在Android中按下提交按钮时使用自定义适配器从列表视图中获取选定的项目

Question

I have created my own adapter, InteractiveArrayAdapter, and set it to as adapter for the listview (MyListView) I have in my xml layout file, let's say MyActivity. Then, from MyActivity activity, I perform this:

MyActivity.java :

        ListView listView = (ListView) findViewById(R.id.MyListView);

        ArrayAdapter<Model> adapter = new InteractiveArrayAdapter(this, model);
        listView.setAdapter(adapter);

My custom adapter uses a custom model, Model, that I have created to work with each element in the listview. Each listview item, consists of a TextView and a CheckBox.

My goal is to obtain the complete list of checked items when user press a submit button placed in the MyActivity xml layout file so I perform the following steps to get rid of this:

1. In my custom adapter, InteractiveArrayAdapter, I declare a private Set to store the identifiers of each element selected in the listview. I use my own identifier insteaf of using the position in the list. This identifier comes from the primary key in the database from where in a previous step I have extracted all the desired items to populate the listview. As identifiers for items cannot be repeated, I use a Set. This Set is updated each time an item in the listview is checked or unchecked. Also I implement a public method to return this set.

    public class InteractiveArrayAdapter extends ArrayAdapter<Model> { ... private Set<Integer> checked = new HashSet<Integer>(); ... @Override public View getView(int position, View convertView, ViewGroup parent) { View view = null; viewHolder.checkbox = (CheckBox) view.findViewById(R.id.interactiveRowCheck); viewHolder.checkbox .setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() { @Override public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) { Model element = (Model) viewHolder.checkbox .getTag(); element.setSelected(buttonView.isChecked()); Integer id = element.getId(); if (isChecked) { checked.add(id); } else if (checked.contains(id)) { checked.remove(id); } } }); } public Set<Integer> getCheckedItems() { return this.checked; } } 

2. in MyActivity.java I handle click on the button and I try to read the list of items checked by doing this:

    public void OnClickCompareButton(View view) { InteractiveArrayAdapter adapter; Set<Integer> checkedItems = adapter.getCheckedItems(); Iterator<Integer> it = checkedItems.iterator(); int size = checkedItems.size(); for (int i = 0; i < size; i++) { // Do whatever... } } 

But Android Studio is throwing an error on compiling:

Error:(82, 37) error: variable adapter might not have been initialized

If then I use the variable adapter initialized at the beginning of this post when setting the adapter, that is:

    public void OnClickCompareButton(View view) {
        Set<Integer> checkedItems = this.adapter.getCheckedItems();
        Iterator<Integer> it = checkedItems.iterator();

        int size = checkedItems.size();
        for (int i = 0; i < size; i++) {
            // Do whatever...
        }
    }

below error is shown on compiling:

Error:(82, 49) error: cannot find symbol method getCheckedItems()

So how to solve this?

Answer 1

Error:(82, 37) error: variable adapter might not have been initialized

This is indeed the case - adapter had been declared just above and not initialised, so would be null anyway if the compiler hadn't got there first.

Your second method seems, at a quick glance, much more likely to work, so:

Error:(82, 49) error: cannot find symbol method getCheckedItems()

This is because the declared type of the Adapter is ListAdapter (see getAdapter() , arguably the preferred way to access fields), not your subclass InteractiveArrayAdapter .

You could cast it:

Set<Integer> checkedItems = ((InteractiveArrayAdapter) getListAdapter()).getCheckedItems();

and to avoid confusion with the ArrayAdapter you could simplify the instantiation / passing to your view:

ListView listView = (ListView) findViewById(R.id.MyListView);
listView.setAdapter(new InteractiveArrayAdapter(this, model));

Also, a couple of minor code style points:

• Probably best to make all your method names start with lower case, as per the official Java standards .
• You can generally remove this. in a method (eg return this.checked; -> return checked; )