带你领略 Google Collections 2

原贴地址：

http://jubin2002.javaeye.com/blog/471698

上篇讲到google collections的几个比较主要的点，今天我们来看看其提供的几个小的但是相当有用的东西。

1，Preconditions

Preconditions 提供了状态校验的方法。

Before:

 public DeliverycreateDelivery(Orderorder,UserdeliveryPerson){

 if (order.getAddress()== null ){
 throw  new NullPointerException( "orderaddress" );
 }

 if (!workSchedule.isOnDuty(deliveryPerson,order.getArrivalTime())){
 throw  new IllegalArgumentException(
 String.format("%sisnotondutyfor%s" ,deliveryPerson,order));
 }

 return  new RealDelivery(order,deliveryPerson);
 }

public Delivery createDelivery(Order order, User deliveryPerson) {
    
    if(order.getAddress() == null) {
        throw new NullPointerException("order address");
    }
    
    if(!workSchedule.isOnDuty(deliveryPerson, order.getArrivalTime())) {
        throw new IllegalArgumentException(
            String.format("%s is not on duty for %s", deliveryPerson, order));
    }

    return new RealDelivery(order, deliveryPerson);
}

After:

 public DeliverycreateDelivery(Orderorder,UserdeliveryPerson){
 Preconditions.checkNotNull(order.getAddress(),"orderaddress" );
 Preconditions.checkArgument(
 workSchedule.isOnDuty(deliveryPerson,order.getArrivalTime()),
 "%sisnotondutyfor%s" ,deliveryPerson,order);

 return  new RealDelivery(order,deliveryPerson);
 }

public Delivery createDelivery(Order order, User deliveryPerson) {
    Preconditions.checkNotNull(order.getAddress(), "order address");
    Preconditions.checkArgument(
        workSchedule.isOnDuty(deliveryPerson, order.getArrivalTime()),
            "%s is not on duty for %s", deliveryPerson, order);

    return new RealDelivery(order, deliveryPerson);
}

2,Iterables.getOnlyElement

Iterables.getOnlyElement 确保你的集合或者迭代器包含了刚好一个元素并且返回该元素。如果他包含0和2+元素，它会抛出RuntimeException。一般在单元测试中使用。

Before：

 public  void testWorkSchedule(){
 workSchedule.scheduleUserOnDuty(jesse,mondayAt430pm,mondayAt1130pm);

 Set<User>usersOnDuty=workSchedule.getUsersOnDuty(mondayAt800pm);
 assertEquals(1 ,usersOnDuty.size());
 assertEquals(jesse,usersOnDuty.iterator().next());
 }

public void testWorkSchedule() {
    workSchedule.scheduleUserOnDuty(jesse, mondayAt430pm, mondayAt1130pm);

    Set<User> usersOnDuty = workSchedule.getUsersOnDuty(mondayAt800pm);
    assertEquals(1, usersOnDuty.size());
    assertEquals(jesse, usersOnDuty.iterator().next());
}

After：

 public  void testWorkSchedule(){
 workSchedule.scheduleUserOnDuty(jesse,mondayAt430pm,mondayAt1130pm);

 Set<User>usersOnDuty=workSchedule.getUsersOnDuty(mondayAt800pm);
 assertEquals(jesse,Iterables.getOnlyElement(usersOnDuty));
 }

public void testWorkSchedule() {
    workSchedule.scheduleUserOnDuty(jesse, mondayAt430pm, mondayAt1130pm);

    Set<User> usersOnDuty = workSchedule.getUsersOnDuty(mondayAt800pm);
    assertEquals(jesse, Iterables.getOnlyElement(usersOnDuty));
}

Iterables.getOnlyElement比Set.iterator().getNext()和List.get(0)描述的更为直接。

3，Objects.equal

Objects.equal(Object,Object) and Objects.hashCode(Object...)提供了内建的null处理，能使你实现equals() 和hashCode()更加简单。

Before：

 public  boolean equals(Objecto){
 if (o instanceof Order){
 Orderthat=(Order)o;

 return (address!= null
 ?address.equals(that.address)
 :that.address==null )
 &&(targetArrivalDate!=null
 ?targetArrivalDate.equals(that.targetArrivalDate)
 :that.targetArrivalDate==null )
 &&lineItems.equals(that.lineItems);
 }else {
 return  false ;
 }
 }

 public  int hashCode(){
 int result= 0 ;
 result=31 *result+(address!= null ?address.hashCode(): 0 );
 result=31 *result+(targetArrivalDate!= null ?targetArrivalDate.hashCode(): 0 );
 result=31 *result+lineItems.hashCode();
 return result;
 }

public boolean equals(Object o) {
    if (o instanceof Order) {
      Order that = (Order)o;

      return (address != null 
              ? address.equals(that.address) 
              : that.address == null) 
          && (targetArrivalDate != null 
              ? targetArrivalDate.equals(that.targetArrivalDate) 
              : that.targetArrivalDate == null)
          && lineItems.equals(that.lineItems);
    } else {
      return false;
    }
}

public int hashCode() {
    int result = 0;
    result = 31 * result + (address != null ? address.hashCode() : 0);
    result = 31 * result + (targetArrivalDate != null ? targetArrivalDate.hashCode() : 0);
    result = 31 * result + lineItems.hashCode();
    return result;
}

