不好意思。我只是不了解如何实现它以与我的代码一起使用。我正在做的是编辑列表视图中的名称。在“ EditDeleteList”中编辑名称并返回上一个活动（ListView）以查看在列表视图中更新的名称时，没有任何反应。我必须完全退出活动才能看到变化。如何获取此更新？我实现了onActivityReult（）方法，但随后收到此错误消息“ java.lang.NullPointerException：尝试在空对象引用上调用接口方法'int java.util.List.size（）'”，因此我将其完全删除。我如何才能在不收到该错误消息的情况下更新列表视图？
ListView.Java
public class ListView extends AppCompatActivity {
private static final String TAG = "ListView";
DatabaseHelper mDatabaseHelper;
Button btnAdd;
private EditText editText;
private android.widget.ListView listView;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list_view);
mDatabaseHelper = new DatabaseHelper(this);
btnAdd = (Button) findViewById(R.id.btnAdd);
editText = (EditText) findViewById(R.id.editText);
listView = (android.widget.ListView) findViewById(R.id.lv);
ArrayList<String> list = getIntent().getStringArrayListExtra("myList");
android.widget.ListView lv = (android.widget.ListView) findViewById(R.id.lv);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, list);
lv.setAdapter(adapter);
//Takes user back to the main activity
ImageView ivBack = (ImageView) findViewById(R.id.ivBackArrow);
ivBack.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d(TAG, "onClick: pressed back arrow");
Intent intent = new Intent(ListView.this, MainActivity.class);
startActivity(intent);
}
});
btnAdd.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String newEntry = editText.getText().toString();
if (editText.length() != 0) {
addData(newEntry);
editText.setText("");
} else {
toastMessage("you must put something in the text field");
}
}
});
populateListView();
}
public void addData(String newEntry) {
boolean insertData = mDatabaseHelper.addData(newEntry);
if (insertData) {
toastMessage("Successfully inserted");
recreate();
} else {
toastMessage("Whoops, something went wrong");
}
}
private void toastMessage(String message) {
Toast.makeText(this, message, Toast.LENGTH_SHORT).show();
}
private void populateListView() {
Log.d(TAG, "populateListView: displaying data in the listview");
//get data and append to list
Cursor data = mDatabaseHelper.getData();
ArrayList<String> listData = new ArrayList<>();
while(data.moveToNext()) {
//get the value from the database in column 1
//set it to the arraylist
listData.add(data.getString(1));
}
//create arraylist and set it to the adapter
ListAdapter adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listData);
listView.setAdapter(adapter);
//set onclick listen to edit activity
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
String name = adapterView.getItemAtPosition(position).toString();
Log.d(TAG, "onItemClick: you clicked on " + name);
Cursor data = mDatabaseHelper.getItemID(name); //get the id associated with that name
int itemID = -1;
while (data.moveToNext()) {
itemID = data.getInt(0);
}
if (itemID > -1) {
Log.d(TAG, "onItemID: the ID is: " + itemID);
Intent editScreenIntent = new Intent(ListView.this, EditDeleteList.class);
editScreenIntent.putExtra("id",itemID);
editScreenIntent.putExtra("name",name);
startActivity(editScreenIntent);
} else {
toastMessage("No ID found");
}
}
});
}
}
EditDeleteList.java
public class EditDeleteList extends AppCompatActivity {
private static final String TAG = "EditDeleteList";
DatabaseHelper mDatabaseHelper;
private ImageView ivDelete;
private ImageView ivApprove;
private EditText editHashtag;
private String selectedName;
private int selectedID;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_edit_delete);
mDatabaseHelper = new DatabaseHelper(this);
editHashtag = (EditText) findViewById(R.id.editHashtag);
ivDelete = (ImageView) findViewById(R.id.ivDelete);
ivApprove = (ImageView) findViewById(R.id.ivApprove);
//get the intent extra from the ListView activity
final Intent receivedIntent = getIntent();
//get item ID passed as an extra
selectedID = receivedIntent.getIntExtra("id", -1);
//get name passed as an extra
selectedName = receivedIntent.getStringExtra("name");
//set text field to selected item text
editHashtag.setText(selectedName);
ivApprove.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String item = editHashtag.getText().toString();
if (!item.equals(null)) {
mDatabaseHelper.updateName(item, selectedID, selectedName);
} else {
toastMessage("you must enter a #hashtag");
}
finish();
}
});
}
private void toastMessage(String message) {
Toast.makeText(this, message, Toast.LENGTH_SHORT).show();
}
}
在EditDeleteList.java中，我有一个onClickListener，可以保存更改并通过使用finish（）返回上一个活动；
ivApprove.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String item = editHashtag.getText().toString();
if (!item.equals(null)) {
mDatabaseHelper.updateName(item, selectedID, selectedName);
} else {
toastMessage("you must enter a #hashtag");
}
finish();
}
});
答案 0 :(得分：2)
在活动生命周期的某些阶段通知适配器。
OnCreate()不应在您返回时运行（这是您必须完全重新创建活动才能看到列表更新的原因），因此您应使用OnRestart/OnStart/OnResume通知适配器检查是否有新的项目。
选中此image
