android - ListView Android 中的 NullPointerException

Question

我有一个 Activity ，其中有一个按钮，点击按钮时应用程序崩溃问题在线 adapter.remove(list.get(i));

Logcat 详细信息在给定行 adapter.remove(list.get(i));

中提及 NullPointerException

   package com.example.veeresh.myphotogallery;

   import android.app.ListActivity;
   import android.os.Bundle;
   import android.util.SparseBooleanArray;
   import android.view.View;
   import android.view.View.OnClickListener;
   import android.widget.ArrayAdapter;
   import android.widget.Button;
   import android.widget.EditText;

   import java.util.ArrayList;

   public class MainActivity extends ListActivity {

/** Items entered by the user is stored in this ArrayList variable */
ArrayList list = new ArrayList();

/** Declaring an ArrayAdapter to set items to ListView */
ArrayAdapter adapter;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    /** Setting a custom layout for the list activity */
    setContentView(R.layout.activity_main);

    /** Reference to the add button of the layout main.xml */
    Button btn = (Button) findViewById(R.id.btnAdd);

    /** Reference to the delete button of the layout main.xml */
    Button btnDel = (Button) findViewById(R.id.btnDel);

    /** Defining the ArrayAdapter to set items to ListView */
    adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_multiple_choice, list);
    adapter.add("Item 1");

    /** Defining a click event listener for the button "Add" */
    OnClickListener listener = new OnClickListener() {
        @Override
        public void onClick(View v) {
            EditText edit = (EditText) findViewById(R.id.txtItem);
            list.add(edit.getText().toString());
            edit.setText("");
            adapter.notifyDataSetChanged();
        }
    };

    /** Defining a click event listener for the button "Delete" */
    OnClickListener listenerDel = new OnClickListener() {
        @Override
        public void onClick(View v) {
            /** Getting the checked items from the listview */
            SparseBooleanArray checkedItemPositions = getListView().getCheckedItemPositions();
            int itemCount = getListView().getCount();

            for(int i=itemCount-1; i >= 0; i--)
            {
                if(checkedItemPositions.get(i)){
                    adapter.remove(list.get(i));
                }
            }
            checkedItemPositions.clear();
            adapter.notifyDataSetChanged();
        }
    };

    /** Setting the event listener for the add button */
    btn.setOnClickListener(listener);

    /** Setting the event listener for the delete button */
    btnDel.setOnClickListener(listenerDel);

    /** Setting the adapter to the ListView */
    setListAdapter(adapter);
}
}

Answer 1

您的适配器包含一个列表，而您的 MainActivity 包含另一个列表。

您正在将项目添加到您的适配器，但是这不会将项目添加到您的 Mainactivity 中的列表。适配器中的列表包含“项目 1”，但 MainActivity 中的列表不包含。

OnClick 你正试图从你的 MainActivity 的列表中检索一个项目，它不包含你试图检索的项目，因此 NullpointerException

您应该从列表中添加或删除项目，然后调用 adapter.notifyDataSetChanged();

尝试:

//add an item
list.add("item 1");
adapter.notifyDataSetChanged();



//remove an item
list.remove(i);
adapter.notifyDataSetChanged();