After：

 public  boolean equals(Objecto){
 if (o instanceof Order){
 Orderthat=(Order)o;

 return Objects.equal(address,that.address)
 &&Objects.equal(targetArrivalDate,that.targetArrivalDate)
 &&Objects.equal(lineItems,that.lineItems);
 }else {
 return  false ;
 }
 }

 public  int hashCode(){
 return Objects.hashCode(address,targetArrivalDate,lineItems);
 }

public boolean equals(Object o) {
    if (o instanceof Order) {
      Order that = (Order)o;

      return Objects.equal(address, that.address)
          && Objects.equal(targetArrivalDate, that.targetArrivalDate)
          && Objects.equal(lineItems, that.lineItems);
    } else {
      return false;
    }
}

public int hashCode() {
    return Objects.hashCode(address, targetArrivalDate, lineItems);
}

4，Iterables.concat()

Iterables.concat() 连结多种集合 (比如ArrayList和HashSet) 以至于你能在一行代码里遍历他们:

Before：

 public  boolean orderContains(Productproduct){
 List<LineItem>allLineItems=new ArrayList<LineItem>();
 allLineItems.addAll(getPurchasedItems());
 allLineItems.addAll(getFreeItems());

 for (LineItemlineItem:allLineItems){
 if (lineItem.getProduct()==product){
 return  true ;
 }
 }

 return  false ;
 }

public boolean orderContains(Product product) {
    List<LineItem> allLineItems = new ArrayList<LineItem>();
    allLineItems.addAll(getPurchasedItems());
    allLineItems.addAll(getFreeItems());

    for (LineItem lineItem : allLineItems) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
}

After：

 public  boolean orderContains(Productproduct){
 for (LineItemlineItem:Iterables.concat(getPurchasedItems(),getFreeItems())){
 if (lineItem.getProduct()==product){
 return  true ;
 }
 }

 return  false ;
 }

public boolean orderContains(Product product) {
    for (LineItem lineItem : Iterables.concat(getPurchasedItems(), getFreeItems())) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
}

5，Join

Join 是用分隔符分割字符串变得非常容易。

Before:

 public  class ShoppingList{
 private List<Item>items=...;

 ...

 public StringtoString(){
 StringBuilderstringBuilder=new StringBuilder();
 for (Iterator<Item>s=items.iterator();s.hasNext();){
 stringBuilder.append(s.next());
 if (s.hasNext()){
 stringBuilder.append("and" );
 }
 }
 return stringBuilder.toString();
 }
 }

public class ShoppingList {
  private List<Item> items = ...;

  ...

  public String toString() {
    StringBuilder stringBuilder = new StringBuilder();
    for (Iterator<Item> s = items.iterator(); s.hasNext(); ) {
      stringBuilder.append(s.next());
      if (s.hasNext()) {
        stringBuilder.append(" and ");
      }
    }
    return stringBuilder.toString();
  }
}

After:

 public  class ShoppingList{
 private List<Item>items=...;

 ...

 public StringtoString(){
 return Joiner.on( "and" ).join(items);
 }
 }

public class ShoppingList {
  private List<Item> items = ...;

  ...

  public String toString() {
    return Joiner.on(" and ").join(items);
  }
}

6,Maps, Sets and Lists

泛型是好的，不过他们有些过于罗嗦。

Before：

 Map<CustomerId,BillingOrderHistory>customerOrderHistoryMap
 =new HashMap<CustomerId,BillingOrderHistory>();

Map<CustomerId, BillingOrderHistory> customerOrderHistoryMap 
    = new HashMap<CustomerId, BillingOrderHistory>();

After：

 Map<CustomerId,BillingOrderHistory>customerOrderHistoryMap
 =Maps.newHashMap();

Map<CustomerId, BillingOrderHistory> customerOrderHistoryMap 
    = Maps.newHashMap();

Maps, Sets and Lists 包含了工厂方法来创建集合对象。

另一个例子，Before：

 Set<String>workdays= new LinkedHashSet<String>();
 workdays.add("Monday" );
 workdays.add("Tuesday" );
 workdays.add("Wednesday" );
 workdays.add("Thursday" );
 workdays.add("Friday" );

Set<String> workdays = new LinkedHashSet<String>();
workdays.add("Monday");
workdays.add("Tuesday");
workdays.add("Wednesday");
workdays.add("Thursday");
workdays.add("Friday");

OR：

 Set<String>workdays= new LinkedHashSet<String>(
 Arrays.asList("Monday" , "Tuesday" , "Wednesday" , "Thursday" , "Friday" ));

Set<String> workdays = new LinkedHashSet<String>(
  Arrays.asList("Monday", "Tuesday", "Wednesday", "Thursday", "Friday"));

After：

 Set<String>workdays=Sets.newLinkedHashSet(
 "Monday" , "Tuesday" , "Wednesday" , "Thursday" , "Friday" );

Set<String> workdays = Sets.newLinkedHashSet(
  "Monday", "Tuesday", "Wednesday", "Thursday", "Friday");

Google Collections 对于Maps, Sets, Lists, Multimaps, Multisets 都提供了工厂方法 。