java.lang.NullPointerException while referencing string variable in code

问题

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)

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

Line where I see the error:

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

Full error:

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)

My /res/values/variables.xml file:

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

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>

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()
    }
}

Here's what I've done so far.

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

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) 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:

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;
}

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;
 }

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

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

回答1:

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

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);
}

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.

回答2:

You need a Context to access your app resources. You're trying to get a resource without having a valid context first. Move your calls to onCreate() method, where your Activity is first created and you have the context:

private String CLIENT_ID;
private String CLIENT_SECRET;

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

    temp = (TextView) findViewById(R.id.temp);
    CLIENT_ID = getString(R.string.MY_CLIENT_ID);
    CLIENT_SECRET = getString(R.string.MY_CLIENT_SECRET);

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

来源：https://stackoverflow.com/questions/31518428/java-lang-nullpointerexception-while-referencing-string-variable-in-code

标签

android

android-resources