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Java-Selection sorting an array of objects by an int key and displaying in table

I am currently working on a project for school and am really struggling. I am supposed to selection sort a group of Student objects and then display them in selection sort order.
Create an array with the size of 10 and assign student details (Name, BroncoId, age and TotalMarks) to the array. Perform the selection sort to sort the students in descending order based on their total marks.
a. Steps:
i. Create the student list (use Random class in java to generate the age (15-25) and total (0-100))
ii. Print the Student List in a table format
iii. Perform selection sort based on the total marks of the students
The place I am stuck at currently is making the selection sort. I understand how to create the selection sort, but I can't seem to translate it for this implementation.
My selection sort code:
public static Student[] selectionSort(Student[] studentList)
{
for(int i = 0; i <studentList.length-1; i++)
{
int minIndex = studentList[i].getGrades();
int pos = i;
for(int j = i + 1; j < studentList.length-2; j++)
{
if(studentList[j].getGrades() > studentList[minIndex].getGrades())
{
minIndex = studentList[j].getGrades();
pos = j;
}
}
int temp = studentList[pos].getGrades();
studentList[pos] = studentList[i];
int k = studentList[i].getGrades();
k = temp;
}
return studentList;
}
When I run this code, the console returns:
I sought tutoring to hopefully fix this problem, but my tutor gave me a few nonfunctional suggestions. We were both stumped at the end of the session.
My code for printing:
public static void printStudentInfo(Student[] students)
{
System.out.println("Name: AGE: idNumber: Score:");
for(Student student: students)
{
if(student.getName().length() <= 49)
System.out.printf("%-50s %-5d %-10s %-4d\n", student.getName(), student.getAge(), student.getID(), student.getGrades() );
else
{
System.out.printf("%-50s %-5d %-10s %-4d\n", student.getName().substring(0,48), student.getAge(), student.getID(), student.getGrades() );
System.out.println();
int i = 0;
while(i <= student.getName().length())
{
System.out.printf("%-50s", student.getName().substring(49 +48*i, 97+48*i) );
System.out.println();
i++;
}
}
}
}
As more of an issue out of passion, I sought to make an interesting print method. My problem is, also that I don't really know how to parse and format a string of 155 characters for instance. What do I put in this while lop to accomplish this?
I want the program to output one object name line like:
49 characters
49 chars
…
What ever is left
It probably won't ever go past three lines, but hey, who says I can't give an example like that? What do I put in the header of the while loop to accomplish this?
PS:
Here is the Student class if you need it.
public class Student
{
private String name;
private int age;
private String idNumber;
private int gradePoints;
public Student(String name, int age, String idNumber, int gradePoints)
{
this.name = name;
this.age = age;
this.idNumber = idNumber;
this.gradePoints = gradePoints;
}
public void setName(String name)
{
this.name = name;
}
public void setAge(int age)
{
this.age = age;
}
public void setidNumber(String idNumber)
{
this.idNumber = idNumber;
}
public void setPoints(int gradePoints)
{
this.gradePoints = gradePoints;
}
public String getName()
{
return name;
}
public int getAge()
{
return age;
}
public String getID()
{
return idNumber;
}
public int getGrades()
{
return gradePoints;
}

Welcome to SO Matthew.
Rather than giving you a solution I thought it might be useful to give you a process for solving the problem yourself.
Good practice in software development is to break your problem down into very small components, make sure each of those work perfectly (through unit testing) and then build your solution from those components.
In line with that practice I suggest you do the following:
list each of the individual steps required to do a selection sort on paper.
Pick the simplest one (e.g. swapping two elements).
Write a unit test that would pass if your swap method worked
run the unit test and verify that it fails
write the simplest code you can to make that test pass
write a new test to cover a more complex scenario that isn't yet supported
keep going until you believe that method works perfectly
move onto the next method
once all the components are working perfectly write the method that calls them all using the same process (i.e. test first then code)
If you follow this process then you will end up with a system that you understand perfectly, works, is maintainable, and that you can refactor. It has another very significant benefit: it means when you come to SO with a question you'll be asking about a specific item that you don't know how to solve rather than a 'why doesn't my code work' question. Specific questions tend to get better and faster responses.
In your case, I would start with methods for swapping items (hint: your code for this doesn't work which you'll discover quickly when you write a unit test) and then move on to finding the smallest item in a sublist. Then a method that uses those two to put the smallest item at the start of a sublist. Finally a method that performs that method for all sublist progressively. Make sure each method is working perfectly, including checking validity of arguments, before you move on to putting them together.

