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.
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)
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 ;)
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.
Related
I am trying to call the array variables in the reference class, try to sort them using a user-defined method and call the method onto the case statement that will be invoked if the user chooses a particular number. I wanted to provide the user the option what attribute of a student will be sorted (i.e. name, course...) and show the sorted one dimensional array called in the case statements and invoked through the main method.
Here's the variables in the Reference class:
class RecordReference {
private int idNumber;
private String firstName = "";
private String middleName = "";
private String lastName = "";
private int age;
private String yearLevel;
private String course = "";
private double gwa;
public RecordReference(int i, String f, String m, String l, int a, String y, String c, double g) {
idNumber = i;
firstName = f;
middleName = m;
lastName = l;
age = a;
yearLevel = y;
course = c;
gwa = g;
}
public int getIdNumber() {
return idNumber;
}
public String getFirstName() {
return firstName;
}
public String getMiddleName() {
return middleName;
}
public String getLastName() {
return lastName;
}
public int getAge() {
return age;
}
public String getYearLevel() {
return yearLevel;
}
public String getCourse() {
return course;
}
public double getGwa() {
return gwa;
}
public void setIdNumber(int idnumber) {
idNumber = idnumber;
}
public void setFirstName(String fName) {
firstName = fName;
}
public void setMiddleName(String mName) {
middleName= mName;
}
public void setLastNameName(String lName) {
lastName= lName;
}
public void setAge(int a) {
age = a;
}
public void setYearLevel(String yLevel) {
yearLevel = yLevel;
}
public void setCourse(String c) {
course = c;
}
public void setGwa(int gwa) {
gwa = gwa;
}
public String toString() {
return String.valueOf(System.out.printf("%-15s%-15s%-15d%-15d%n",
firstName, course , yearLevel ,gwa));
}
} // end of class
And I am trying to call it in this sort method, but I don't know how to reference it.
public static void sortFirstNameArray(String[] f){
for (int i = 0; i < f.length - 1; i++) {
for (int j = i + 1; j < f.length; j++) {
if (f[i].compareToIgnoreCase(f[j]) > 0) {
String temp = f[i];
f[i] = f[j];
f[j] = temp;
}
}
}
}
After the sorting is successfully done, I'll call it in a switch case statements that will be invoked once the user chooses a particular number. This part has 5 case statements (Name, Age, Course, General Weighted Average and the option to sort it all - I plan to add more student attributes if this works)
(I don't know if I should store this in another method and call it in the main method or just put it in the main method like that)
public RecordReference Process(RecordReference[] f, RecordReference[] a) {
// for loop?
for (int x = 0; x < f.length; x++) {
switch (choice) {
case 1:
System.out.println("Sorted array of first name: ");
sortFirstNameArray(f[x].getFirstName());
System.out.printf("%-15s%n", Arrays.toString(f));
break;
case 2:
System.out.println("Sorted array of age: ");
// invokes the age method
sortAgeArray(a[x].getAge());
System.out.printf("%-15s%n", Arrays.toString(a));
break;
}
}
}
If it is in another method, what param do I include when I call it in the main method?
I tried this but it doesn't work, I don't know what to do
System.out.print("Please choose what student attribute you want to
sort :");
choice = keyboard.nextInt();
// calling the process method here, but I receive syntax error
Process(f,a); // Here, I want to pass the sorted values back into the array but I get an error.
If you can help me out that would be great. Thank you in advance.
I'm just a first year student and I am eager to learn in solving this error.
It's good to see that you have attempted the problem yourself and corrected your question to make it clearer, because of that I am willing to help out.
I have tried to keep the solution to the problem as close to your solution as possible, so that you are able to understand it. There may be better ways of solving this problem but that is not the focus here.
