Question

当我按下按钮打开另一个包含2个文本字段和保存并取消按钮的活动时，会弹出此错误消息。正在向保存功能中的这部分代码指出错误 -

listView.setAdapter(adapter);

完全错误

Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

保存功能的Java

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add);

    Button btnSave = (Button) findViewById(R.id.btnSave);
    ListView listView = (ListView) findViewById(R.id.listDisp);
    arrayList = new ArrayList<>();
    edName = (EditText) findViewById(R.id.editName);
    edMobile = (EditText) findViewById(R.id.editMobile);

    adapter = new ArrayAdapter<Classes>(this, R.layout.activity_add, R.id.txtItem, arrayList);
    Log.i("test", " " + arrayList.size());
    listView.setAdapter(adapter);

    btnSave.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Classes class1 = new Classes(edName.getText().toString(), edMobile.getText().toString());
            arrayList.add(class1);
            adapter.notifyDataSetChanged();


            Log.i("test", " " + arrayList.size());

            try {

                FileOutputStream fos = openFileOutput("Saves.tim", Context.MODE_PRIVATE);

                ObjectOutputStream oos = new ObjectOutputStream(fos);
                oos.writeObject(arrayList);
                oos.close();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    });

用于加载功能的Java

private ViewPager viewPager;
private SectionsPagerAdapter mSectionsPagerAdapter;
ListView lv;
public static ArrayList<Classes> arrayList;
private ArrayAdapter<Classes> adapter;
private EditText etName, etMobile;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    lv = (ListView) findViewById(R.id.listDisp);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    etName = (EditText) findViewById(R.id.editName);
    etMobile = (EditText) findViewById(R.id.editMobile);
    Button btSave = (Button) findViewById(R.id.btnSave);
    arrayList = new ArrayList<>();
    try {
        FileInputStream fis =  this.openFileInput("Saves.tim");
        ObjectInputStream ois = new ObjectInputStream(fis);
        arrayList = (ArrayList<Classes>) ois.readObject();
        ois.close();


    } catch (IOException ex) {
        ex.printStackTrace();
    } catch(ClassNotFoundException ex){
        ex.printStackTrace();
    }

启动活动的方法

    public void loadAddActivity (View v)
{
    startActivity(new Intent(this,AddActivity.class));
}

Answer 1

您的错误正在发生，因为listView未初始化。我看到你试图初始化它。我的猜测是，R.id.listDisp ID不存在，或者不是R.layout.activity_add的一部分，这是您在此课程中夸大的布局。

TL; DR。由于这一行而发生错误：

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

listDisp不是activity_add布局

的一部分

Answer 2

您的布局activity_add可能没有ID为listDisp

的列表视图

我试图在Android工作室中创建一个简单的保存/加载应用程序但我得到一个空对象引用错误（下面的完整错误）

2 个答案: