如何从Java中的另一个类获取变量？

Question

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

MainActivity.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在MainActivity类ListFood中使用的类MainActivity id_categories中获取变量：

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

但都没有成功。

Answer 1

1. 您将其添加到意图。

    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); 

   2. 无论何时需要，都可以在ListFood类中打开它。

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

Answer 2

在您的MainActivity类中定义变量id_categories示例：

public class MainActivity {

 private int id_categories;
  ...

更改此行代码

 int id_categories = item.getId();

在

 id_categories = item.getId();

并在类MainActivity中创建一个函数

public int getIdcategories(){
    return id_categories;
}

Answer 3

把它放在主要活动的意图中，就像这样

itent.putExtra("id", item.getId());

并在列表食物中检索它像这样

int id=getIntent().getIntExtra("id",0); // 0 is default value if id is not found in intent

Answer 4

声明id_categories作为public static int id_categories在MainActivity的onCreate之前。 然后，您可以在任何类中访问它作为MainActivity.id_categories 。