JAVA基础教程8.ppt

1、Holding your objects,华南理工大学 计算机科学与工程学院 韦佳 ,You need to create any number of objects, anytime, anywhere. The java.util library has a reasonably complete set of container classes to solve this problem, the basic type of which are List, Set, Queue, and Map.,Generics and type-safe containers,One of the

2、problems of using pre-Java SE5 containers was that the compiler allowed you to insert an incorrect type in to a container. /ApplesAndOrangesWithoutGenerics.java With generics, youre prevented, at compile time, from putting the wrong types of objects to a container. /ApplesAndOrangesWithGenerics.java

3、,You are not limited to putting the exact type of objects into a container when you specify that type as a generic parameter. Upcasting works the same with generics as it does with other types. /GenericsAndUpcasting.java,Basic concepts of container,Collection: a sequence of individual elements with

4、one or more rules applied to them. List: must hold the elements in the way they were inserted; Set: cannot have duplicate elements; Queue: produces the elements in the order determined by a queuing discipline (FIFO).,Map: a group of key-value object pairs, allowing you to look up a value using a key

5、. A map allows you to look up an object using another object. Its also called an associative array.,Adding groups of elements,Arrays.asList(): takes either an array or a comma-separated list of elements and turn it into a List object. Collections.addAll(): takes a Collection object and either an arr

6、ay or a comma-separated list and adds the elements to the Collection. /AddingGroups.java,A limitation of Arrays.asList() is that it takes a best guess about the resulting type of the List, and doesnt pay attention to what youre assigning it to. /AsListInference.java,Printing containers,You can print

7、 containers without any help. /PrintingContainers.java List: ArrayList, LinkedList. Set: HashSet, TreeSet, LinkedHashSet. Map: HashMap, TreeMap, LinkedHashMap.,List,There are two types of List: ArrayList, which excels at randomly accessing elements. LinkedList, which excels at inserting and deleting

8、 elements from the middle of the List. /ListFeatures.java,Iterator,An iterator is an object whose job is to move through a sequence and select each object in that sequence without the client programmer knowing or caring about the underlying structure of that sequence. The Java Iterator can move in o

9、nly one direction.,Theres not much you can do with an Iterator except: Ask a Collection to hand you an Iterator using a method called iterator(). Get the next object in the sequence with next(). See if there are any more objects with hasNext(). Remove the last element returned by the iterator with r

10、emove(). /SimpleIteration.java /CrossContainerIteration.java,ListIterator,The ListIterator is a more powerful subtype of Iterator that is produced only by List classes. While Iterator can only move forward, ListIterator is bidirectional. /ListIteration.java,LinkedList,The LinkedList performs inserti

11、on and removal in the middle of the List more efficiently than ArrayList. Conversely, the LinkedList is less efficient for random-access operations than ArrayList. /LinkedListFeatures.java,Stack,A stack is sometimes referred to as a “last-in, first out” (LIFO) container. LinkedList has methods that

12、directly implement stack functionality, so you can also just use a LinkedList rather than making a stack class. /Stack.java; StackTest.java,Set,A Set refuses to hold more than one instance of each object value. /SetOfInteger.java If you want the results to be sorted, one approach is to use a TreeSet instead of a HashSet. /SortedSetOfInteger.java; SetOperations.java,Map,The ability to map objects to other objects can be an powerful way to solve programming problems. A Map easily solves the problem. /Statistics.java; PetMap.java