Java programming - Class and method

I am new to Java programming and I had a task in class.
So the main thing was I had to make a public class named BankAccount with these 3: int number, String owner, int amount.
Then I had to make an array of 10 with this class and fill it up (the owner names were Name1, Name2, etc.).
Then I had to make a String array of 5 with 5 names in it (for example: "James", "Jack", etc.)
and then I had to kind of "modify" the owner names so that my program will attach one of the 5 names randomly to the end of the current owner names.
So it will be like this for example: Name2Jack, Name3James, etc.
I successfully did all of this.
But then.
My teacher told me to make another method which will decide, how many names do I have out of the 10 ownernames in which name X is present.
So I did this:
public static int Count(BankAccount[] accounts, String name){
int number=0;
for (i=0; i<accounts.length, i++){
if(accounts[i].owner.contains(name)==true)
number++;
}
return number;
}
At least if I remember correctly, it was this. Or something similar like that.
And this worked as well.
But then, my teacher said, how would I do it with not .contains but with .equals ?
And if I would do it with that, would I need 1 or 2 "=" marks?
I had no idea what she means, like I dont know how to do it with .equals... because the owner names are like Name1Jack for example..
She told me I would need 1 "=" mark instead of 2, and that I should look after this for the next class.
Can you guys actually tell me what she meant with this ".equals" method instead of the .contains one?
How could I do this with .equals , and I dont get it why would I need 1 "=" mark instead of 2 whatsoever.
Any help would be really appreciated!

It appears that you need to just change this one line:
From:
if (accounts[i].owner.contains(name) == true)
To:
if (accounts[i].owner.equals(name))
Here is a sample code with both variations:
public class BankAccountDemo {
public static void main(String[] args) {
BankAccount[] bankAccounts = new BankAccount[5];
bankAccounts[0] = (new BankAccount(1, "Name1James", 1000));
bankAccounts[1] = (new BankAccount(2, "Name2Jack", 2000));
bankAccounts[2] = (new BankAccount(3, "Name3Henry", 3000));
bankAccounts[3] = (new BankAccount(4, "Name4Jack", 4000));
bankAccounts[4] = (new BankAccount(5, "Name5James", 5000));
System.out.println("Check A:");
System.out.println(BankAccountDemo.Count(bankAccounts, "James"));
System.out.println(BankAccountDemo.Count2(bankAccounts, "James"));
System.out.println("Check B:");
System.out.println(BankAccountDemo.Count(bankAccounts, "Name5James"));
System.out.println(BankAccountDemo.Count2(bankAccounts, "Name5James"));
}
public static int Count(BankAccount[] accounts, String name) {
int number = 0;
for (int i = 0; i < accounts.length; i++) {
if (accounts[i].owner.contains(name) == true) {
number++;
}
}
return number;
}
public static int Count2(BankAccount[] accounts, String name) {
int number = 0;
for (int i = 0; i < accounts.length; i++) {
if (accounts[i].owner.equals(name)) {
number++;
}
}
return number;
}
}
Run output is:
Check A:
2
0
Check B:
1
1

