Collections

一、Collection Hierarchy

 

     List:An ordered data items.

     Set:Unique data itme.There is no duplicate element

     Map:maps the keys to values.

二、Fetures of collections

 

      Performance:Accessing a giving data item,Insertion and Deletion a giving data items.ArrayList spend a constant time accessing a giving data item,its deletion/insertion is linear in time.If delete or insert data item frequently,it's better to use LinkedList

     Sorted/ordered:The data structure is ordered if the data itmes is in some order.For example,ArrayList is ordered by its index.

     Uniqueness:There is no duplicate data item.Such as Set,Map

     Synchronize:support synchronization for thread safety.Such as Vector,HashTable.

 

三、Iterator

      Iterator is  a safe way to remove an element from a collection when traversing it.Traversing by index or enumeration is not safe way.

       A collection is create as bellow:  ArrayList ls = new ArrayList(); ls.add("I"); ls.add(" "); ls.add(" "); ls.add("am"); ls.add(" "); ls.add("Emmy");

      Delete empty String by enumeration:  for(int i = 0;i<ls.size();i++){ System.out.println(ls.get(i)); if(ls.get(i).equals(" ")) ls.remove(i); } for(int i = 0;i<ls.size();i++){ System.out.print(ls.get(i)); } The result: I am I amEmmy2009-08-05

      Delete empty String by iterator:  Iterator it = ls.iterator(); String item; while(it.hasNext()){ item = (String) it.next(); if(item.equals(" ")) it.remove(); } for(int i = 0;i<ls.size();i++){ System.out.print(ls.get(i)); } The result: IamEmmy2009-08-05

发现第一中删除元素的方法并不正确，因为当删除了一个元素以后，集合的长度和要访问的下一个元素的index已经变成被删除元素的index。

四、hashCode()和equals()

      Two objects are equals,determined by equals() method,must return the same hashCode.The reverse is not required.

      HashCode specify the uniqueness of a data item in a data structure.

FinalScore s0 = new FinalScore(77.7,77.7); FinalScore s1 = new FinalScore(77.7,77.7);

      According to the equals()  method in object class,s1 and s0 is not equal because they don't refer to the same object.However ,you might like to see they as equal because you might use s0 as key when storing a data item,while use s1 as key when you want to retrieve.Hope this to happen,we need to override equals() and hashCode() method.   

package com.citibank.aimy.examples.HashSet; import java.io.Serializable; import java.util.HashSet; import java.util.Set; public class Apple implements Serializable{ public String name; public String color; public transient String owner; public Apple(String nm,String cl,String or){ name = nm; color = cl; owner = or; } public Apple(String nm,String cl){ name = nm; color = cl; } public int hashCode(){ return name==null?0:name.hashCode(); } public boolean equals(Object o){ return true; } public static void main(String[] args){ Apple a1 = new Apple("Emmy","red"); Apple a2 = new Apple("Emmy","yello"); System.out.println("a1==a2:::"+(a1==a2)); System.out.println("a1.equals(a2):::"+a1.equals(a2)); System.out.println("---------------"); Set<Apple> s1 = new HashSet<Apple>(); s1.add(a1); System.out.println("Didn't add a2,Contains a2 already::"+s1.contains(a2)); s1.add(a2); System.out.println(s1.size()); Apple res = s1.iterator().next(); System.out.println(res.color); } }

五、Object ordering

      Ordering improve the perfomance of retrieval operation such as search.

      Natural ordering

      You can sort a collection in natural ordering by using the following method.      Collections.sort(ls);

     The classes of the data items should implement comparable interface,thereby support natural ordering.

