Simple ArrayList program-JAVA

Question

I'm trying to solve this question:

Define a class Student with attributes studentID and marks in the module 1030Y. The class Student must also contain a default constructor along with a constructor to initialize an object of type Student with user-defined values, mutator and accessor methods for each attribute and a display method. Write a test program that maintains an ArrayList of Student objects (with IDs in the range 701-799 and marks in the range 0.0 to 100.0). The program will allow the user to input the student id number and the mark for each student. After all the input has been done, the program will display the ids of the students with the highest and the lowest marks.

For some reason the part where i'm trying to get the lowest and highest mark of the students is not working.

Here's my codes:
Student.java:

package Number5;

public class Student {
private int studentID;
private float mark;

public Student()
{
    studentID = 0;
    mark = 0;
}

public Student(int id, float marks)
{
    this.studentID = id;
    this.mark = marks;
}

public void setID(int id)
{
    this.studentID = id;
}

public int getID()
{
    return studentID;
}

public void setMark(float marks)
{
    this.mark = marks;
}

public float getMark()
{
    return mark;
}

public void display()
{
    System.out.println("Student ID: "+getID());
    System.out.println("Marks: "+getMark());
}
}

testStudent.java:

package Number5;
import java.util.ArrayList;
import java.util.Scanner;

public class testStudent {

public static void main(String[] args) {
    ArrayList<Student> students = new ArrayList<Student>();
    int id=0; float mark=0;
    Scanner input = new Scanner(System.in);

    do
    {
        System.out.print("Enter the student ID: ");
        id = input.nextInt();
        System.out.print("Enter the marks: ");
        mark = input.nextFloat();
        students.add(new Student(id,mark));
    }
    while(id != 0 || mark != 0);

    int smallest = 9999, largest = -9999;

    for(int i=0; i<students.size(); i++)
    {
        while(smallest > students.get(i).getMark())
        {
            smallest = students.get(i).getID();
        }

        while(largest < students.get(i).getMark())
        {
            largest = students.get(i).getID();
        }
    }

    System.out.println("Smallest is "+smallest);
    System.out.println("Largest is "+largest);
}
}

The program just stops after reading the user input. It doesn't even go until the for loop.

Obviously my codes could be wrong, right? If i knew exactly where i wouldn't be asking. — Manisha Singh Sanoo
– Manisha Singh Sanoo, Commented Apr 30, 2015 at 21:08
@downvoter Can you explain why please? So that i don't make the same mistake hereafter. Is being a beginner in java wrong? — Manisha Singh Sanoo
– Manisha Singh Sanoo, Commented Apr 30, 2015 at 21:13

BlackHatSamurai · Accepted Answer · 2015-04-30 21:19:33Z

Your problem appears to be the fact that you are entering into infinite loops when you check using your while statement. Use if statements like so:

for(int i=0; i<students.size(); i++)
    {
        if(students.get(i).getMark() < smallest)
        {

            smallest = students.get(i).getID();
        }

        if(students.get(i).getMark() > largest)
        {

            largest = students.get(i).getID();
        }
    }

However, this is going to leave you with another problem, in that you are comparing the mark to the id. You need to check the value of mark against mark, then assign the id. like so:

int largestMark = 0;
inst smallestMark = 9999;
for(int i=0; i<students.size(); i++)
    {
        if(students.get(i).getMark() < smallestMark)
        {
            smallestMark = students.get(i).getMark();
            smallest = students.get(i).getID();
        }

        if(students.get(i).getMark() > largestMark)
        {
            largestMark = students.get(i).getMark();
            largest = students.get(i).getID();
        }
    }

nullPointer · Accepted Answer · 2015-04-30 21:12:46Z

1

Use:

if(smallest > students.get(i).getMark())
{
    smallest = students.get(i).getMark();
}

if(largest < students.get(i).getMark())
{
    largest = students.get(i).getMark();
}

You were comparing marks but then assigning id. To keep track of the student who had the most or least marks, you can add:

studentWithMostMarks = students.get(i).getId()

answered Apr 30, 2015 at 21:12

nullPointer

1337 bronze badges

Comments

Marcello Davi · Accepted Answer · 2015-04-30 21:15:43Z

1

This part of code is probably causing an infinite loop:

for(int i=0; i<students.size(); i++)
{
    while(smallest > students.get(i).getMark())
    {
        smallest = students.get(i).getID();
    }

    while(largest < students.get(i).getMark())
    {
        largest = students.get(i).getID();
    }
}

You have to change it:

for(int i=0; i<students.size(); i++)
{
    if(smallest > students.get(i).getMark())
    {
        smallest = students.get(i).getID();
    }

    if(largest < students.get(i).getMark())
    {
        largest = students.get(i).getID();
    }
}

answered Apr 30, 2015 at 21:15

Marcello Davi

4435 silver badges17 bronze badges

Comments

Haifeng Zhang · Accepted Answer · 2015-04-30 21:19:13Z

1

The program will allow the user to input the student id number and the mark for each student. After all the input has been done, the program will display the ids of the students with the highest and the lowest marks.

while(smallest > students.get(i).getMark()) {
    smallest = students.get(i).getID();
}

while(largest < students.get(i).getMark()) {
  largest = students.get(i).getID();
}

You should assign your mark to smallest or largest, not Id.

You can also tracking the index number of your students which has highest or lowest marks, then get the id and mark from your students arraylist

edited Apr 30, 2015 at 21:19

answered Apr 30, 2015 at 21:13

Haifeng Zhang

32.1k21 gold badges95 silver badges133 bronze badges

6 Comments

nullPointer Over a year ago

even though the while loop works in this case, I'd replace it with an if-statement as it makes the code more readable

Haifeng Zhang Over a year ago

thanks, agree. only pointed the wrong part, not optimize it yet.

Haifeng Zhang Over a year ago

@downvoter. can you explain why downvote? In my opinion, we need to point out OP's logic error and help him out, code optimization is making code more readable and works more efficient, here it is optional.

BlackHatSamurai Over a year ago

This doesn't solve the problem. You didn't change any of the code. It won't print out the ID with the highest or lowest mark. You are also comparing marks to ID's.

Haifeng Zhang Over a year ago

@BlackHatSamurai that's OP's code, not mine, I was pointing out where was not correct. My explanation is underneath

|

xerx593 · Accepted Answer · 2015-05-05 22:01:52Z

1

You need an additional variable:

Currently you store your best/worst student id in smallest/largest, while you compare it with the mark. You need something like:

smallestId;
smallestMark;//initialize biig
largestId;
largestMark;//initialize looow

edited May 5, 2015 at 22:01

answered Apr 30, 2015 at 21:14

xerx593

13.4k5 gold badges38 silver badges77 bronze badges

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