I have been asked to use arrays to store and manage objects

I have to create a storage system whenever a user gets fuel in their car. The data that needs to be stored is the date, car mileage, number of litres and the cost per litre. a separate class should be created to record these.
I should be able to add in the details of every transaction each time the person gets fuel.
Can anyone help get along the right lines and help me get started? below is my fuel logger class and i do not know how to create the fuel transaction class i was talking about.
public class FuelLogger
{
public static void main (String [] arguments)
{
FuelTransaction Ft1 = new FuelTransaction("10/01/2016", 500, 10, 0.99);
FuelTransaction Ft2 = new FuelTransaction("15/01/2016", 560, 10, 0.99);
FuelTransaction Ft3 = new FuelTransaction();
Ft3.setDate("24/01/2016");
Ft3.setCarMileage(670);
Ft3.setNumberOfLitres(15);
Ft3.setCostPerLitre(1.01);
Ft1.displayDetails();
Ft2.displayDetails();
Ft3.displayDetails();
//Amount of fuel bought between 2 dates
//System.out.println("The total amount of fuel between the two dates is " + FuelTransaction.getFuelAmount(Ft1, Ft3));
System.out.println("The total number of FuelTransactions is " + FuelTransaction.getTotalNum());
}
}

You can create a new Log object and add detail to the object in constructor and store it.

Maybe something like this:
public class FuelTransaction {
// Class variables
String date;
int mileage, numberOfLitres, costPerLitre;
// Constructor where we instantiate the FuelTransaction object
public FuelTransaction(String date, int mileage, int numberOfLitres, int costPerLitre) {
// Takes all the variables passed in and stores them to the class variable
this.date = date;
this.mileage = mileage;
this.numberOfLitres = numberOfLitres;
this.costPerLitre = costPerLitre;
}
public FuelTransaction() {
// Empty constructor, sets everything to "" or 0
this.date = "";
mileage = numberOfLitres = costPerLitre = 0;
}
public void setDate(String date) {
// Setter to set the date
this.date = date;
}
}
This isn't the complete class, you'll have to add to it but these are the basics. Notice how there are 2 constructors, this lets you initialize the FuelTransaction class by specifying all of the variables like new FuelTransaction(DATE, MILEAGE, NUMBEROFLITRES, COSTPERLITRE) but it also lets you call new FuelTransaction() and then manually add the data after instantiating with the setDate() setter:
FuelTransaction ft = new FuelTransaction();
ft.setDate("01-23-45");

How to expand a randomHuman constructor to take two types of objects?

I am a java-noob as I recently started to learn in a course.
I have created a class:Humans which have ability to store their name and age, and also a subclass Students which extends Humans and adds the Year they began there studies.
I have constructed a randomHuman constructor where I call it(in my main class) and create a list with the humans(with random name and age).
My problem is when i want to random 5 human non-students and 5 students and create this list, I'm not sure how to find out what type of object is sent to the random constructor, so i know if i should give it a year or not. And what type to tell the constructor to return.
I am sorry that this turned into an essay, but if anyone would be so kind to help then I would greatly appreciate it.
TLDR; How to expand a randomHuman constructor to take two types of objects?
Here is my main class:
public class Main {
public static void main(String []args){
Human newHuman = new Human( 18, "Tommy");
System.out.println("Age: " + newHuman.getAge());
System.out.println("Name: " + newHuman.getName());
System.out.println(newHuman.toString());
Human putte = new Human (25,"Putte");
System.out.println(putte);
//Varför blir det så?
//kanske lokal variabel
//Array RandomHumans
System.out.println(" ");
System.out.println("Array Human");
ArrayList<Human> myAl = new ArrayList();
for(int i = 0; i<15; i++){
Human xx =Human.randomHuman();
myAl.add(xx);
}
//Array RandomFysiker
for(int j = 0; j<myAl.size(); j++){
Human var = myAl.get(j);
System.out.println(var.toString());
}
System.out.println(" ");
System.out.println("Array Fysiker");
ArrayList<Fysiker> myAl2 = new ArrayList();
//puts the Fysiker in an array
for(int i = 0; i<15; i++){
Fysiker xx =Fysiker.randomHuman();
myAl2.add(xx);
}
//prints teh array
for(int j = 0; j<myAl2.size(); j++){
Fysiker var = myAl2.get(j);
System.out.println(var.toString());
}
}
}
and my Human class:
public class Human {
public String name;
public int age;
Human(int ageIn, String nameIn){ //Constructor
age=ageIn;
name=nameIn;
}
public int getAge(){
return age;
}
public String getName(){
return name;
}
public String toString(){
return "Name: " + getName() +"," + " Age: " + getAge();
}
//Random human
// Behöver ändra konstruktorn så att den kan kolla
// om objectet är Fysiker eller Human och sedan,
// Behandla dom olika
//Problem1: Hur kollar man? Föreslag if(obj instanceof Fysiker), men vad ska jag ha som obj
//Problem2: Vilken returtyp ska man då ha?
public static Human randomHuman(){
String[] anArrayOfStrings={"Tom", "Jon", "Chris","Julian","Roberto","Sam","Lisa","Roxanne","Rebecca","Anton","Johannes","Antonella","Bianca"};
int randomAge = (int) (100*Math.random());
String randomName = anArrayOfStrings[(int)(Math.random()*anArrayOfStrings.length)];
int RandomYear = (int) (Math.random()*(2013-1932) + 1932);
// if(xx instanceof Fysiker){
//
// }
return new Human(randomAge,randomName);
}
}
and the subclass Fysiker(aka student):
/**
*
* #author Julian
*/
public class Fysiker extends Human{
public int schoolYear;
public Fysiker(int startYear,int ageIn, String nameIn){
super(ageIn, nameIn);
if (age>15){
if (startYear>2013){
} else if (startYear<1932){
} else {
schoolYear = startYear;
}
} else {
}
}
public int getYear(){
return schoolYear;
}
public String toString(){
return super.toString() +","+" Startyear: " +getYear();
}
}

