java 如何为链表编写 peek 方法
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How to code a peek method for Linked List
提问by Brock
I am creating a linked list implementation for a stack. I've got the pop and push method done, but I can't seem to get the peek methods right. The code I have now in there, returns the memory address I think.
我正在为堆栈创建一个链表实现。我已经完成了 pop 和 push 方法，但我似乎无法正确使用 peek 方法。我现在在那里的代码，返回我认为的内存地址。
Here is my code:
这是我的代码：
public class LinkedStack<T> implements StackADT<T> {
private int count;
private LinearNode<T> contents;
public LinkedStack() {
count = 0;
contents = null;
}
@Override
public void push(T element) {
LinearNode<T> top = new LinearNode<T>(element);
if (contents == null) {
contents = top;
} else {
LinearNode<T> current = contents;
while (current.getNext() != null) {
current = current.getNext();
}
current.setNext(top);
}
count++;
}
@SuppressWarnings("unchecked")
@Override
public T pop() {
T item = (T) contents;
contents = contents.getNext();
count--;
return item;
}
@SuppressWarnings("unchecked")
@Override
public T peek() throws NoSuchOperationException {
T top = (T) contents;
if(top == null){
throw new NoSuchOperationException();
}
return top;
}
@Override
public boolean isEmpty() {
// TODO Auto-generated method stub
return false;
}
@Override
public int size() {
return count;
}
}
This is what it outputs when I call the peek method. I used my push method to add an object, and I tested it with the size method. It showed that I added an element. Then I called my pop method and displayed the size again to make sure that worked.
这是我调用 peek 方法时输出的内容。我用我的push方法添加了一个对象，我用size方法测试了它。它表明我添加了一个元素。然后我调用我的 pop 方法并再次显示大小以确保它有效。
This is my output of the peek method:
这是我的 peek 方法的输出：
LinearNode@33f42b49
线性节点@33f42b49
Here is my LinearNode class:
这是我的 LinearNode 类：
public class LinearNode<T> {
private T element;
private LinearNode<T> next;
public LinearNode() {
this.element = null;
this.next = null;
}
public LinearNode(T element) {
this.element = element;
this.next = null;
}
public T getElement() {
return element;
}
public void setElement(T _element) {
this.element = _element;
}
public LinearNode<T> getNext() {
return next;
}
public void setNext(LinearNode<T> next) {
this.next = next;
}
}
}
回答by Jim Kiley
Sounds like the LinearNode class needs a toString() method.
听起来 LinearNode 类需要一个 toString() 方法。
回答by KarlP
pop() and peek() should return what the LinearNode references, not the actual LinkedNode.
pop() 和 peek() 应该返回 LinearNode 引用的内容，而不是实际的 LinkedNode。
The @SupressWarnings indicate that you are not doing generics right. Remove them, and look at what you are returning - it does not look like you are returning a `T' but the LinkedNode directly. You should not need any SupressWarnings in this code.
@SupressWarnings 表明您没有正确使用泛型。删除它们，然后查看您返回的内容 - 看起来您返回的不是“T”而是直接返回 LinkedNode。您不应该在此代码中需要任何 SupressWarnings。
If you want to store Strings then you should define the stack like this:
如果你想存储字符串，那么你应该像这样定义堆栈：
LinkedStack<String> stack = new LinkedStack<String>();
You can think that the generic type Twill then represent a String inside your stack class.
您可以认为泛型类型T将在您的堆栈类中表示一个字符串。
In that case, the peek()methods signature will then return a String.
在这种情况下，peek()方法签名将返回一个字符串。
Also: Check your pop and push methods, both should be able to do in O(1), however, you are traversing the list. You have got that backwards.
另外：检查您的 pop 和 push 方法，两者都应该能够在 O(1) 中完成，但是，您正在遍历列表。你已经倒退了。
回答by Jorge A Vega Vigo
You can simple peek by using: System.out.println("The element on top is: " + top.getElement); and return top.getElement. Remember in top you are actually removing a Node from the top of the stack and its element. Since peek only actually sees the top element of the stack you don't need to use top = top.getNext or count--;. Hope it helps!
您可以使用以下方法简单查看： System.out.println("The element on top is: " + top.getElement); 并返回 top.getElement。请记住，在 top 中，您实际上是从堆栈及其元素的顶部删除一个节点。由于 peek 实际上只看到堆栈的顶部元素，因此您不需要使用 top = top.getNext 或 count--;。希望能帮助到你！
