page74-泛型可迭代的基础集合数据类型的API-Bag+Queue+Stack

【泛型可迭代的基础集合数据类型的API】

背包：就是一种不支持从中删除元素的集合数据类型——它的目的就是帮助用例收集元素并迭代遍历所有收集到的元素。（用例也可以检查背包是否为空， 或者获取背包中元素的数量）

　　public class Bag<Item> implements Iterable<Item>

　　Bag() 创建一个空背包

　　void add(Item item) 添加一个元素

　　boolean isEmpty() 背包是否为空

　　int size() 背包中的元素数量

使用Bag的API， 用例可以将元素添加到背包，并通过foreach循环来访问所有元素。用例也可以使用栈或者队列， 但使用Bag可以说明元素的处理顺序不重要 。

---

先进先出（FIFO）队列：

public class Queue<Item> implements Iterable<Item>

　　Queue() 创建一个空队列

　　void enqueue(Item item) 添加一个元素

　　Item dequeue() 删除最近添加的元素

　　boolean isEmpty() 队列是否为空

　　int size()  队列中的元素数量

---

下压（后进先出，LIFO）栈

　　public class Stack<Item> implements Iterable<Item>

　　Stack() 创建一个空栈

　　void push(Item item) 添加一个元素

　　Item pop() 删除最近添加的元素

　　boolean isEmpty()  栈是否为空

　　int size() 栈中元素数量

---

Stats类是Bag的一个典型用例。

它的任务是简单地计算标准输入中的所有double值的平均值和样本标准差。数的计算顺序和结果无关，因此我们将它们保存在一个Bag对象中， 并使用foreach循环来计算每个和。

用Bag对象保存所有数字是更复杂的统计计算所必须的 。

背包的典型用例：Stats.java

public class Stats {

    public static void main(String[] args) {

        Bag<Double> numbers = new Bag<Double>();
        while(!StdIn.isEmpty())
            numbers.add(StdIn.readDouble());
        int N = numbers.size();
        double sum = 0.0;
        for (Double x : numbers)
            sum += x;
        double mean = sum / N;
        sum = 0.0;

        for (Double x : numbers)
            sum += (x-mean)*(x-mean);
        double std = Math.sqrt(sum/(N-1));

        StdOut.printf("mean : %.2f\n", mean);
        StdOut.printf("Std dev : %.2f\n", std);

    }
}

【打印结果】

【Bag.java】

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Bag<Item> implements Iterable<Item> {

    private int N;               // number of elements in bag
    private Node<Item> first;    // beginning of bag

    // helper linked list class
    private static class Node<Item> {//嵌套类
        private Item item;
        private Node<Item> next;
    }

    /**
     * Initializes an empty bag.
     */
    public Bag() {
        first = null;
        N = 0;
    }

    /**
     * Is this bag empty?
     * @return true if this bag is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in this bag.
     * @return the number of items in this bag
     */
    public int size() {
        return N;
    }

    /**
     * Adds the item to this bag.
     * @param item the item to add to this bag
     */
    public void add(Item item) {//由add()方法可以看出，是头插法。
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        N++;
    }

    /**
     * Returns an iterator that iterates over the items in the bag in arbitrary order.
     * @return an iterator that iterates over the items in the bag in arbitrary order
     */
    public Iterator<Item> iterator()  {
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }
    }

    /**
     * Unit tests the <tt>Bag</tt> data type.
     */
    public static void main(String[] args) {
        Bag<String> bag = new Bag<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            bag.add(item);
        }

        StdOut.println("size of bag = " + bag.size());
        for (String s : bag) {
            StdOut.println(s);
        }
    }

}

---

【Queue.java】

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Queue<Item> implements Iterable<Item> {
    private int N;               // number of elements on queue
    private Node<Item> first;    // beginning of queue
    private Node<Item> last;     // end of queue

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    /**
     * Initializes an empty queue.
     */
    public Queue() {
        first = null;
        last  = null;
        N = 0;
    }

    /**
     * Is this queue empty?
     * @return true if this queue is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in this queue.
     * @return the number of items in this queue
     */
    public int size() {
        return N;
    }