Actually, your randomHuman() method, as mentioned in the comments, is not a constructor at all. It's a static factory method, although I'm sure you're not aware of what that means as yet.
Basically, a constructor is not a method at all and doesn't have a return type. What a constructor does is provide an initialization for a new instance of the class, created by using new, although it can do things that don't strictly initiate the fields of that object.
A method, in contrast, can return something. And in your particular case, the last line actually tells you exactly what it returns - it's calling new for the class Human, so it will return an object of class Human, never a Student.
In fact, the class Human is not aware of the class Student. In principle, you could write a subclass for a class, years after the parent class has been written. Parent classes don't need to know about their descendents. They just decide what they allow those descendents to change and what they don't allow them to change.
You could, in theory, put a method in Human that creates a Student instance. But I'm pretty sure that's not needed in the current situation.
What you probably want to do is fill a list of humans outside the definition of either Human or Student. Filling a random list is probably not part of "being a human" or "being a student" is all about, so you should just do it in your Main class, calling new Human() or new Student() as you wish and filling them as appropriate. Since you know which new you called, you also know whether or not to use a random year.
You could do it in a static method in your Main class, to signify that this is something you do for testing, and not really part of the logic of either a Human or a Student.
As for being able to tell which object you now got from the list - you can do that with instanceof. But you'll also need to typecast it to Student if you want to access its getYear() method.
However - and this is the neat thing about polymorphism - if you just call the toString() method, and don't even check the type of the object, you'll get it with the year if it's really a Student object, and without it if it's a plain Human object.
Let's assume your teachers actually want you to extend the randomHuman method so that it sometimes gives Human instances, and sometimes Students. When it gives Student, it should of course provide it with a year.
As I said above, this is called Tight Coupling between parent and subclass, and is not recommended. If I wanted to build another human subclass, such as Politician, I'd have to call you and ask you to release a new version of Human that also sometimes gives random Politicians. So, under protest, I'll explain how to do it.
Your existing function is:
public static Human randomHuman(){
String[] anArrayOfStrings={"Tom", "Jon", "Chris","Julian","Roberto","Sam","Lisa","Roxanne","Rebecca","Anton","Johannes","Antonella","Bianca"};
int randomAge = (int) (100*Math.random());
String randomName = anArrayOfStrings[(int)(Math.random()*anArrayOfStrings.length)];
int RandomYear = (int) (Math.random()*(2013-1932) + 1932);
// if(xx instanceof Fysiker){
//
// }
return new Human(randomAge,randomName);
}
We change it like so:
public static Human randomHuman(){
String[] anArrayOfStrings={"Tom", "Jon", "Chris","Julian","Roberto","Sam","Lisa","Roxanne","Rebecca","Anton","Johannes","Antonella","Bianca"};
int randomAge = (int) (100*Math.random());
String randomName = anArrayOfStrings[(int)(Math.random()*anArrayOfStrings.length)];
Human result = null;
if ( Math.random() < 0.5 ) {
// With a probability of 50%, create a plain human
result = new Human( randomAge, randomName );
} else {
// Create a student. Start by calculating a random year.
int randomYear = (int) (Math.random()*(2013-1932) + 1932);
result = new Fysiker( randomYear, randomAge, randomName );
}
return result;
}
So, you decide that you want to make a plain human, and within the scope of that decision, you create it with new Human(...) and assign to the result variable.
If you decide to make a student, within the scope of that decision, you calculate a random year, and create it with new Fysiker(). You can assign it to the variable result because polymorphically, it's Human. But in reality, internally, it's a Student.
You return the result variable, which may contain either a Human or a Student at this point.

