A Collection Framework Question in Java: Multiple Solution Approaches

Question:A class30, the student number of the student is20070301-20070330, all students have taken the Java Programming Design course, and all students' grades are given (random numbers can be generated, ranging60-100), please write a program to sort and print the grades of all students in this class from low to high.

Requirements:Implement with List, Map, and Set respectively, the printed information includes student number, name, and grade.

1Using the List collection to implement

import java.util.ArrayList; 
import java.util.Collections; 
import java.util.Comparator; 
import java.util.Iterator; 
import java.util.LinkedList; 
import java.util.TreeMap; 
public class Test2{ 
  public static void main(String[] args){ 
    /* Here ArrayList is used 
     * 
     * ArrayList<Student>al=new ArrayList<Student>(); 
    for(int i=20070301,j=10;i<=20070330;i++,j++) 
    { 
      al.add(new Student(i,(int) (40*Math.random()+60), "Student"+j)); 
    } 
    //ArrayList sorting is implemented using the sort() method in Collections. 
    Collections.sort(al, new Sortbygrade()); 
    for(Student sd:al) 
    System.out.println(sd); 
    */ 
    LinkedList<Student> lt=new LinkedList<Student>(); 
    for(int i=20070301,j=10;i<=20070330;i++,j++) 
    { 
      lt.add(new Student(i,(int) (40*Math.random()+60), "Student"+j)); 
    } 
    //Sort the linked list 
    Collections.sort(lt, new Sortbygrade()); 
    //Output the linked list 
    for(Student sd:lt) 
      System.out.println(sd); 
  } 
} 
//Student class 
class Student{ 
  int num,grade; 
  String name; 
  //Constructor 
  public Student(int num,int grade,String name){ 
    this.num=num; 
    this.name=name; 
    this.grade=grade; 
  } 
  //Here it is necessary to override 
  public String toString(){ 
//   System.out.println("hi"); 
    return "Student ID:"+this.num+"\t"+"Name:"+this.name+"  "+"成绩:"+this.grade; 
  } 
} 
//Create a comparator class 
class Sortbygrade implements Comparator<Student>{ 
  @Override 
  public int compare(Student s1, Student s2) { 
    if(s1.grade>s2.grade) 
      return 1; 
    if(s1.grade<s2.grade) 
      return -1; 
    if(s1.grade==s2.grade) 
      return s1.name.compareTo(s2.name); 
    return 0;     
  } 
} 

　The output result is shown in the figure:

On the List collection framework'sSummary:

1In fact, the List collection is a dynamic array, elements can be directly retrieved through a for loop without iteration.
2, when outputting a List collection, the default method called is the toString() method of the objects stored in the collection, so it needs to be overridden in the class.
If the toString() method is not overridden, it must be used

for(int i=0;i<lt.size();i++) 
{ 
  Student s=lt.get(i); 
  System.out.println("学号："+s.num+"  Name:",+s.name+"  Score:",+s.grade); 
} 

3, sorting of List collection requires the use of the Collections utility class, i.e., Collections.Sort(list, new Comparator class()) method. Therefore, a custom comparator class needs to be defined, defining your own comparison rules.

2, to implement using Set collection
(1to implement using TreeSet

package com.package1; 
import java.util.*; 
public class StuScore { 
public static void main(String[] args) { 
  TreeSet<Student> ts=new TreeSet<Student>(new Com()); 
  //Add elements into 
  for(int i=20070301,j=1;i<=20070330;i++,j++) 
  { 
    ts.add(new Student(i,"Student"+j,(int) (40*Math.random()+60))); 
  } 
  //Iteratively loop to extract 
  Iterator<Student> it=ts.iterator(); 
  while(it.hasNext()) 
  { 
    Student o1=it.next(); 
    System.out.println("学号："+o1.num+" ",+"姓名："+o1.name+" ",+" ",+"Score:",+o1.grade); 
  } 
} 
} 
//Student class 
class Student  
{ 
int num; 
int grade; 
String name; 
public Student(int num, String name, int grade) 
{ 
  this.num=num; 
  this.name=name; 
  this.grade=grade; 
} 
} 
class Com implements Comparator 
{ 
@Override 
public int compare(Object o1, Object o2) { 
  Student s1=(Student) o1; 
  Student s2=(Student) o2; 
  if(s1.grade>s2.grade) 
    return 1; 
  if(s1.grade<s2.grade) 
    return -1; 
  if(s1.grade==s2.grade) 
  { 
    return new Integer(s1.num).compareTo(new Integer(s2.num)); 
  } 
  return 0; 
} 
} 