First of all, let's create a class named BubbleSorter that will hold methods for sorting:
public class BubbleSorter
{
//Explicitly provide an empty constructor for good practice.
public BubbleSorter(){}
//Method that accepts a variable of type RecordReference[], sorts the
//Array based on the firstName variable within each RecordReference
//and returns a sorted RecordReference[].
public RecordReference[] SortByFirstName(RecordReference[] recordReferencesList)
{
for (int i = 0; i < recordReferencesList.length - 1; i++) {
for (int j = i + 1; j < recordReferencesList.length; j++) {
if (recordReferencesList[i].getFirstName().compareToIgnoreCase
(recordReferencesList[j].getFirstName()) > 0) {
RecordReference temp = recordReferencesList[i];
recordReferencesList[i] = recordReferencesList[j];
recordReferencesList[j] = temp;
}
}
}
return recordReferencesList;
}
}
That gives us a class that we can instantiate, where methods can be added to be used for sorting. I have added one of those methods which takes a RecordReference[] as a parameter and sorts the RecordReference[] based on the firstName class variable within each RecordReference. You will need to add more of your own methods for sorting other class variables.
Now for the main class:
class Main {
public static void main(String[] args) {
//Get a mock array from the GetMockArray() function.
RecordReference[] refArray = GetMockArray();
//Instantiate an instance of BubbleSorter.
BubbleSorter sorter = new BubbleSorter();
//Invoke the SortByFirstName method contained within the BubbleSorter
//and store the sorted array in a variable of type RecordReference[] named
//sortedResult.
RecordReference[] sortedResult = sorter.SortByFirstName(refArray);
//Print out the results in the sorted array to check if they are in the correct
//order.
//This for loop is not required and is just so that we can see within the
//console what order the objects in the sortedResult are in.
for(int i = 0; i < sortedResult.length; i++)
{
System.out.println(sortedResult[i].getFirstName());
}
}
public static RecordReference[] GetMockArray()
{
//Instantiate a few RecordReferences with a different parameter for
//the firstName in each reference.
RecordReference ref1 = new RecordReference(0, "Ada", "Test", "Test", 22, "First",
"Computer Science", 1.0f);
RecordReference ref2 = new RecordReference(0, "Bob", "Test", "Test", 22, "First",
"Computer Science", 1.0f);
RecordReference ref3 = new RecordReference(0, "David", "Test", "Test", 22,
"First", "Computer Science", 1.0f);
//Create a variable of type RecordReference[] and add the RecordReferences
//Instantiated above in the wrong order alphabetically (Based on their firstName)
//class variables.
RecordReference[] refArray = {
ref2, ref3, ref1
};
return refArray;
}
}
In the main class I have provided verbose comments to explain exactly what is happening. One thing I would like to point out is that I have added a method named GetMockArray(). This is just in place to provide a RecordReference[] for testing and you probably want to do that somewhere else of your choosing.
If anything is not clear or you need some more assistance then just comment on this answer and I will try to help you further.
Thanks.
As part of the curriculum at my school, we are working on some CodeHS Java.
There is one problem that I'm stuck on:
Taking our Student and Classroom example from earlier, you should fill in the method getMostImprovedStudent, as well as the method getExamRange. The most improved student is the one with the largest exam score range.
To compute the exam score range, you must subtract the minimum exam score from the maximum exam score.
For example, if the exam scores were 90, 75, and 84, the range would be 90 - 75 = 15.
This is the Student class which I added my method getExamRange().
import java.util.*;
public class Student
{
private static final int NUM_EXAMS = 4;
private String firstName;
private String lastName;
private int gradeLevel;
private double gpa;
private int[] exams;
private int numExamsTaken;
public static int[] examRange = new int[Classroom.numStudentsAdded];
private int i = 0;
/**
* This is a constructor. A constructor is a method
* that creates an object -- it creates an instance
* of the class. What that means is it takes the input
* parameters and sets the instance variables (or fields)
* to the proper values.
*
* Check out StudentTester.java for an example of how to use
* this constructor.