    /**
     * Returns the item least recently added to this queue.
     * @return the item least recently added to this queue
     * @throws java.util.NoSuchElementException if this queue is empty
     */
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        return first.item;
    }

    /**
     * Adds the item to this queue.
     * @param item the item to add
     */
    public void enqueue(Item item) {
        Node<Item> oldlast = last;
        last = new Node<Item>();
        last.item = item;
        last.next = null;
        if (isEmpty()) first = last;
        else           oldlast.next = last;
        N++;
    }

    /**
     * Removes and returns the item on this queue that was least recently added.
     * @return the item on this queue that was least recently added
     * @throws java.util.NoSuchElementException if this queue is empty
     */
    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = first.item;
        first = first.next;
        N--;
        if (isEmpty()) last = null;   // to avoid loitering
        return item;
    }

    /**
     * Returns a string representation of this queue.
     * @return the sequence of items in FIFO order, separated by spaces
     */
    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this)
            s.append(item + " ");
        return s.toString();
    } 

    /**
     * Returns an iterator that iterates over the items in this queue in FIFO order.
     * @return an iterator that iterates over the items in this queue in FIFO order
     */
    public Iterator<Item> iterator()  {
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }
    }

    /**
     * Unit tests the <tt>Queue</tt> data type.
     */
    public static void main(String[] args) {
        Queue<String> q = new Queue<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-")) q.enqueue(item);
            else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
        }
        StdOut.println("(" + q.size() + " left on queue)");
    }
}

【Stack.java】

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Stack<Item> implements Iterable<Item> {
    private int N;                // size of the stack
    private Node<Item> first;     // top of stack

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    /**
     * Initializes an empty stack.
     */
    public Stack() {
        first = null;
        N = 0;
    }

    /**
     * Is this stack empty?
     * @return true if this stack is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in the stack.
     * @return the number of items in the stack
     */
    public int size() {
        return N;
    }

    /**
     * Adds the item to this stack.
     * @param item the item to add
     */
    public void push(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        N++;
    }

    /**
     * Removes and returns the item most recently added to this stack.
     * @return the item most recently added
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;        // save item to return
        first = first.next;            // delete first node
        N--;
        return item;                   // return the saved item
    }

    /**
     * Returns (but does not remove) the item most recently added to this stack.
     * @return the item most recently added to this stack
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        return first.item;
    }

    /**
     * Returns a string representation of this stack.
     * @return the sequence of items in the stack in LIFO order, separated by spaces
     */
    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this)
            s.append(item + " ");
        return s.toString();
    }

    /**
     * Returns an iterator to this stack that iterates through the items in LIFO order.
     * @return an iterator to this stack that iterates through the items in LIFO order.
     */
    public Iterator<Item> iterator() {
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }
        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }
    }

    /**
     * Unit tests the <tt>Stack</tt> data type.
     */
    public static void main(String[] args) {
        Stack<String> s = new Stack<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-")) s.push(item);
            else if (!s.isEmpty()) StdOut.print(s.pop() + " ");
        }
        StdOut.println("(" + s.size() + " left on stack)");
    }
}

【Queue测试用例 QueueCase.java】

public class QueueCase {

    public static void main(String[] args) {

        int[] temp = readInts(name);
        for (int i = 0; i < temp.length; i++)
            System.out.println(temp[i] + "  ");
    }

    public static int[] readInts(String name){
        In in = new In(name);
        Queue<Integer> q = new Queue<Integer>();
        while(!in.isEmpty())
            q.enqueue(in.readInt());
        int N = q.size();
        int[] a = new int[N];
        for (int i = 0; i < N; i++)
            a[i] = q.dequeue();
        return a;
    }
}

【Stack 测试用例 StackCase.java】

public class StackCase {

    public static void main(String[] args) {

        In in = new In(args[0]);
        Stack<Integer> s = new Stack<Integer>();
        while(!StdIn.isEmpty())
            s.push(StdIn.readInt());
        for (int i : s) {
            System.out.println(i +  "  ");
        }
    }
}