The output result is:

Student ID:20070307　 Name: Student16　　　 Score:60
Student ID:20070309　 Name: Student18　　　 Score:60
Student ID:20070314　 Name: Student23　　　 Score:61
Student ID:20070318　 Name: Student27　　　 Score:61
Student ID:20070322　 Name: Student31　　　 Score:61
Student ID:20070306　 Name: Student15　　　 Score:62
Student ID:20070310　 Name: Student19　　　 Score:64
Student ID:20070302　 Name: Student11　　　 Score:66
Student ID:20070308　 Name: Student17　　　 Score:68
Student ID:20070321　 Name: Student30　 Score:68
Student ID:20070330　 Name: Student39　　　 Score:69
Student ID:20070303　 Name: Student12　　　 Score:70
Student ID:20070320　 Name: Student29　　　 Score:70
Student ID:20070323　 Name: Student32　　　 Score:77
Student ID:20070313　 Name: Student22　　　 Score:78
Student ID:20070304　 Name: Student13　　　 Score:79
Student ID:20070324　 Name: Student33　　　 Score:83
Student ID:20070326　 Name: Student35　　　 Score:84
Student ID:20070327　 Name: Student36　　　 Score:85
Student ID:20070311　 Name: Student20　 Score:88
Student ID:20070305　 Name: Student14　　　 Score:89
Student ID:20070329　 Name: Student38　　　 Score:89
Student ID:20070316　 Name: Student25　　　 Score:90
Student ID:20070301　 Name: Student10　　　 Score:95
Student ID:20070312　 Name: Student21　　　 Score:96
Student ID:20070317　 Name: Student26　　　 Score:97
Student ID:20070319　 Name: Student28　　　 Score:97
Student ID:20070325　 Name: Student34　　　 Score:98
Student ID:20070315　 Name: Student24　　　 Score:99
Student ID:20070328　 Name: Student37　　　 Score:99

forTreeSetSummary:
1, elements cannot be repeated, and TreeSet is ordered.
2, two sorting methods:
(1Customize a comparator class, for example class Com implements Comparator{ }, and implement compare(Object o1, Object o2method, where the comparison rules are defined.
(2to make the elements themselves comparable.
Steps: Implement the Comparable interface for the elements added to the TreeSet and override the compareTo method. This order is also the natural order of the elements, or what is called the default order.

method1and method2The difference is:

Both methods have their advantages and disadvantages. Using Comparable is simple, as objects that implement the Comparable interface become directly comparable without the need to modify the source code.

The advantage of using Comparator is that you do not need to modify the source code. Instead, you implement another comparator. When a custom object needs to be compared, you can pass the comparator and the object together to compare sizes. And in the Comparator, users can implement complex and general logic themselves, so that it can match some simple objects, which can save a lot of repetitive work.

