在日常的开发工作中,我们有的时候需要构造像Map<K, List<V>>或者Map<K, Set<V>>这样比较复杂的集合类型的数据结构,以便做相应的业务逻辑处理。例如:
import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; import org.junit.Test; public class MultimapTest { Map<String, List<StudentScore>> StudentScoreMap = new HashMap<String, List<StudentScore>>(); @Test public void testStudentScore(){ for(int i=10;i<20;i++){ StudentScore studentScore=new StudentScore(); studentScore.CourseId=1001+i; studentScore.score=100-i; addStudentScore("peida",studentScore); } System.out.println("StudentScoreMap:"+StudentScoreMap.size()); System.out.println("StudentScoreMap:"+StudentScoreMap.containsKey("peida")); System.out.println("StudentScoreMap:"+StudentScoreMap.containsKey("jerry")); System.out.println("StudentScoreMap:"+StudentScoreMap.size()); System.out.println("StudentScoreMap:"+StudentScoreMap.get("peida").size()); List<StudentScore> StudentScoreList=StudentScoreMap.get("peida"); if(StudentScoreList!=null&&StudentScoreList.size()>0){ for(StudentScore stuScore:StudentScoreList){ System.out.println("stuScore one:"+stuScore.CourseId+" score:"+stuScore.score); } } } public void addStudentScore(final String stuName,final StudentScore studentScore) { List<StudentScore> stuScore = StudentScoreMap.get(stuName); if (stuScore == null) { stuScore = new ArrayList<StudentScore>(); StudentScoreMap.put(stuName, stuScore); } stuScore.add(studentScore); } } class StudentScore{ int CourseId; int score; } 说明:想 Map<String, List<StudentScore>> StudentScoreMap = new HashMap<String, List<StudentScore>>()这样的数据结构,自己实现起来太麻烦,你需要检查key是否存在,不存在时则创建一个,存在时在List后面添加上一个。这个过程是比较痛苦的,如果你希望检查List中的对象是否存在,删除一个对象,或者遍历整个数据结构,那么则需要更多的代码来实现。 Multimap Guava的Multimap就提供了一个方便地把一个键对应到多个值的数据结构。让我们可以简单优雅的实现上面复杂的数据结构,让我们的精力和时间放在实现业务逻辑上,而不是在数据结构上,下面我们具体来看看Multimap的相关知识点。 上面的代码和数据结构用Multimap来实现,代码结构清晰简单了很多吧,具体代码如下: @Test public void teststuScoreMultimap(){ Multimap<String,StudentScore> scoreMultimap = ArrayListMultimap.create(); for(int i=10;i<20;i++){ StudentScore studentScore=new StudentScore(); studentScore.CourseId=1001+i; studentScore.score=100-i; scoreMultimap.put("peida",studentScore); } System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.keys()); } 调用Multimap.get(key)会返回这个键对应的值的集合的视图(view),没有对应集合就返回空集合。对于 ListMultimap来说,这个方法会返回一个List,对于SetMultimap来说,这个方法就返回一个Set。修改数据是通过修改底层 Multimap来实现的。例如: @Test public void teststuScoreMultimap(){ Multimap<String,StudentScore> scoreMultimap = ArrayListMultimap.create(); for(int i=10;i<20;i++){ StudentScore studentScore=new StudentScore(); studentScore.CourseId=1001+i; studentScore.score=100-i; scoreMultimap.put("peida",studentScore); } System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.keys()); Collection<StudentScore> studentScore = scoreMultimap.get("peida"); studentScore.clear(); StudentScore studentScoreNew=new StudentScore(); studentScoreNew.CourseId=1034; studentScoreNew.score=67; studentScore.add(studentScoreNew); System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.keys()); } Multimap也支持一系列强大的视图功能: 尽管Multimap的实现用到了Map,但Multimap<K, V>不是Map<K, Collection<V>>。因为两者有明显区别: @Test public void teststuScoreMultimap(){ Multimap<String,StudentScore> scoreMultimap = ArrayListMultimap.create(); for(int i=10;i<20;i++){ StudentScore studentScore=new StudentScore(); studentScore.CourseId=1001+i; studentScore.score=100-i; scoreMultimap.put("peida",studentScore); } System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.keys()); Collection<StudentScore> studentScore = scoreMultimap.get("peida"); StudentScore studentScore1=new StudentScore(); studentScore1.CourseId=1034; studentScore1.score=67; studentScore.add(studentScore1); StudentScore studentScore2=new StudentScore(); studentScore2.CourseId=1045; studentScore2.score=56; scoreMultimap.put("jerry",studentScore2); System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.keys()); for(StudentScore stuScore : scoreMultimap.values()) { System.out.println("stuScore one:"+stuScore.CourseId+" score:"+stuScore.score); } scoreMultimap.remove("jerry",studentScore2); System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.get("jerry")); scoreMultimap.put("harry",studentScore2); scoreMultimap.removeAll("harry"); System.out.println("scoreMultimap:"+scoreMultimap.size()); System.out.println("scoreMultimap:"+scoreMultimap.get("harry")); } Multimap的实现 Multimap提供了丰富的实现,所以你可以用它来替代程序里的Map<K, Collection<V>>,具体的实现如下: 2.LinkedHashMultimap维持插入的顺序,以及键的插入顺序。 |
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来自: 贾朋亮博客 > 《Guava学习笔记》