*/
public Student(String fName, String lName, int grade)
{
firstName = fName;
lastName = lName;
gradeLevel = grade;
exams = new int[NUM_EXAMS];
numExamsTaken = 0;
}
public int getExamRange()
{
Arrays.sort(exams);
examRange[i] = exams[exams.length-1] - exams[0];
i++;
return exams[exams.length-1] - exams[0];
}
public String getName()
{
return firstName + " " + lastName;
}
public void addExamScore(int score)
{
exams[numExamsTaken] = score;
numExamsTaken++;
}
// This is a setter method to set the GPA for the Student.
public void setGPA(double theGPA)
{
gpa = theGPA;
}
/**
* This is a toString for the Student class. It returns a String
* representation of the object, which includes the fields
* in that object.
*/
public String toString()
{
return firstName + " " + lastName + " is in grade: " + gradeLevel;
}
}
And this is the Classroom class in which I added the method getMostImprovedStudent().
import java.util.*;
public class Classroom
{
Student[] students;
static int numStudentsAdded;
public Classroom(int numStudents)
{
students = new Student[numStudents];
numStudentsAdded = 0;
}
public Student getMostImprovedStudent()
{
Arrays.sort(Student.examRange);
//return Student.examRange[0];
}
public void addStudent(Student s)
{
students[numStudentsAdded] = s;
numStudentsAdded++;
}
public void printStudents()
{
for(int i = 0; i < numStudentsAdded; i++)
{
System.out.println(students[i]);
}
}
}
I can get the exam Range by sorting the exams array then subtracting the smallest from the biggest, but once I do this, how do I find the student with the biggest exam range, and return it?
The way you would do this is looping through students, and have a variable to hold the biggest difference in score, and the most improved student:
public Student getMostImprovedStudent()
{
Student mostImproved = students[0];
int biggest = student[i].getExamRange();
for(int i = 1; i < students.length; i++) {
if(students[i].getExamRange() > biggest) {
mostImproved = students[i];
biggest = students[i].getExamRange();
}
}
return mostImproved;
}
However Java 8+ we can do:
public Student getMostImprovedStudent()
{
return Arrays.stream(students)
.max(Comparator.comparing(Student::getExamRange))
.get();
}
Which is assuming that students is not empty
As I explained in the comment above you can do it this way:
public Student getMostImprovedStudent() {
Student maxRangeStudent = null;
int maxRange = 0;
for (Student student: students) {
int curExamRange = student.getExamRange();
if (curExamRange > maxRange){
maxRangeStudent = student;
maxRange = curExamRange;
}
}
return maxRangeStudent;
}
I'm trying to take data out of a txt file and create comparable objects out of the data and add them to an array. After that array is created, I want to make a 2d array that stores a 1 in a slot if two options meet the requirements. I keep getting a String index out of range: 0 error though and I do not know where it comes from.
import java.util.*;
import java.io.*;
public class CourseScheduler
{
public int numberOfCourses;
public int[][] adjacent;
public Course[] courses;
public CourseScheduler(String filename)
{
File file = new File(filename);
try{
Scanner scan = new Scanner(file);
numberOfCourses = scan.nextInt();
courses = new Course[numberOfCourses];
adjacent = new int[numberOfCourses][numberOfCourses];
scan.useDelimiter(",|\\n");
for(int i = 0; i < numberOfCourses;i ++){
if(scan.hasNext()){
String dept = scan.next();
String num = scan.next();
String building = scan.next();
String room = scan.next();
String instruct = scan.next();
courses[i] = new Course(dept, num, building, room, instruct);
}
}
}
catch(FileNotFoundException ex){
System.out.println("File was not found");
}
for(int x = 0;x<numberOfCourses;x++){
for(int y = 0;y<numberOfCourses;y++){
adjacent[x][y] = (courses[x].compare(courses[y]));
}
}
}
This is the code for the main class
public class Course{
String department;
String courseNum;
String buildingCode;
String roomCode;
String instructorName;
public Course(String dept, String number, String building, String room, String instructor){
department = dept;
courseNum = number;
buildingCode = building;
roomCode = room;
instructorName = instructor;
}
public String getDept(){
return department;
}
public String getCourse(){
return courseNum;
}
public String getBuilding(){
return buildingCode;
}
public String getRoom(){
return roomCode;
}
public String getInstructor(){
return instructorName;
}
public String toString(){
return department + ";" + courseNum + ";" + buildingCode + ";" + roomCode + ";" + instructorName;
}
public int compare(Course comp){
int ans = 1;
String compNum = comp.getCourse();
if(instructorName == comp.getInstructor())
ans = 0;
if(buildingCode == comp.getBuilding()){
if(roomCode == comp.getRoom())
ans = 0;
}
if(department == comp.getDept()){
if(courseNum.charAt(0) == compNum.charAt(0))
ans = 0;
}
return ans;
}
}
this is the code for the course class
Educated guess: Most likely your error is coming from this line:
if(courseNum.charAt(0) == compNum.charAt(0))
ans = 0;
Either courseNum or compNum are empty.
