[英]Printing out a linked list using toString
Ok guys, so I am trying to learn how to print out a linked list.好的伙计们,所以我正在尝试学习如何打印链表。 I have all the methods that I would need to use for the list, but I can't figure out how to display the values of the nodes.
我拥有列表所需的所有方法,但我不知道如何显示节点的值。 Right now there is nothing in my main method because I kept getting errors trying to call non static methods in the main.
现在我的主要方法中没有任何内容,因为我一直在尝试调用主要方法中的非 static 方法时出错。 I have a toString method that displays the contents of the list.
我有一个显示列表内容的 toString 方法。 How would I go about calling this toString to display the value of each node?
我 go 如何调用此 toString 来显示每个节点的值? Any advice will be greatly appreciated.
任何建议将不胜感激。
Here is the node class:这是节点 class:
public class LinkedListNode
{
private int data;
private LinkedListNode next;
public LinkedListNode(int data)
{
this.data = data;
this.next = null;
}
public int getData()
{
return data;
}
public void setData(int d)
{
data = d;
}
public LinkedListNode getNext()
{
return next;
}
public void setNext(LinkedListNode n)
{
next = n;
}
}
Here is the LinkedList class that contains the main and methods to manipulate the list:这是 LinkedList class,其中包含操作列表的主要方法和方法:
public class LinkedList {
public LinkedListNode head;
public static void main(String[] args) {
LinkedList l = new LinkedList();
l.insertFront(0);
System.out.println(l.toString());
}
public LinkedList() {
this.head = null;
}
public int removeFront(){
if(head == null){
System.out.println("Error - Attempting to call removeFront() on empty list");
return 0;
}else{
int temp = head.getData();
head = head.getNext();
return temp;
}
}
public void insertFront(int data){
if(head == null){
head = new LinkedListNode(data);
}else{
LinkedListNode newNode = new LinkedListNode(data);
newNode.setNext(head);
head = newNode;
}
}
public void insertBack(int data){
if(head == null){
head = new LinkedListNode(data);
}else{
LinkedListNode newNode = new LinkedListNode(data);
LinkedListNode current = head;
while(current.getNext() != null){
current = current.getNext();
}
current.setNext(newNode);
}
}
public int removeBack(){
if(head == null){
System.out.println("Error - Attempting to call removeBack() on empty list");
return 0;
}else if (head.getNext() == null){
int temp = head.getData();
head = null;
return temp;
}else{
LinkedListNode current = head;
while(current.getNext().getNext() != null){
current = current.getNext();
}
int temp = current.getNext().getData();
current.setNext(null);
return temp;
}
}
public String toString(){
String retStr = "Contents:\n";
LinkedListNode current = head;
while(current != null){
retStr += current.getData() + "\n";
current = current.getNext();
}
return retStr;
}
public LinkedListNode getHead() {
return head;
}
public void setHead(LinkedListNode head) {
this.head = head;
}
}
public static void main(String[] args) {
LinkedList list = new LinkedList();
list.insertFront(1);
list.insertFront(2);
list.insertFront(3);
System.out.println(list.toString());
}
String toString() {
String result = "";
LinkedListNode current = head;
while(current.getNext() != null){
result += current.getData();
if(current.getNext() != null){
result += ", ";
}
current = current.getNext();
}
return "List: " + result;
}
As has been pointed out in some other answers and comments, what you are missing here is a call to the JVM System class to print out the string generated by your toString() method. 正如在其他一些答案和注释中所指出的,这里缺少的是调用JVM System类来打印由toString()方法生成的字符串。
LinkedList myLinkedList = new LinkedList();
System.out.println(myLinkedList.toString());
This will get the job done, but I wouldn't recommend doing it that way. 这将完成工作,但我不建议这样做。 If we take a look at the javadocs for the Object class, we find this description for toString():
如果我们看一下Object类的javadocs,我们会找到toString()的这个描述:
Returns a string representation of the object.
返回对象的字符串表示形式。 In general, the toString method returns a string that "textually represents" this object.
通常,toString方法返回一个“文本表示”此对象的字符串。 The result should be a concise but informative representation that is easy for a person to read.
结果应该是一个简洁但信息丰富的表示 ,便于人们阅读。 It is recommended that all subclasses override this method.
建议所有子类都重写此方法。
The emphasis added there is my own. 这里强调的是我自己的。 You are creating a string that contains the entire state of the linked list, which somebody using your class is probably not expecting.
您正在创建一个包含链表的整个状态的字符串,有人使用您的类可能没有预料到。 I would recommend the following changes:
我建议进行以下更改:
In LinkedListNode: 在LinkedListNode中:
public String toString(){
return "LinkedListNode with data: " + getData();
}
In LinkedList: 在LinkedList中:
public int size(){
int currentSize = 0;
LinkedListNode current = head;
while(current != null){
currentSize = currentSize + 1;
current = current.getNext();
}
return currentSize;
}
public String toString(){
return "LinkedList with " + size() + "elements.";
}
public void printList(){
System.out.println("Contents of " + toString());
LinkedListNode current = head;
while(current != null){
System.out.println(current.toString());
current = current.getNext();
}
}
When the JVM
tries to run your application, it calls your main method statically; 当
JVM
尝试运行您的应用程序时,它会静态调用您的main方法; something like this: 这样的事情:
LinkedList.main();
That means there is no instance of your LinkedList
class. 这意味着您没有
LinkedList
类的实例。 In order to call your toString()
method, you can create a new instance of your LinkedList
class. 为了调用
toString()
方法,您可以创建LinkedList
类的新实例。
So the body of your main
method should be like this: 所以
main
方法的main
应该是这样的:
public static void main(String[] args){
// creating an instance of LinkedList class
LinkedList ll = new LinkedList();
// adding some data to the list
ll.insertFront(1);
ll.insertFront(2);
ll.insertFront(3);
ll.insertBack(4);
System.out.println(ll.toString());
}
I do it the following way: 我这样做的方式如下:
public static void main(String[] args) {
LinkedList list = new LinkedList();
list.insertFront(1);
list.insertFront(2);
list.insertFront(3);
System.out.println(list.toString());
}
String toString() {
StringBuilder result = new StringBuilder();
for(Object item:this) {
result.append(item.toString());
result.append("\n"); //optional
}
return result.toString();
}
A very simple solution is to override
the toString()
method in the Node
. 一个非常简单的解决方案是
override
Node
的toString()
方法。 Then, you can call print by passing LinkedList
's head
. 然后,您可以通过传递
LinkedList
的head
调用print。 You don't need to implement any kind of loop. 您不需要实现任何类型的循环。
Code: 码:
public class LinkedListNode {
...
//New
@Override
public String toString() {
return String.format("Node(%d, next = %s)", data, next);
}
}
public class LinkedList {
public static void main(String[] args) {
LinkedList l = new LinkedList();
l.insertFront(0);
l.insertFront(1);
l.insertFront(2);
l.insertFront(3);
//New
System.out.println(l.head);
}
}
For the @Marin code, it doesnt cover if the list is empty or the list contains only 1 node.对于@Marin 代码,如果列表为空或列表仅包含 1 个节点,则不涵盖。 So here the improved code:
所以这里是改进的代码:
@Override
public String toString() {
String result = "";
LinkedListNode dummy = head;
if (dummy == null) { //if list is empty return result
return result;
}else if(dummy.getNext() == null){ //if the list contains only 1 node
result += dummy.getData();
return result;
}else{
while(dummy != null){
result += dummy.getData();
if(dummy.getNext()!= null){
result += " ";
}
dummy = dummy.getNext();
}
return result;
}
}
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