如何将空数组列表添加到列表视图？尝试在空对象引用上调用接口方法 'int java.util.List.size()'

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.

数组列表起初是空的，但随后用户可以填充它。

我该怎么做才能解决此错误？

Answer 1

这是代码：

@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 );
}