如何将空数组列表添加到 ListView？尝试在空对象引用上调用接口方法“int java.util.List.size()”[重复]

Question

@Override
protected void onCreate(Bundle savedInstanceState) {    
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    NoteTaking_Activity noteTaking_activity = new NoteTaking_Activity();

    arrayList = noteTaking_activity.getHeadlineArray();

    ListView listView = (ListView) findViewById(R.id.List_Of_notes);
    ArrayAdapter<String> adapter = new ArrayAdapter<String>( this, android.R.layout.simple_list_item_1, arrayList );

    listView.setAdapter( adapter );

执行上面显示的代码会导致错误消息

Attempt to invoke interface method 'int java.util.List.size()' on a null object reference.

数组列表一开始是空的，后来用户可以填充它。

我可以做些什么来解决这个错误？

Answer 1

代码如下：

    @Override
    protected void onCreate(Bundle savedInstanceState) {    
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        NoteTaking_Activity noteTaking_activity = new NoteTaking_Activity();

        arrayList = noteTaking_activity.getHeadlineArray();

        ListView listView = (ListView) findViewById(R.id.List_Of_notes);
if(arrayList!=null){
        ArrayAdapter<String> adapter = new ArrayAdapter<String>( this, android.R.layout.simple_list_item_1, arrayList );

        listView.setAdapter( adapter );
}