Java中的链接列表参考

Question

If I have a ListNode Class, which is

public class ListNode{
  int data;
  ListNode nextNode;
}

and a List Class, which is

public class List{
  private ListNode firstNode;
  private ListNode lastNode;
  private String name;

public List(String listName){
  name = listName;
  firstNode = lastNode = null; 
  }
}

what does the last statement "firstNode = lastNode = null" mean? Whether firstNode.data = null or firstNode.nextNode = null?

Answer 1

Edit: It appears I answered the wrong question. As the OP mentioned in comments below:

Sorry, my question may not be clear enough, what I am truly confusing is that when executing "firstNode = null", wether firstNode.data = null or firstNode.nextNode = null because firstNode is a object of type ListNode, which has int data and listNode type instances.

After the assignment firstNode = null , there is no .data or .nextNode . You'd have to assign firstNode to a new ListNode() first.

Once you do that, its data member will be initialized to 0 and its nextNode member will be initialized to null , as those are the default initial values for member fields if no explicit initialization is done. But again, you need to instantiate a ListNode and assign it to firstNode first, otherwise you'll get a NullPointerException if you try and access it while it is null .

---

Original answer, re: firstNode = lastNode = null

It assigns both firstNode and lastNode to null .

It is a commonly used syntactic trick that takes advantage of the fact that an assignment expression as a whole evaluates to the value of the variable after assignment:

    a = b = 1   
=>  a = (b = 1)            (b = 1) assigns b then evaluates to 1
=>  a = 1

You can chain as many together as you want, as long as the types are compatible:

    a = b = c = d = 1
=>  a = b = c = (d = 1)    (d = 1) assigns d then evaluates to 1
=>  a = b = (c = 1)        (c = 1) assigns c then evaluates to 1
=>  a = (b = 1)            (b = 1) assigns b then evaluates to 1
=>  a = 1

Incompatible types will result in an error:

Integer x;
Double y;
x = y = null; // error: cannot convert from Double to Integer

It's the same trick you use when you do stuff like:

int ch;
while ((ch = input.read()) != -1) {
   ...
}

From JLS 15.26 :

At run time, the result of the assignment expression is the value of the variable after the assignment has occurred.

Answer 2

这是空LinkedList的“开始逻辑”，当实例化List类型的对象时，它最初是空的，因此没有firstNode和lastNode ，因此它们都为null