首先来看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。