I did not try to compile and run it but its seems that the exception comes from this line
if(courseNum.charAt(0) == compNum.charAt(0))
If a string is empty, charAt(0) will throw exactly the given exception.
Tip: if you don't know how to use a debugger, use the old fashioned System.out.println(). Put println() here and there in your code to understand how it works.
package chapter10;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class Customer {
private String name;
private String streetAddress;
private String phoneNumber;
private int total;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getStreetAddress(){
return streetAddress;
}
public void setStreetAddress(String streetAddress) {
this.streetAddress = streetAddress;
}
public String getPhoneNumber() {
return phoneNumber;
}
public void setPhoneNumber(String phoneNumber) {
this.phoneNumber = phoneNumber;
}
public int getTotal(){
return total;
}
public void setTotal(int total){
this.total = total;
}
public static void assign(){
int a = (int) (Math.random() + 10);
int r = (int) (Math.random() + 10);
int f = (int) (Math.random() + 10);
System.out.println(a + r + f + "x" + "x" + "x");
}
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
ArrayList <Customer > customerList = new ArrayList <Customer>();
char ans;
do
{
Customer customer = new Customer();
System.out.print("Customer name ");
customer.setName(in.next());
int i = 0;
++i;
System.out.print("Street Address ");
customer.setStreetAddress(in.next());
System.out.print("Phone Number ");
customer.setPhoneNumber(in.next());
customerList.add(customer);
System.out.println("Enter total sales ");
customer.setTotal(in.nextInt());
System.out.println("Would you like to enter in a new customer ( y/n)? ");
String answer = in.next();
ans = answer.charAt(0);
((String) customerList).concat("")
} while(ans == 'y');
for(Customer c: customerList){
System.out.print(c.getName() + "\n" + "Phone number is " +c .getPhoneNumber() +"\n" + "Total sales is "+ c.getTotal() + "\n" + "Address is"+ c.getStreetAddress());
}
for(int i=0; i<customerList.size(); i++){
//System.out.print(customerList.get(i).getName());
}
}
}
I need to assign a number to each value in the arraylist but i am getting an error that says that I have to convert to string (arraylist). How do I add it?
If what I gather from the comments is correct then I believe this is what you want:
Your current assign() is incorrect if you want random values 1-10, it should look like this instead:
public String assign(){
Random rand = new Random();
int a = rand.nextInt(10) + 1;
int r = rand.nextInt(10) + 1;
int f = rand.nextInt(10) + 1;
return a+r+f+"xxx";
}
Customer will look like this:
public class Customer {
private String name;
private String customerNumber;
private String streetAddress;
private String phoneNumber;
private int total;
...
...
...
public String getCustomerNumber() { return this.customerNumber; }
public void setCustomerNumber(String cNumber) { this.customerNumber = cNumber; }
And assigning the numbers should look like this:
for(Customer c : customerList) {
c.setCustomerNumber(assign());
}
Also avoid this line of code, it's a really bad idea:
((String) customerList).concat("")
You should probably rename the customerNumber to customerID.