(2）usingHashSetto implement

package com.package1; 
import java.util.*; 
public class StuScore { 
  public static void main(String[] args) { 
    HashSet<Student> hs=new HashSet<Student>(); 
    //Add elements into 
    for(int i=20070301,j=1;i<=20070330;i++,j++) 
    { 
      hs.add(new Student(i,"Classmate"+j,(int)  
(40*Math.random()+60))); 
    } 
    ArrayList<Student>li=new ArrayList(hs); 
    Collections.sort(li, new Sortbygrade()); 
    for(Student ss:li) 
      System.out.println(ss); 
  } 
} 
//Student class 
class Student  
{ 
  int num; 
  int grade; 
  String name; 
  public Student(int num, String name, int grade) 
  { 
    this.num=num; 
    this.name=name; 
    this.grade=grade; 
  } 
  public String toString(){ 
    //System.out.println("hi"); 
    return "Student ID:"+this.num+"\t"+"Name:"+this.name 
+"  "+"成绩:"+this.grade; 
  } 
} 
class Sortbygrade implements Comparator{ 
  @Override 
  public int compare(Object o1, Object o2) { 
    Student s1=(Student) o1; 
    Student s2=(Student) o2; 
    if(s1.grade>s2.grade) 
      return 1; 
    if(s1.grade<s2.grade) 
      return -1; 
//   if(s1.grade==s2.grade) 
    return 0; 
  } 
} 

The output result is as follows:
Student ID:20070310 Name: Classmate19　　　 Score:60
Student ID:20070330 Name: Classmate39　　　 Score:62
Student ID:20070326 Name: Classmate35　　　 Score:63
Student ID:20070317 Name: Classmate26　　　 Score:64
Student ID:20070318 Name: Classmate27　　　 Score:65
Student ID:20070322 Name: Classmate31　　　 Score:65
Student ID:20070301 Name: Classmate10　　　 Score:67
Student ID:20070328 Name: Classmate37　　　 Score:68
Student ID:20070304 Name: Classmate13　　　 Score:68
Student ID:20070319 Name: Classmate28　　　 Score:69
Student ID:20070313 Name: Classmate22　　　 Score:70
Student ID:20070303 Name: Classmate12　　　 Score:71
Student ID:20070312 Name: Classmate21　　　 Score:71
Student ID:20070329 Name: Classmate38　　　 Score:72
Student ID:20070306 Name: Classmate15　　　 Score:72
Student ID:20070324 Name: Classmate33　　　 Score:72
Student ID:20070305 Name: Classmate14　　　 Score:75
Student ID:20070315 Name: Classmate24　　　 Score:75
Student ID:20070314 Name: Classmate23　　　 Score:78
Student ID:20070307 Name: Classmate16　　　 Score:80
Student ID:20070311 Name: Classmate20 Score:81
Student ID:20070302 Name: Classmate11　　　 Score:83
Student ID:20070309 Name: Classmate18　　　 Score:84
Student ID:20070320 Name: Classmate29　　　 Score:85
Student ID:20070321 Name: Classmate30 Score:85
Student ID:20070316 Name: Classmate25　　　 Score:86
Student ID:20070327 Name: Classmate36　　　 Score:90
Student ID:20070308 Name: Classmate17　　　 Score:94
Student ID:20070323 Name: Classmate32　　　 Score:94
Student ID:20070325 Name: Classmate34　　　 Score:95

forHashSetSummary:
1Elements in HashSet cannot be duplicated. If a duplicate is added, only one will be displayed.
The principle is as follows:
HashSet: The underlying data structure is a hash table. It is not thread-safe and is not synchronized.
2How does HashSet ensure the uniqueness of elements?
答：是通过元素的两个方法，hashCode和equals来完成。
如果元素的HashCode值相同，才会判断equals是否为true。如果元素的hashcode值不同，不会调用equals。
3、对HashSet的排序，通过将Set集合转换为List集合，借助Collections.sort()方法实现排序。

