我有一个UI屏幕（CardViewActivity），带有一堆EditText行，供用户输入数据。用户完成后，单击UI上的“保存”按钮，将字符串数据输入保存到CardView，然后将其添加到RecyclerView列表中。当我尝试在Recycler活动（ListContactsActivity）中添加“保存”按钮引用（R.id.saveButtonRV）时，应用程序因按钮单击而崩溃。
ListContactsActivity
...
public class ListContactsActivity extends AppCompatActivity {
private ListContactsAdapter mContactsAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final RecyclerView mRecyclerView;
...
Button saveButton = (Button) findViewById(R.id.saveButtonRV);
// the below line caused the app to crash with NPE
saveButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Contact contact = new Contact("", "");
mContactsAdapter.addItem(contact);
mRecyclerView.scrollToPosition(0);
}
});
CardViewActivity (for user input)
...
public class CardViewActivity extends AppCompatActivity {
private ListenerEditText cListenerEditText;
...
public void onClickSave(View v) {
int stringToDo = cListenerEditText.getText().toString().replace(" ", "").length();
else if (stringToDo > 0 && stringNotes1 == 0 && stringNotes2 == 0 &&
stringDueDate == 0 && stringDueTime ==0) {
cListenerEditText.requestFocus();
InputMethodManager imm = (InputMethodManager)
getSystemService(Context.INPUT_METHOD_SERVICE);
imm.hideSoftInputFromWindow(cListenerEditText.getWindowToken(), 0);
cListenerEditText.clearFocus();
Button saveButton = (Button)findViewById(R.id.saveButtonRV);
saveButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Can I reference Contact, the adapter and the RecyclerView
// here to addItem (the CardView with string data) to the
// ReyclerView list?
}
});
}
}
xml referencing the saveButtonRV
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/cardviewTwobuttons"
android:orientation="horizontal" >
<Space
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="14" />
<LinearLayout
android:id="@+id/LinearLayout4"
android:layout_marginTop="1dp"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="52"
style="?android:attr/borderlessButtonStyle"
android:background="@drawable/rect_forbuttons">
<Button
android:id="@+id/clearButton"
android:text="Clear"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="start|center"
android:textColor="#FFFFFF"
android:textStyle="bold"
style="?android:attr/borderlessButtonStyle"
android:textAppearance="?android:attr/textAppearanceLarge"
android:background="@drawable/clearbutton_rounded"
android:drawableStart="@drawable/ic_clear_white_24dp"
android:drawableLeft="@drawable/ic_clear_white_24dp"
android:paddingStart="2dp"
android:paddingLeft="2dp"
android:paddingEnd="2dp"
android:paddingRight="2dp"
android:layout_marginEnd="12dp"
android:layout_marginRight="12dp"
android:layout_marginTop="4dp"
android:layout_marginBottom="4dp"
android:onClick="onClickClear"
android:nextFocusRight="@+id/saveButtonRV" />
<Button
android:id="@+id/saveButtonRV"
android:text="Save"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end|center"
android:textColor="#FFFFFF"
android:textStyle="bold"
style="?android:attr/borderlessButtonStyle"
android:textAppearance="?android:attr/textAppearanceLarge"
android:background="@drawable/savebutton_rounded"
android:drawableStart="@drawable/ic_save_white_24dp"
android:drawableLeft="@drawable/ic_save_white_24dp"
android:layout_marginStart="12dp"
android:layout_marginLeft="12dp"
android:layout_marginTop="4dp"
android:layout_marginBottom="4dp"
android:onClick="onClickSave" />
</LinearLayout>
<Space
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="14" />
来自 ListContactsActivity 的logcat：
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.jdw.seventhscreen/com.example.jdw.seventhscreen.ListContactsActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
at android.app.ActivityThread.access$600(ActivityThread.java:130)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.jdw.seventhscreen.ListContactsActivity.onCreate(ListContactsActivity.java:58)
at android.app.Activity.performCreate(Activity.java:5008)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
at android.app.ActivityThread.access$600(ActivityThread.java:130)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
02-02 23:47:10.529 15871-15871/com.example.jdw.seventhscreen I/Process: Sending signal.
答案 0 :(得分：1)
检查ListContactsActivity.java的第58行。这是您错误的来源。
您似乎使用ID R.id.saveButtonRV创建了一个按钮。你说你发布的xml是由cardviewactivity使用的。这意味着，您无法在其他活动中使用R.id.saveButtonRV。这就是您null saveButton ListContactsActivity值null的原因。因此，将onclick侦听器设置为null pointer exception值将导致startActivityForResult()
如果要将数据从一个活动传递到另一个活动，则应使用startActivityForResult()。查看this thread和this documentation/tutorial。
基本上，您使用startActivity()代替Intent returnIntent = new Intent();
returnIntent.putExtra(someTag, userInputString);
setResult(Activity.RESULT_OK, returnIntent);
。然后在你的CardViewActivity中你需要创建另一个intent然后把字符串放在那个intent对象中，这样做：
ListContactsActivity然后在onActivityResult中，覆盖List<String>以获取意图中的数据。
由于您没有发布有关您的recyclerview的足够数据，我将假设您对recyclerview的行有一个单独的布局，并且您的recyclerview适配器中有List<String>。
获取用户输入并将其传递回包含recyclerview的活动后，您只需将数据添加到recyclerview适配器，很可能是{{1}}。然后检查this有关如何将数据添加到recyclerview的信息。