java - 如何从 Java 中的另一个类获取变量？

Question

我想获取类 MainActivity 类型中的 id_categories 变量，以便在类 ListFood 的方法 LoadListFoodbyId (int id) 中使用。

主 Activity .java

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lv = (ListView) findViewById(R.id.list_store_View);
        LoadCategories();
        lv.setOnItemClickListener(new CategoriesClickListener());
}
    // The click listener for ListView in Store List
    private class CategoriesClickListener implements
            ListView.OnItemClickListener {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position,
                long id) {

            Categories item = cateAdapter.getItem(position);
            Intent itent = new Intent(getApplicationContext(), ListFood.class);
            itent.putExtra("item", item);
            int id_categories = item.getId(); //i want to get this variable
            String log = "ID: " + id_categories;
            startActivity(itent);
            Log.d("ID: ", log);

        }
    }

ListFood.java

public class ListFood extends Activity {
@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.list_dish);
        LoadListFoodbyId(4); //use variable in here
}
    public void LoadListFoodbyId(int i) {
        DataApdapter mDbHelper = new DataApdapter(this);
        mDbHelper.createDatabase();
        mDbHelper.open();
        ArrayList<Food> listFoodById = mDbHelper.getAllFoodById(i);
        listFoodAdapter = new ListFoodArrayAdapter(this, R.layout.list_dish,
                listFoodById);
        lv.setAdapter(listFoodAdapter);

        mDbHelper.close();

    }

我尝试通过 ListFood 在类 mainActivity 中调用方法 LoadListFoodbyId (int i):

ListFood lf = new ListFood(); 

lf.LoadListFoodbyId (id_categories)

并尝试另一种方法是获取 MainActivity 类 ListFood 中使用的类 MainActivity id_categories 中的变量:

MainActivity main = new MainActivity(); 
LoadListFoodbyId (main.getId ()), 

但都没有成功。

Answer 1

1. 您将其添加到 Intent 中。

   Categories item = cateAdapter.getItem(position);
   Intent itent = new Intent(getApplicationContext(), ListFood.class);
   itent.putExtra("item", item);
   int id_categories = item.getId(); //i want to get this variable
   String log = "ID: " + id_categories;
   itent.putExtra("id_categories", id_categories) //you can call your variable whatever you want, I have called it "id_categories" for convenience.
   startActivity(itent);
   Log.d("ID: ", log);
   

   1. 随时在 ListFood 类中打开它。

   Bundle b = getIntent().getExtras();
int id_categories = bundle.getInt("id_categories");