我使用以下代码创建一个不可变队列。
import java.util.NoSuchElementException;
import java.util.Queue;
public class ImmutableQueue<E> {
//Two stacks are used. One is to add items to the queue(enqueue) and
//other is to remove them(dequeue)
private ImmutableQueue(ReversableStack<E> order, ReversableStack<E> reverse) {
this.order = order;
this.reverse = reverse;
}
//initially both stacks are empty
public ImmutableQueue() {
this.order = ReversableStack.emptyStack();
this.reverse = ReversableStack.emptyStack();
}
public ImmutableQueue<E> enqueue(E e) {
if (null == e)
throw new IllegalArgumentException();
return new ImmutableQueue<E>(this.order.push(e), this.reverse);
}
public ImmutableQueue<E> dequeue() {
if (this.isEmpty())
throw new NoSuchElementException();
if (!this.reverse.isEmpty()) {
return new ImmutableQueue<E>(this.order, this.reverse.tail);
} else {
return new ImmutableQueue<E>(ReversableStack.emptyStack(),
this.order.getReverseStack().tail);
}
}
private static class ReversableStack<E> {
private E head; //top of original stack
private ReversableStack<E> tail; //top of reversed stack
private int size;
//initializing stack parameters
private ReversableStack(E obj, ReversableStack<E> tail) {
this.head = obj;
this.tail = tail;
this.size = tail.size + 1;
}
//returns a new empty stack
public static ReversableStack emptyStack() {
return new ReversableStack();
}
private ReversableStack() {
this.head = null;
this.tail = null;
this.size = 0;
}
//Reverses the original stack
public ReversableStack<E> getReverseStack() {
ReversableStack<E> stack = new ReversableStack<E>();
ReversableStack<E> tail = this;
while (!tail.isEmpty()) {
stack = stack.push(tail.head);
tail = tail.tail;
}
return stack;
}
public boolean isEmpty() {
return this.size == 0;
}
public ReversableStack<E> push(E obj) {
return new ReversableStack<E>(obj, this);
}
}
private ReversableStack<E> order;
private ReversableStack<E> reverse;
private void normaliseQueue() {
this.reverse = this.order.getReverseStack();
this.order = ReversableStack.emptyStack();
}
public E peek() {
if (this.isEmpty())
throw new NoSuchElementException();
if (this.reverse.isEmpty())
normaliseQueue();
return this.reverse.head;
}
public boolean isEmpty() {
return size() == 0;
}
//returns the number of items currently in the queue
public int size() {
return this.order.size + this.reverse.size;
}
public static void main(String[] args)
{
ImmutableQueue<Integer> newQueue = new ImmutableQueue<Integer>();
newQueue.enqueue(5);
newQueue.enqueue(10);
newQueue.enqueue(15);
int x = newQueue.size();
//ImmutableQueue<Integer> x = newQueue.dequeue();
System.out.println(x);
}
}
但每当我尝试出队时,我都会收到NoSuchElementException。此外,newQueue.size函数也返回0。 我究竟做错了什么 ? 感谢
答案 0 :(得分:2)
您缺少新的ImmutableQueue
参考..
因为enqueue()
方法返回ImmutableQueue
public ImmutableQueue<E> enqueue(E e) {
if (null == e)
throw new IllegalArgumentException();
return new ImmutableQueue<E>(this.order.push(e), this.reverse);
}
但是在你的主要方法上你丢弃了那个对象
public static void main(String[] args)
{
ImmutableQueue<Integer> newQueue = new ImmutableQueue<Integer>();
newQueue.enqueue(5);
newQueue.enqueue(10);
newQueue.enqueue(15);
int x = newQueue.size();
//ImmutableQueue<Integer> x = newQueue.dequeue();
System.out.println(x);
}
将您的电话改为:
newQueue = newQueue.enqueue(5);
int x = newQueue.size();
System.out.println(x);
您会看到尺寸会发生变化