3、使用TreeMap来实现

package com.package1;  
import java.util.Comparator; 
import java.util.Iterator; 
import java.util.Set; 
import java.util.TreeMap; 
public class TestTreeMap { 
  public static void main(String[] args) { 
    //1.创建集合 
    TreeMap<Student, Integer> tm = new TreeMap<Student, Integer>(); 
    for(int i=20070301,j=10;i<=20070330;i++,j++) 
    { 
      int grade = (int) (40*Math.random()+60); 
       //2、向集合对象中添加元素 
       tm.put(new Student(grade,"同学"+j),i); 
    } 
    //3.遍历集合，排序完成  
    Set<Student> k = tm.keySet(); 
    Iterator<Student> it = k.iterator(); 
    while(it.hasNext()){ 
      Student key = it.next(); 
      Integer num = tm.get(key); 
      System.out.println("学号："+num+"  "+"姓名："+key.name+"  "+"成绩:"+key.grade); 
    } 
  } 
} 
class Student implements Comparable<Student> { 
  int grade; 
  String name; 
  public Student(int grade, String name) { 
    this.grade = grade; 
    this.name=name; 
  } 
  @Override 
  public int compareTo(Student o) { 
    if(this.grade>o.grade)  
      return 1;  
      if(this.grade==o.grade)  
      { //Sort by name when the scores are the same 
       return this.name.compareTo(o.name);  
      }  
      return -1;  
  } 
} 

　The output result is:

Student ID:20070303　　　 Name: Student12　　　 Score:61
Student ID:20070323　　　 Name: Student32　　　 Score:61
Student ID:20070317　　　 Name: Student26　　　 Score:62
Student ID:20070309　　　 Name: Student18　　　 Score:64
Student ID:20070301　　　 Name: Student10　　　 Score:67
Student ID:20070304　　　 Name: Student13　　　 Score:69
Student ID:20070322　　　 Name: Student31　　　 Score:69
Student ID:20070328　　　 Name: Student37　　　 Score:70
Student ID:20070305　　　 Name: Student14　　　 Score:71
Student ID:20070319　　　 Name: Student28　　　 Score:73
Student ID:20070321　　　 Name: Student30 Score:74
Student ID:20070310　　　 Name: Student19　　　 Score:81
Student ID:20070315　　　 Name: Student24　　　 Score:82
Student ID:20070307　　　 Name: Student16　　　 Score:84
Student ID:20070330 Name: Student39　　　 Score:84
Student ID:20070312　　　 Name: Student21　　　 Score:85
Student ID:20070324　　　 Name: Student33　　　 Score:87
Student ID:20070306　　　 Name: Student15　　　 Score:88
Student ID:20070308　　　 Name: Student17　　　 Score:90
Student ID:20070327　　　 Name: Student36　　　 Score:90
Student ID:20070318　　　 Name: Student27　　　 Score:91
Student ID:20070316　　　 Name: Student25　　　 Score:92
Student ID:20070320 Name: Student29　　　 Score:92
Student ID:20070314　　　 Name: Student23　　　 Score:93
Student ID:20070313　　　 Name: Student22　　　 Score:94
Student ID:20070302　　　 Name: Student11　　　 Score:95
Student ID:20070325　　　 Name: Student34　　　 Score:95
Student ID:20070329　　　 Name: Student38　　　 Score:97
Student ID:20070326　　　 Name: Student35　　　 Score:98
Student ID:20070311　　　 Name: Student20 Score:99

forTreeMapSummary:
1The TreeMap default sorts the key, so you can put a custom object into the key, and put the integer representing the student number into the value. When sorting the key, you can specify a certain attribute of the custom object to sort.
2The Map collection uses the put() method to add elements.
3The principle of extracting from the Map collection: convert the map collection into a set collection and then retrieve it through the iterator. The two ways to retrieve the map collection are:
(1Set<k> keySet:Store all the keys in the map into the Set collection. Because the set has an iterator, all keys can be retrieved in an iterative manner, and the value corresponding to each key can be obtained by the get method.
(2Set<Map.Entry<k,v>> entrySet:The mapping relationship in the map collection is stored in the set collection, and the data type of this relationship is: Map.Entry

That's all for this article. I hope it will be helpful to everyone's learning, and I also hope everyone will support the Shouting Tutorial more.

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