Java中的HashSet

Question

I study programming java, and i have problem about java.util.HashSet my question, How to show size() in hashset this my code

// class Student

package name;
public class Student
{
   private String name;
   public String getName()
   {
      return this.name;
   }
   public void setName(String name)
   {
     this.name = name;
   }
}
// Entry Point
package client;
import name.Student;
import java.util.Set;
import java.util.HashSet;
import java.util.Scanner;
public class Client1
{
    public static void main (String[] args)
    {
       Scanner scan = new Scanner(System.in);
       Set<Student> students = new HashSet<Student>();
       Student student = new Student();

       int totalStudent = 0;
       System.out.print("TypeTotal Student : ");
       totalStudent = Integer.parseInt(scan.nextLine());

       for(int i = 0; i < totalStudent ; i++)
       {
           System.out.print("Name : ");
           String name = scan.nextLine();
           student.setName(name);
           students.add(student);
       }
       System.out.println("Element Total In Set :" students.size());
       for(Student std: students)
       {
           System.out.println(std.getName());
       }
    }
}

if i run this code in terminal student.size() not increase. i need advice

Answer 1

That's because you're adding the same student everytime in hashset. And HashSet doesn't allow duplicate values.

Try this way.

// class Student

package name;
public class Student
{
   private String name;
   public String getName()
   {
      return this.name;
   }
   public void setName(String name)
   {
     this.name = name;
   }
}
// Entry Point
package client;
import name.Student;
import java.util.Set;
import java.util.HashSet;
import java.util.Scanner;
public class Client1
{
    public static void main (String[] args)
    {
       Scanner scan = new Scanner(System.in);
       Set<Student> students = null;
       students = new HashSet<Student>();
       Student student;

       int totalStudent = 0;
       System.out.print("TypeTotal Student : ");
       totalStudent = Integer.parseInt(scan.nextLine());

       for(int i = 0; i < totalStudent ; i++)
       {
           System.out.print("Name : ");
           String name = scan.nextLine();
           student = new Student();
           student.setName(name);
           students.add(student);
       }
       System.out.println("Element Total In Set :" students.size());
       for(Student std: students)
       {
           System.out.println(std.getName());
       }
    }
}

Answer 2

If you use this block:

{
     System.out.print("Name : ");
     String name = scan.nextLine();
     student.setName(name);
     students.add(student);
}

you are using always same object and Hashset does not allow duplicate (even if you change one property of the object).

Using this block it works fine, because you are creating a new object every interaction:

{
    System.out.print("Name : ");
    String name = scan.nextLine();
    student = new Student();
    student.setName(name);
    students.add(student);
}

even if the variable is the same (student) you create a new instance with new

Answer 3

In your code, Student student is a reference to exactly one object (ever). Since you do not allocate a new Student each time you add an object to the students set, you are, instead, adding the same object to the Set multiple times.

Note that I used the words "same object" and "Set". A Set allows no more than one instance of an object.

Your code is pretending to add multiple students, but is actually only adding one student.

The solution is as follows:

 // really add a student to the set.
 System.out.print("Name : ");
 String name = scan.nextLine();
 student = new Student();// note this line.
 student.setName(name);
 students.add(student);