Hiiii
As i understand you are trying to to add number to each value to arrayList ,And same time you are creating arrayList of customer object, So first understand about arrayList of object,
Customer c1 = new Customer();
Customer c2 = new Customer();
ArrayList<Customer> al = new ArrayList();
al.add(c1);
al.add(c2);
Here this ArrayList object save only address of customer object so how can you change the address of Customer object ; You can not add number in Customer type ArrayList Object,
There is another way typecast your ArrayList to Object type and then no need to typecast again , you can add any type of object in ArrayList
like this,
Customer c1 = new Customer();
Customer c2 = new Customer();
ArrayList<Object> al = new ArrayList();
al.add(c1);
al.add(c2);
In your code there's the following line:
((String) customerList).concat("")
Trying to cast a List to a String is doomed to failure - I'm not sure why do you think it should work.
If you want a String representation of the list, you should implement a toString() method in class Customer and then you can do something like:
System.out.println(Arrays.toString(customerList.toArray()));
Instead of using ArrayList, you can use Map. In which you can have the number as key and value as Customer.
http://examples.javacodegeeks.com/java-basics/java-map-example/ Contains the example of using Map
Answer in Storing a new object as the value of a hashmap? contains info about how to use Object as value in HashMap
I have the problem that I can't take a number from the arraylist and make it into a int or double which I have to do to cacluate the BMI using weight and height. Please have a look!
The assignment is to put in guests' weight, length, and name and sort the ones with a bad length to height ratio out. In the main I create an array with a couple of guests and when I run it says:
"Exception in thread "main" java.lang.NumberFormatException: For input string "name"
and
java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:492)
at java.lang.Integer.parseInt(Integer.java:527)
at Diet.putOnDiet(Diet.java:12)
at TestDiet.main(TestDiet.java:7)
The Diet class is as follows:
public class Diet{
public static ArrayList<Guest> putOnDiet(ArrayList <Guest> list){
ArrayList<Guest> namn = new ArrayList<Guest>();
ArrayList<Guest> hej = new ArrayList<Guest>();
for(int i = 0; i<=list.size()/3; i = i+3){
int langd = Integer.parseInt(list.get(i+1).toString()); //I dont know how to make this work
double vikt = Double.parseDouble(list.get(i).toString());
String name = list.get(i+2).toString();
if ((vikt) > 1.08*(0.9*(langd -100))){
namn.add(new Guest(vikt, langd, name));
}
}
return namn;
}
}
And the Guest class:
public class Guest {
private double weight; private double length; private String name;
public Guest(double weight, double length, String name){
this.name = name; this.weight = weight; this.length = length; // Maybe the problem is here. How do you modify the class to make it work?
}
public String getName() {
return name;
}
public double getWeight()
{
return weight;
}
public double getLength() {
return length;
}
public void setName(String name) {
this.name = name;
}
public void setWeight(double weight) {
this.weight = weight;
}
public void setLength(double length)
{ this.length = length;
}
public boolean isSlim() {
if (weight >= 1.08 * 0.9 * (length - 100)) {
return false;
}
else
return true;
}
public String toString() {
return name + "\n" + weight + "\n" + length;
}
}
Are you sure that the you are parsing an integer?
Well number parsing exception is thrown when it can't parse the number. When the string is not a number like "somthing#$%^&". So try replacing this line
int langd = Integer.parseInt(list.get(i+1).toString());
with this
try {
int langd = Integer.parseInt(list.get(i+1).toString());
} catch (NumberFormatException e) {
System.out.println(list.get(i+1).toString() +" : This is not a number");
System.out.println(e.getMessage());
}
EDIT After reading WOUNDEDStevenJones answer I also think you should not be even using toString() or parsing methods. See WOUNDEDStevenJones answer for more details.
It looks like you'll want to change it to
for (int i=0; i<list.size(); i++) {
double langd = list.get(i).getLength();
double vikt = list.get(i).getWeight();
String name = list.get(i).getName();
}
and kind of ignore your getString() method for this purpose
Note: I'm not sure what you're trying to do with your different indexes, but they'll probably all be .get(i)
The method list.get(i) will return an object of type Guest. Guest has methods, getWeight() and getLength().
list.get(i).getWeight()
This would actually give you a double value in return.
And,
Integer.parseInt(list.get(i).getWeight().toString())
This should be able to parse.
Hope this helps.
_san