如何使用其上下文从另一个类调用函数（android，java）

Question

每当用户从viewPager中按下Item内的按钮时，我都会尝试重新加载viewPager。本案的相关类别是SetupActivity和Adapter。在activity类中，我已经使两个函数重新加载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);
    }

所以基本上，deleteSession()从数据库中删除一个特定的项目。 refreshAdapter()从数据库中读取更新的数据，并为viewPager设置一个新的适配器。如果我从SetupActivity类中的OnClick事件调用它们，这些函数工作正常。但是，如果我尝试从Adapter类中的OnClick事件调用它们，则应用程序崩溃时会出现以下错误：

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)

这是我的SetupActiviy课程（缩减到相关部分）：

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);
    }
}

这是我的Adapter课程（也简化为相关部分）：

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;
    }
}

我认为它与上下文有关。我是否必须将上下文从SetupActiity传递到Adapter类，如果是这样，我该怎么做？或者这是不可能的，我需要在Adapter类中捕获SetupActivity类中的OnClick事件？我是java编程的新手，如果你们中的任何人能帮助我解决这个问题，我将非常感激！

Answer 1

像这样改变你的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;
    }
}

为什么错误？

你将Context从SetupActivity传递到你的Adapter，你在场上保存那个引用（context），但是你错误地创建了一个新的SetupActivity实例。

这就是你得到NullPointerException的原因。活动是你的Context。它继承了它。所以只需将它转换为构造函数并稍后使用它。