Android - 在 .java 文件中引用 strings.xml 中的字符串

Question

i have this code in MainActivity.java:

String passName = new String(getString(R.string.name)); 

where name is a string in strings.xml:

<string name="name">My name</string>

it does not give me an error, but the app keeps crashing, how can i do this properly?

basically i want to save my name as a string variable in MainActivity.java, but store the actual text in strings.xml

i originally had:

String passName = new String("my name");

and i was able to successfully pass it to the second activity, but i want the text to be stored in strings.xml, not the.java file

edit: i have provided more of my code for context:

public class MainActivity extends AppCompatActivity {

    Button button;
    //code that will eventually point to my name in strings.xml
    String passName = new String("my name");

    //String passName = getResources().getString(R.string.name);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //code that senses when the button is clicked
        button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
            //code that executes when the button is clicked

                Toast.makeText(getApplicationContext(),"Moving to second activity...",Toast.LENGTH_SHORT).show();
                
                //code that passes my name to the second activity
                Intent intent = new Intent(MainActivity.this, SecondActivity.class);
                intent.putExtra("key", passName);
                startActivity(intent);
            }

        });

    }

}

as for a crash log, there doesn't seem to be one. when i launch the app it opens and then immediately closes again.

the message it gives me is this:

01/19 15:21:11: Launching 'app' on Pixel 4 XL API 30 (test).
Install successfully finished in 388 ms.
$ adb shell am start -n "my.name.n0000.lab_n0000/my.name.n0000.lab_n0000.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER

edit: i have added the proper logcat dialogue:

2023-01-19 15:32:41.587 7562-7562/my.name.n00000.lab_n00000 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: my.name.n00000.lab_n00000, PID: 7562
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{my.name.n00000.lab_n00000/my.name.n00000.lab_n00000.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:3365)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     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:97)
        at android.view.ContextThemeWrapper.getResourcesInternal(ContextThemeWrapper.java:134)
        at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:128)
        at androidx.appcompat.app.AppCompatActivity.getResources(AppCompatActivity.java:612)
        at my.name.n00000.lab_n00000.MainActivity.<init>(MainActivity.java:17)
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1253)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3353)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2023-01-19 15:32:41.615 7562-7562/my.name.n00000.lab_n00000 I/Process: Sending signal. PID: 7562 SIG: 9

Answer 1

There is a lifecycle involved when creating activities. Some things should not be accessed before the onCreate method is called. So you should be able to declare passName prior and then inside onCreate , you can initialise it.

public class MainActivity extends AppCompatActivity {

    Button button;
    String passName;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //code that senses when the button is clicked
        button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
            //code that executes when the button is clicked

                Toast.makeText(getApplicationContext(),"Moving to second activity...",Toast.LENGTH_SHORT).show();
                
                //code that passes my name to the second activity
                Intent intent = new Intent(MainActivity.this, SecondActivity.class);
                intent.putExtra("key", passName);
                startActivity(intent);
            }

        });
        passName = getResources().getString(R.string.name);

    }

}

You can read more about the activity lifecycle here

And in particular in this paragraph , it talks about instantiating class scope variables.

For example, your implementation of onCreate() might bind data to lists, associate the activity with a ViewModel, and instantiate some class-scope variables.