Cannot invoke "java.util.ArrayList.add(Object)"
时间: 2024-05-05 20:17:14 浏览: 12
as the ArrayList is not initialized. You need to create an instance of ArrayList before you can add elements to it.
Example:
```
ArrayList<String> list = new ArrayList<>();
list.add("element 1");
list.add("element 2");
```
In this example, we create an ArrayList of String type and then add two elements to it using the add() method.
相关问题
Exception in thread main java.lang.NullPointerException: Cannot invoke java.util.List.add(Object) because this.WorkerAL is null
这个错误是因为你在尝试向一个空的列表中添加元素,导致了空指针异常。你需要在使用 `add()` 方法之前,先初始化这个列表,即创建一个新的 `ArrayList` 对象,如下所示:
```
List<Object> WorkerAL = new ArrayList<Object>();
```
或者检查一下这个列表是否在其他地方被正确地初始化了。
Cannot invoke "java.util.ArrayList.isEmpty()" because "list" is null
This error message means that you are trying to call the isEmpty() method on an ArrayList object that is null, meaning it has not been initialized or assigned to any value. It is important to make sure that you have properly initialized the ArrayList object before calling any of its methods.
To fix this error, you should first check if the ArrayList object is null before calling the isEmpty() method. You can do this using an if-statement:
```
if (list != null && !list.isEmpty()) {
// list is not null and not empty, do something
} else {
// list is null or empty, handle the error
}
```
Alternatively, you can initialize the ArrayList object with an empty list when you declare it:
```
ArrayList<String> list = new ArrayList<>();
```
This way, even if you forget to add any elements to the list, it will not be null and you can call its methods without any errors.