse.finaldbparse.SelectLanguage}: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String android.content.SharedPreferences.getString(java.lang.String, java.lang.String)' on a null object reference
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread.access0(ActivityThread.java:144)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.os.Looper.loop(Looper.java:135)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread.main(ActivityThread.java:5221)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at java.lang.reflect.Method.invoke(Native Method)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at java.lang.reflect.Method.invoke(Method.java:372)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-05 04:21:35.055: E/AndroidRuntime(2665): Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String android.content.SharedPreferences.getString(java.lang.String, java.lang.String)' on a null object reference
01-05 04:21:35.055: E/AndroidRuntime(2665):     at com.finaldbparse.finaldbparse.SelectLanguage.onCreate(SelectLanguage.java:55)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.Activity.performCreate(Activity.java:5933)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
01-05 04:21:35.055: E/AndroidRuntime(2665):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
01-05 04:21:35.055: E/AndroidRuntime(2665):     ... 10 more

Answer 1

你得到 NPE 因为 pref=NULL 在

chk = pref.getString("key_name", null);

在使用前初始化pref。喜欢

pref = getApplicationContext().getSharedPreferences("MyPref", 0); in onCreate(....)

Answer 2

这里：

chk = pref.getString("key_name", null);

1. 您不是在初始化 SharedPreferences 的 pref 实例，而是在调用 getString 方法。这样做：

pref = getApplicationContext().getSharedPreferences("MyPref", 0); 
chk = pref.getString("key_name", null);

2. 使用 String.equals 来比较字符串而不是 ==

3. 在 onCreate 方法中初始化 pref 然后不需要像目前在 setOnCheckedChangeListener[=19 中那样再次初始化=]

Answer 3

因为您已经实施了一个 onCheckedChangeListener 事件，每当您的 activity 启动时，它总是会自动调用。所以出现的问题是，虽然你的 sharedpreference 值是正确的，但在那之后你的 CheckedChangeListener 立即被调用，所以你从你的偏好中获得的值被重置并且你得到不正确的结果。

这就是为什么我在您要检查另一个单选按钮时删除旧值的原因。

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

    group.setOnCheckedChangeListener(new OnCheckedChangeListener() {

        @Override
        public void onCheckedChanged(RadioGroup group, int checkedId) {
            // TODO Auto-generated method stub
            pref = getApplicationContext().getSharedPreferences("MyPref", 0);
            Editor editor1 = pref.edit();
            editor1.remove("key_name");
            if (checkedId == R.id.radio0) {
                lang = "Gujarati";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0);
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                // chk = pref.getString("key_name", null);
                txtV.setText(lang);

            } else if (checkedId == R.id.radio1) {
                lang = "Hindi";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0);
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                // chk = pref.getString("key_name", null);
                txtV.setText(lang);
            } else if (checkedId == R.id.radio2) {
                lang = "English";
                pref = getApplicationContext().getSharedPreferences("MyPref", 0);
                Editor editor = pref.edit();
                editor.putString("key_name", lang);
                editor.commit();
                // chk = pref.getString("key_name", null);
                txtV.setText(lang);

            }
        }

    });

    pref = getApplicationContext().getSharedPreferences("MyPref", 0);
    chk = pref.getString("key_name", null);
    System.err.println("Get value is=====" + chk);
    if (chk != null) {
        if (chk.equals("Gujarati")) {
            gujarati.setChecked(true);
        }
        if (chk.equals("Hindi")) {
            hindi.setChecked(true);
        }
        if (chk.equals("English")) {
            english.setChecked(true);
        }
    }
}

在 android 应用程序中出现错误：尝试在空对象引用上调用接口方法

getting error in android application: Attempt to invoke interface method on a null object reference

android

nullpointerexception

sharedpreferences

android-radiogroup

android-radiobutton