For determining what type the object instance is use either object instanceof class or object.getClass().equals(Clazz.getSimpleName())
For return type just use the superClass (or interface). You can always cast it to the child if needed.

If you want to create 5 class of each you need a boolean in the method declaration and call it 5 times each to be sure u will have 5 instances of each class.
public static Human randomHuman(boolean isHuman){
If this is not important u can add a random boolean and then call the constructor:
boolean isHuman = Math.random() < 0.5;
if(!isHuman){
int RandomYear = (int) (Math.random()*(2013-1932) + 1932);
// create student
} else {
// create human
}

Am i even doing this remotely right? Java Methods

Task at hand:Consider a class ratingScore that represents a numeric rating for some thing such as a move. Attributes: A description of what is being rated, The maximum possible rating, rating.
It will have methods to: get rating from ta user, Return the maximum rating posisble, return the rating, return a string showing the rating in a format suitable for display.
a. write a method heading for each method
b. write pre and post conditions for each method
c. write some java statements to test the class
d. implement the class.
I think i did what i was supposed to do, but it is a method and i am not sure that i put enough room for it to be changed much, this is what i have so far.
import java.util.*;
public class MovieRating
{
// instance variables
private String description = " A movie that shows how racism affect our lives and choices";
private int maxRating = 10;
private int rating;
// methods
//precondition: Must have maxRating, rating and description before you post it back to the user.
//rating between 1 and 10, maxRating is set to 10, description of a movie
public void writeOutput()
{
System.out.println("The max rating is: " + maxRating );
System.out.println("Your rating is: " + rating );
System.out.println("The rating for" + description + " is " + rating);
System.out.println("while the max rating was " + maxRating);
}
// PostCondition: Will write maxRating, rating and description to the user.
//Precondition: description, enter the rating
public void readInput()
{
Scanner keyboard = new Scanner(System.in);
System.out.println("What would you rate the movie \"American History x\" out of ten");
System.out.println(description);
rating = keyboard.nextInt();
}
//postcondition: rating will be set to user's input for the movie American History x.
}
This is my Tester program.. not much so far
public class MovieRatingTester
{
public static void main(String[] args)
{
//object of the class MovieRating
MovieRating rating1 = new MovieRating();
rating1.readInput();
rating1.writeOutput();
}
}
SO did i cover what was told to cover? i think i did but i think i did it the wrong way, let me know please.

