bubble sorting an array of a class

Question

I wrote a program that is supposed to read in records from a file and enter them into an array of a Student class. I then need to sort them by name.

import java.io.*;
import java.util.Scanner;

public class StudentTest 
{

public static void main(String[] args) 
{
    String name;
    String address;
    String major;
    double gpa;
    int classLevel;
    int college;
    String blank;
    String idNumber;

    Scanner fileIn = null;
    try
    {
        fileIn = new Scanner (new FileInputStream("student.dat"));
    }
    catch (FileNotFoundException e)
    {
        System.out.println("File not found");
        System.exit(0);
    }

    Student[] aStudent = new Student[15];
    int index = 0;

    for (index=0; index <= 15; index++)
    {
        while (fileIn.hasNext())
        {
        name = fileIn.nextLine();
        address = fileIn.nextLine();
        major = fileIn.nextLine();
        gpa = fileIn.nextDouble();
        classLevel = fileIn.nextInt();
        college = fileIn.nextInt();
        fileIn.nextLine();
        idNumber = fileIn.nextLine();
        aStudent[index] = new Student(name, address, major, gpa, classLevel, college, idNumber);
        aStudent[index].Display();
        }
    }
    Student temp = null;
    for (int pass = 0; pass < (index-1); pass++)
    {
        for (int c = 0; c < (index - 1); c++)
        {
            if  (aStudent[].getName() > aStudent[c+1].getName())
            {
                temp = aStudent[];
                aStudent[]=aStudent[+1];
                aStudent[+1]=temp;
            }
        }
    }
}
}

import java.util.Scanner;
public class Student 
{
private String name;
private String address;
private String major;
private double gpa;
private int classLevel;
private int college;
private String idNumber;
Scanner keyboard = new Scanner(System.in);

    public Student(String name, String address, String major, double gpa, int classLevel, int coll, String idNum)
    {
        this.name = name;
        this.address = address;
        this.gpa = gpa;
        this.major = major;
        this.classLevel = classLevel;
        this.college = coll;
        this.idNumber = idNum;

    }
    public String getName()
    {
        return name;
    }
    public String getAddress()
    {
        return address;
    }
    public String getMajor()
    {
        return major;
    }
    public double getGPA()
    {
        return gpa;
    }
    public int getClassLevel()
    {
        return classLevel;
    }
    public int getCollege()
    {
        return college;
    }
    public String getID()
    {
        return idNumber;
    }
    public void setAddress(String address)
    {
    }
    public void setMajor(String maj)
    {
    }
    public void setCollege(int coll)
    {
    }
    public void Display()
    {
        System.out.println("Name: "+getName());
        System.out.println("Address: "+getAddress());
        System.out.println("Major: " + getMajor());
        System.out.println("GPA: "+getGPA()+" Class Level: "+getClassLevel()+" College: "+getCollege());
        System.out.println("ID: "+getID());
        System.out.println("===============================");
    }
}

I wrote the sort the way my proffessor described it in class, but I am still getting errors that "the > operator is undefined for the argument type(s) java.laungage.String"

Any help would be greatly appreciated.

thanks

Edit: I used Ashan's suggestion and now it looks like this.

    for (int pass = 0; pass < (index-1); pass++)
    {
        for (int c = 0; c < (index - 1); c++)
        {
            if  (aStudent[c].getName().compareTo(aStudent[c+1].getName()) > 0)
            {
                temp = aStudent[c];
                aStudent[c]=aStudent[+1];
                aStudent[+1]=temp;

That cleared up that error. However, I am now getting a NullPointerException.

Answer 1

You cannot compare strings using operator such as < , > . To compare strings there is a methodd provided in String class called compareTo . This method compares two strings lexicographically.

compareTo returns

• 0 incase both the strings are lexicographically equal
• -1 if the calling string is lexicographically smaller than the input string
• 1 if the calling string is lexicographically larger than the input stirng

You can replace the following condition

    if  (aStudent[].getName() > aStudent[c+1].getName())

using compareTo method as:

    if  (aStudent[].getName().compareTo(aStudent[c+1].getName()) > 0)

Answer 2

I think error is because you cannot compare how big is the name or how small it is. Making a bubble search to sort names by alphabetical order, you need to check their first characters ASCII . Which is a pretty easy thing to do. I am not good at Java , but C++ . So algorithms are same ;) Good luck ;)

Answer 3

You may want to use compareTo() of String. > and < are used for int, float numbers, characters and etc., not for objects like strings. If objects support compare operations, it must implement the Comparable interface, in which you will define the compareTo() method.

This method will return -1 if it is less than the other, 0 if they are equal and 1 if it is greater than the other object.