Commons Collections - Bag组

先来看Bag组。

 

Bag

HashBag

BagUtils

 

Bag是在org.apache.commons.collections包中定义的接口，它extends java.util.Collection，而它的实现类都被放在下面的bag包中。之所以有这样一组类型，是因为我们有时候需要在Collection中存放多个相同对象的拷贝，并且需要很方便的取得该对象拷贝的个数。需要注意的一点是它虽然extends Collection，但是如果真把它完全当作java.util.Collection来用会遇到语义上的问题，详细信息参考Javadoc。

 

HashBag是Bag接口的一个标准实现。而BagUtils提供一组static的方法让调用者获取经过不同装饰后的Bag实例。

 

还是举例子来看：

 

/** Book.java */

 

package sean.study.commons.collections;

 

import org.apache.commons.lang.builder.ToStringBuilder;

import org.apache.commons.lang.builder.ToStringStyle;

 

public class Book {

   

    private String name;

    private String isbn;

    private double retailPrice;

   

    public Book() {

    }

   

    public Book(String name, String isbn, double retailPrice) {

        this.name = name;

        this.isbn = isbn;

        this.retailPrice = retailPrice;

    }

   

    public String toString() {

        return new ToStringBuilder(this, ToStringStyle.MULTI_LINE_STYLE)

        .append("name", name)

        .append("ISBN", isbn)

        .append("retailPrice", retailPrice)

        .toString();

    }

 

    public String getIsbn() {

        return isbn;

    }

 

    public void setIsbn(String isbn) {

        this.isbn = isbn;

    }

 

    public String getName() {

        return name;

    }

 

    public void setName(String name) {

        this.name = name;

    }

 

    public double getRetailPrice() {

        return retailPrice;

    }

 

    public void setRetailPrice(double retailPrice) {

        this.retailPrice = retailPrice;

    }

   

}

 

/** BagUsage.java */

 

package sean.study.commons.collections;

 

import org.apache.commons.collections.Bag;

import org.apache.commons.collections.BagUtils;

import org.apache.commons.collections.bag.HashBag;

import org.apache.commons.lang.StringUtils;

 

public class BagUsage {

 

    public static void main(String[] args) {

        demoBagUsage();

    }

 

    public static void demoBagUsage() {

 

        System.out.println(StringUtils.center(" demoBagUsage ", 40, "="));

 

        // data setup

        Book book1 = new Book("Refactoring Workbook", "7-5083-2208-8", 29.8);

        Book book2 = new Book("J2EE Design Patterns", "7-5083-3099-4", 45);

        Book book3 = new Book("Agile Software Development", "7-5083-1503-0", 59);

 

        // create a bag

        Bag myBag = BagUtils.typedBag(new HashBag(), Book.class);

        myBag.add(book1, 360);

        myBag.add(book2, 500);

        myBag.add(book3, 170);

 

        // calculations for a bag

        double price1 = book1.getRetailPrice();

        double price2 = book2.getRetailPrice();

        double price3 = book3.getRetailPrice();

        int book1Count = myBag.getCount(book1);

        int book2Count = myBag.getCount(book2);

        int book3Count = myBag.getCount(book3);

        double totalValue = (price1 * book1Count) + (price2 * book2Count)

                + (price3 * book3Count);

 

        // dispaly results

        System.out.println("There are " + book1Count + " copies of "

                + book1.getName() + ".");

        System.out.println("There are " + book2Count + " copies of "

                + book2.getName() + ".");

        System.out.println("There are " + book3Count + " copies of "

                + book3.getName() + ".");

        System.out.println("The total value of these books is: " + totalValue);

 

        System.out.println();

 

    }

 

}

 

以下是运行结果：

 

============= demoBagUsage =============

There are 360 copies of Refactoring Workbook.

There are 500 copies of J2EE Design Patterns.

There are 170 copies of Agile Software Development.

The total value of these books is: 43258.0

 

需要说明的是，以上的代码仅仅为了演示如何使用Bag，实际应用不建议像这样硬编码。