引用代码中的字符串变量时出现java.lang.NullPointerException [英] java.lang.NullPointerException while referencing string variable in code

问题描述

我正在尝试引用存储在xml中的一些字符串.我收到此错误:

I'm trying to reference a few strings that I've stored in an xml. I'm getting this error:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 
'android.content.res.Resourcesandroid.content.Context.getResources()' 
on a null object reference at
android.content.ContextWrapper.getResources(ContextWrapper.java:86)

在我使用getString从xml引用变量的行中的

.

on the line where I'm using getString to reference the variable from xml.

看到错误的行:

private String CLIENT_ID = getString(R.string.MY_CLIENT_ID);

完全错误:

com.android.example.helloworld E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.android.example.helloworld, PID: 8408
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.android.example.helloworld/com.android.example.helloworld.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
    at android.app.ActivityThread.access$800(ActivityThread.java:151)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
    at android.content.ContextWrapper.getResources(ContextWrapper.java:86)
    at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:74)
    at android.content.Context.getString(Context.java:377)
    at com.android.example.helloworld.MainActivity.<init>(MainActivity.java:55)
    at java.lang.reflect.Constructor.newInstance(Native Method)
    at java.lang.Class.newInstance(Class.java:1606)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
    at android.app.ActivityThread.access$800(ActivityThread.java:151)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

---

我的/res/values/variables.xml文件:

---

My /res/values/variables.xml file:

<string name="MY_CLIENT_ID">foo1</string>
<string name="MY_CLIENT_SECRET">bar1</string>

我的AndroidManifest.xml文件

My AndroidManifest.xml file

<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="Hello!"
    android:theme="@style/AppTheme" >
    <meta-data
        android:name="CLIENT_ID"
        android:value="@string/MY_CLIENT_ID" />
    <meta-data
        android:name="CLIENT_SECRET"
        android:value="@string/MY_CLIENT_SECRET" />

    <activity
        android:name="com.android.example.helloworld.MainActivity"
        android:label="Hello!" >
        <intent-filter android:label="@string/app_name" >
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

我对MainActivity.java文件的部分代码:

My partial code for MainActivity.java file:

public class MainActivity extends ActionBarActivity implements myListener  {

/* One of the very first lines that are executed throw the nullpointerexception */

    private String CLIENT_ID = getString(R.string.MY_CLIENT_ID);
    private String CLIENT_SECRET = getString(R.string.MY_CLIENT_SECRET);

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

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

        obj = new foo(CLIENT_ID, CLIENT_SECRET, this);
        obj.bar()
    }
}

---

这是我到目前为止所做的.

---

Here's what I've done so far.

1)我正在使用 http://developer.android. com/guide/topics/resources/string-resource.html 作为参考.

1) I'm using http://developer.android.com/guide/topics/resources/string-resource.html as a reference.

2)如果我在开始时对值进行硬编码并运行该应用程序，则它可以正常工作.当我尝试从xml引用值时，只会收到上述错误.

2) If I hardcode the values in the beginning and run the app it works. I only get the said error when I try to reference the values from xml.

3)我在google和stackexchange上对此主题进行了大量搜索，并尝试了stackexchange链接上建议的几种方法，例如:

3) I have searched quite a bit on this topic on google and stackexchange and have tried several approaches suggested on the stackexchange links such as:

• 创建并调用如下方法.我在此方法中遇到了同样的错误.

• creating and calling a method like below. I get that same error inside this method.

public String getStringResourceByName(String resourceName) {
    String packageName = "com.android.example.helloworld";
    Log.i("-- package name --",packageName);
    int resId = this.getResources().getIdentifier(resourceName, "string", packageName);
    String resName = getString(resId);
    Log.i("-- resource name -- ",resName);
    return resName;
}

使用MainActivity.this专门在上述方法中

Using MainActivity.this specifically in the above method

public String getStringResourceByName(String resourceName) {
    String packageName = "com.android.example.helloworld";
    Log.i("-- package name --",packageName);
    int resId = MainActivity.this.getResources().getIdentifier(resourceName, "string", packageName);
    String resName = getString(resId);
    Log.i("-- resource name -- ",resName);
    return resName;
 }

所有更改都向我抛出相同的错误.

All of the changes throw the same error back at me.

有人可以指导我我在这里想念什么吗?希望我已经提供了足够的数据.

Can someone guide me on what am I missing here? I hope I've provided with sufficient data.

推荐答案

private String CLIENT_ID = getString(R.string.MY_CLIENT_ID);

如果在类初始化时初始化字段，则在onCreate()之前调用getString()，因此上下文尚未初始化.最好像这样使用它

If you initialize the field on class initialization, you are calling getString() before onCreate(), and thus the Context has not initialized. Better use it like this

public void onCreate(final Bundle bundle) {
    // Other code    
    CLIENT_ID = getString(R.string.MY_CLIENT_ID);
}

我也强烈建议您不要缓存字符串，因为如果语言发生更改，您可能会得到错误的翻译(这个问题已经发生在我身上，因此我从经验中就可以这么说).而是在需要时使用getString().

Also I strongly advise against caching the strings, because you can get wrong translations if the language has changed (this problem already happened to me so I say this from experience). Instead, use getString() whenever you need it.

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