package com.citibank.aimy.example.collections; public class FinalScore implements Comparable{ private double math; private double liter; public FinalScore(double math, double liter) { super(); this.math = math; this.liter = liter; } public double getLiter() { return liter; } public void setLiter(double liter) { this.liter = liter; } public double getMath() { return math; } public void setMath(double math) { this.math = math; } public int compareTo(Object o) { // TODO Auto-generated method stub double d1 = math+liter; FinalScore s = (FinalScore) o; double d2 = s.getMath()+s.getLiter(); if(d1>d2) return 1; else if(d1==d2) return 0; else return -1; } } package com.citibank.aimy.example.collections; import java.util.ArrayList; import java.util.Collections; public class OrderObject { private enum Colors{RED,BLUE,REEN,YELLOW,GRAY;} /** * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub FinalScore s0 = new FinalScore(77.7,77.8); FinalScore s1 = new FinalScore(77.7,77.7); FinalScore s2 = new FinalScore(88.8,88.8); FinalScore s3 = new FinalScore(99.9,99.9); FinalScore s4 = new FinalScore(88.2,78.9); ArrayList a = new ArrayList(); a.add(s0); a.add(s1); a.add(s2); a.add(s3); a.add(s4); Collections.sort(a); // NaturalOrder order = new NaturalOrder(); // System.out.println("s2 exists:"+Collections.binarySearch(a, s0)); // Collections.sort(a,order); // System.out.println("s2 exists:"+Collections.binarySearch(a, s0)); for(int i = 0;i<a.size();i++){ FinalScore s = (FinalScore) a.get(i); System.out.println("FinalScoure"+i+" Math:"+s.getMath()+" Liter:"+s.getLiter()); } } }

       If FinalScore class don't implement Comparable interface,a exception will be thrown while

Collections.sort(a); was executed.

Exception in thread "main" java.lang.ClassCastException: com.citibank.aimy.example.collections.FinalScore at java.util.Arrays.mergeSort(Arrays.java:1156) at java.util.Arrays.sort(Arrays.java:1080) at java.util.Collections.sort(Collections.java:117) at com.citibank.aimy.example.collections.OrderObject.main(OrderObject.java:24)

     We can also sort finalScore collection whose element don't implementComparable interface with the help of Comparator.

package com.citibank.aimy.example.collections; import java.util.Comparator; public class NaturalOrder implements Comparator { public int compare(Object o1, Object o2) { // TODO Auto-generated method stub FinalScore s1 = (FinalScore) o1; FinalScore s2 = (FinalScore) o2; double d1 = s1.getMath()+s1.getLiter(); double d2 = s2.getMath()+s2.getLiter(); if(d1>d2) return 1; else if(d1==d2) return 0; else return -1; } }

Test Code:

NaturalOrder order = new NaturalOrder(); Collections.sort(a,order);

  Either the class of the object doesn't implement Comparable interface or the order in which you want to sort the elements other than natural ordering.In this case,you want to use Comparator intertace.

  Change NaturalOrder code into reverse way:

public class NaturalOrder implements Comparator { public int compare(Object o1, Object o2) { // TODO Auto-generated method stub FinalScore s1 = (FinalScore) o1; FinalScore s2 = (FinalScore) o2; double d1 = s1.getMath()+s1.getLiter(); double d2 = s2.getMath()+s2.getLiter(); if(d1>d2) return -1; else if(d1==d2) return 0; else return 1; } }

  The result of  Collections.sort(a,order); as below:

FinalScoure0 Math:99.9 Liter:99.9 FinalScoure1 Math:88.8 Liter:88.8 FinalScoure2 Math:88.2 Liter:78.9 FinalScoure3 Math:77.7 Liter:77.8 FinalScoure4 Math:77.7 Liter:77.7

 

六、Binary Search:

     The binary search algorithm seached for specified element in a sorted list.The algorithm is implemented in two forms.

     Collections.binarySearch(list, key)//sorted list,searched key. Collections.binarySearch(list, key, c)//list,searched key,comparator

    The first form assuems the collection is sorted into ascending order according to its natural ordering of the data items.If search a specified item in a collection without ordering,it will return -1 even though the specified item existed.

      package com.citibank.aimy.example.collections; import java.util.ArrayList; import java.util.Collections; public class OrderObject { private enum Colors{RED,BLUE,REEN,YELLOW,GRAY;} /** * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub FinalScore s0 = new FinalScore(77.7,77.8); FinalScore s1 = new FinalScore(77.7,77.7); FinalScore s2 = new FinalScore(88.8,88.8); FinalScore s3 = new FinalScore(99.9,99.9); FinalScore s4 = new FinalScore(88.2,78.9); ArrayList a = new ArrayList(); a.add(s0); a.add(s1); a.add(s2); a.add(s3); a.add(s4); System.out.println("s2 exists:"+Collections.binarySearch(a, s1)); } } The result: s2 exists:-1

  The second form take a comparator in addition to a list and a search key.It assumes the list is sorted into acsending order according to the specified comparator.The sort algorithm can be used to sort the list prior to calling the search algorithm.