Ok, my point of view is:
Your class, MovieRating is missing some basic elements of OOP, and that is what I think you suppose to learn in this homework.
The first element missing is a constructor method, what you did is automatically assigning each new MovieRating the same description. The job of the constructor function is giving a unique values to the Object when it first built in the system.
The constructor method is special, it is public and has the exact same name is the class, as we said, in this method you suppose to assign values to your object variables.
the second thing will be to put getters/setters, these are methods who has access to your private values and will be used to assign/get the values from them. Note the use of them in the code:
import java.util.*;
public class MovieRating
{
// instance variables
private String description;
private int maxRating;
private int rating;
/*This is the constructor
Note the use of .this - the expression is used to call the class form withing
itself*/
public MovieRating(String description, int maxRating, int rating) {
this.setDescription(description);
this.setMaxRating(maxRating);
this.setRating(rating);
}
/*These are the getters and setters - get is used for getting the value
and set is used for assigning a value to it*/
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public int getMaxRating() {
return maxRating;
}
public void setMaxRating(int maxRating) {
this.maxRating = maxRating;
}
public int getRating() {
return rating;
}
public void setRating(int rating) {
this.rating = rating;
}
//This is a method for the printing commands - notice the use of the get methods//
public void printRatings()
{
System.out.println("The max rating is: " + this.getMaxRating() );
System.out.println("Your rating is: " + this.getRating() );
System.out.println("The rating for" + this.getDescription() + " is " +
this.getRating());
System.out.println("while the max rating was " + this.getMaxRating();
}
// PostCondition: Will write maxRating, rating and description to the user.
/*Precondition: description, enter the rating
Note the use of this.setRating()*/
public void readInput()
{
Scanner keyboard = new Scanner(System.in);
System.out.println("What would you rate the movie \"American History x\" out of ten");
System.out.println(description);
this.setRating(keyboard.nextInt());
}
//postcondition: rating will be set to user's input for the movie American History x.
}
Using the constructor, you can create a different rating from your tester program
MovieRating rating1 = new MovieRating("description 1", 10, 5);
MovieRating rating2 = new MovieRating("description 2", 9, 7);

You should not ask / print the data from the Ratings class. These ratings can come from user input, but also from database, web, etc.
1 Add getters and setters for properties of MovieRating
2 Pass the read and write methods to the main. Something like
System.out.println("The rating for the movie |" + rating1.getTitle() + "| is " + rating1.getRating());
3 You are not aggregating ratings to a movie. You can't have two rating to the same movie (v.g., by different users) together. Convert the rating attribute into a Vector to solve it. Change setRating for addRating
There are many other things, but obviously this is a starters exercise and I do not want you to get confused. Work on these issues and check with your teacher.

Java (and OO in general) is all about abstractions. You want to keep your objects as general as possible so that you extend your programs functionality without modifying existing code. This may be beyond what your professor was looking for but here are my suggestions:
1) Rating - separate this into its own class
Again, the rating is totally separate from the movie - songs can have ratings, tv shows can have ratings. Today ratings can be 1-10, tomorrow ratings can up thumbs up or thumbs down, etc. A Movie "has a" rating. Let Rating decide how to prompt the user and how to display itself.
2) Now that you have a separate Movie class, I would take away the hard-coded title, description in my Movie class (this will let me create many movies and rate them).
Then I would eliminate System.out.println in writeOutput method (you can pass in the OutputStream to the function)
By hard-coding in System.in you are forcing implementation. What if tomorrow your professor says "now, instead of printing to the console, print to a file or a database"? You have to modify the code. Actually, instead of writeOutput, I would override the toString method that all Objects have and then just call System.in(movie.toString()) in main.
3) Your test method doesn't "test" anything - it is just executing a statement. Typically a test method will simulate input, execute the statements, and check for the proper state at the end. A good way to signal that the state is improper (if your test fails, like maybe your Movie Rating is -1), then you throw an exception.
4) This is un-OO related and just a preference, but I would put both Pre and Post conditions before the methods. This just makes it easier to find in my opinion.
The idea of OO is that you separate responsibilities/concerns into separate classes. Each class is responsible for itself. This helps to keep your code more flexible and maintainable. Good luck on the assignment!