[英]How to call a function from another class with its context (android, java)
每當用戶在viewPager中的Item內按下按鈕時,我都試圖重新加載viewPager。 這種情況下的相關類是SetupActivity
和Adapter
。 在活動類中,我已經制作了兩個函數來重新加載viewPager:
public void deleteSession(String title) {
dbHelper = new DatabaseHelper(this);
((DatabaseHelper) dbHelper).deleteRow(title);
}
和
public void refreshAdapter () {
dbHelper = new DatabaseHelper(this);
Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();
models = new ArrayList<>();
while (dataCursor.moveToNext()) {
models.add(new Model(
dataCursor.getString(1),
dataCursor.getInt(2),
dataCursor.getInt(3),
dataCursor.getInt(4)
));
}
adapter = new Adapter(models, this);
viewPager.setAdapter(adapter);
viewPager.setCurrentItem(0, true);
}
因此,基本上, deleteSession()
從數據庫中刪除特定項目。 refreshAdapter()
從數據庫中讀取更新的數據,並將新的適配器設置為viewPager。 如果我從SetupActivity
類的OnClick事件中調用它們,這些函數將正常工作。 但是,如果我嘗試從Adapter
類的OnClick事件中調用它們,則應用程序將崩潰,並出現以下錯誤:
java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getDatabasePath(java.lang.String)' on a null object reference
at android.content.ContextWrapper.getDatabasePath(ContextWrapper.java:327)
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:352)
at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:322)
at com.example.android.slink.DatabaseHelper.getData(DatabaseHelper.java:69)
at com.example.android.slink.SetupActivity.refreshAdapter(SetupActivity.java:436)
at com.example.android.slink.Adapter$3.onClick(Adapter.java:111)
at android.view.View.performClick(View.java:6669)
at android.view.View.performClickInternal(View.java:6638)
at android.view.View.access$3100(View.java:789)
at android.view.View$PerformClick.run(View.java:26145)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6863)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:537)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
這是我的SetupActiviy
類(簡化為相關部分):
package com.example.android.slink;
public class SetupActivity extends AppCompatActivity {
private SQLiteOpenHelper dbHelper;
private SQLiteDatabase database;
ViewPager viewPager;
Adapter adapter;
static ArrayList<Model> models;
public void deleteSession(String title) {
dbHelper = new DatabaseHelper(this);
((DatabaseHelper) dbHelper).deleteRow(title);
}
public void refreshAdapter () {
dbHelper = new DatabaseHelper(this);
Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();
models = new ArrayList<>();
models.add(new Model("", 0,0,0));
while (dataCursor.moveToNext()) {
models.add(new Model(
dataCursor.getString(1),
dataCursor.getInt(2),
dataCursor.getInt(3),
dataCursor.getInt(4)
));
}
adapter = new Adapter(models, this);
viewPager.setAdapter(adapter);
viewPager.setCurrentItem(0, true);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_setup);
editTextScreenTitle = findViewById(R.id.editTextScreenTitle);
relativeLayoutCard = findViewById(R.id.relativeLayoutCard);
viewPager = findViewById(R.id.viewPager);
dbHelper = new DatabaseHelper(this);
Cursor dataCursor = ((DatabaseHelper) dbHelper).getData();
models = new ArrayList<>();
models.add(new Model("", 0,0,0));
while (dataCursor.moveToNext()) {
models.add(new Model(
dataCursor.getString(1),
dataCursor.getInt(2),
dataCursor.getInt(3),
dataCursor.getInt(4)
));
}
adapter = new Adapter(models, this);
viewPager.setAdapter(adapter);
}
}
這是我的Adapter
類(也簡化為相關部分):
package com.example.android.slink;
public class Adapter extends PagerAdapter {
private List<Model> models;
private LayoutInflater layoutInflater;
private Context context;
SetupActivity setupActivity = new SetupActivity();
public Adapter(List<Model> models, Context context) {
this.models = models;
this.context = context;
}
@NonNull
@Override
public Object instantiateItem(@NonNull final ViewGroup container, final int position) {
layoutInflater = LayoutInflater.from(context);
final View view = layoutInflater.inflate(R.layout.item, container, false);
TextView buttonDeleteCard = view.findViewById(R.id.textViewDeleteCard);
buttonDeleteCard.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
setupActivity.deleteSession([...some title...]);
setupActivity.refreshAdapter();
}
});
container.addView(view, 0);
return view;
}
}
我認為這與上下文有關。 我是否必須將上下文從SetupActiity
傳遞給Adapter
類,如果是,該怎么做? 還是不可能,我需要從SetupActivity
類的Adapter
類中捕獲OnClick事件? 我是Java編程的新手,如果您能幫助我解決此問題,將不勝感激!
像這樣更改您的Adapter
:
public class Adapter extends PagerAdapter {
private List<Model> models;
private LayoutInflater layoutInflater;
private Context context;
private SetupActivity setupActivity;
public Adapter(List<Model> models, Context context) {
this.models = models;
this.context = context;
setupActivity = (SetupActivity) context;
}
@NonNull
@Override
public Object instantiateItem(@NonNull final ViewGroup container, final int position) {
layoutInflater = LayoutInflater.from(context);
final View view = layoutInflater.inflate(R.layout.item, container, false);
TextView buttonDeleteCard = view.findViewById(R.id.textViewDeleteCard);
buttonDeleteCard.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
setupActivity.deleteSession([...some title...]);
setupActivity.refreshAdapter();
}
});
container.addView(view, 0);
return view;
}
}
為什么會出錯?
您將Context
從SetupActivity
到您的Adapter
,將該引用保存在字段( context
)中,但是您錯誤地創建了SetupActivity
的新實例。
這就是為什么您得到NullPointerException
的原因。 活動是您的Context
。 它繼承自它。 因此,只需將其轉換為構造函數,然后再使用即可。
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