Develop Reference

Why am I getting this IndexOutOfBoundsException error?

hi so im currently trying to get past this error in my code, if anyone could explain where I went wrong, would be greatly appreciated.
public class Lab07vst100SD
{
public static void main (String[] args)
{
System.out.println();
int size = 10;
School bhs = new School(size);
System.out.println(bhs);
System.out.println(bhs.linearSearch("Meg"));
System.out.println(bhs.linearSearch("Sid"));
System.out.println();
bhs.selectionSort();
System.out.println(bhs);
System.out.println(bhs.binarySearch("Meg"));
System.out.println(bhs.binarySearch("Sid"));
System.out.println();
}
}
class School
{
private ArrayList<Student> students;
private int size;
public School (int s)
{
students = new ArrayList<Student>();
size = s;
}
public void addData()
{
String [] name = {"Tom","Ann","Bob","Jan","Joe","Sue","Jay","Meg","Art","Deb"};
int[] age = {21,34,18,45,27,19,30,38,40,35};
double[] gpa = {1.685,3.875,2.5,4.0,2.975,3.225,3.65,2.0,3.999,2.125};
for(int i = 0; i < name.length; i++)
{
students.add(new Student(name[i], age[i], gpa[i]));
}
size = students.size();
}
public void selectionSort ()
{
for(int h = 0; h < students.size(); h++)
{
int index = h;
Student least = students.get(h);
for (int t = 0; t < size; t++) {
if (students.get(t).equals(least)) {
least = students.get(t);
index = t;
}
Student temp = students.get(h);
students.set(h, least);
students.set(t, temp);
}
}
}
public int linearSearch (String str)
{
// new arraylist
ArrayList<String> names = new ArrayList<String>();
for (int q = 0; q < size; q++) {
names.add(students.get(q).getName());
}
//comparison
for (int y = 0; y < size; y++) {
if (names.get(y).equals(str))
return y;
}
return -1;
};
public int binarySearch (String str) {
// new arraylist and variables
ArrayList<String> names = new ArrayList<String>();
Boolean found = false;
int lo = 0;
int hi = size;
int mid = (lo + hi) / 2;
//for loop for to transverse the array.
for (int m = 0; m < size; m++) {
names.add(students.get(m).getName());
}
while (lo <= hi && !found) {
if (names.get(mid).compareTo(str) == 0)
{
found = true;
return mid;
}
if (names.get(mid).compareTo(str) < 0) {
lo = mid + 1;
mid = (lo + hi) / 2;
}
else {
hi = mid -1;
mid = (lo + hi) / 2;
}
}
if (found)
return mid;
else
return -1;
}
public String toString() {
String temp = "";
for (int s = 0; s < students.size(); s++) {
temp += students.get(s);
}
return temp;
}
}
also, I should mention this uses the student class.
here
public class Student
{
private String name;
private int age;
private double gpa;
public Student (String n, int a, double g)
{
name = n;
age = a;
gpa = g;
}
public String getName() {
return name; }
public int getAge() {
return age; }
public double getGPA() {
return gpa; }
public String toString()
{
String temp = name + " " + age + " " + gpa + "\n";
return temp;
}
}
the school class calls to the student class.
this is what comes back.
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
at java.base/java.util.Objects.checkIndex(Objects.java:359)
at java.base/java.util.ArrayList.get(ArrayList.java:427)
at School.linearSearch(Lab07vst100SD.java:78)
at Lab07vst100SD.main(Lab07vst100SD.java:16)
I'm completely confused on why this is happening, I think it may have to do with the ArrayList, other than that, I'm not sure.
please help, and thank you
p.s. I'm new so please bear with my horrible format.

You need call addData:
public static void main (String[] args)
{
System.out.println();
int size = 10;
School bhs = new School(size);
bhs.addData(); // here
System.out.println(bhs);
System.out.println(bhs.linearSearch("Meg"));
System.out.println(bhs.linearSearch("Sid"));
System.out.println();
bhs.selectionSort();
System.out.println(bhs);
System.out.println(bhs.binarySearch("Meg"));
System.out.println(bhs.binarySearch("Sid"));
System.out.println();
}

...
class School
{
private ArrayList<Student> students;
private int size;
public School (int s)
{
students = new ArrayList<Student>(); // Here, it can throw IndexOutOfBoundsException
size = s;
}
...
Please see https://www.tutorialspoint.com/java/util/arraylist_add_index.htm
The capacity of ArrayList must be initialized before ArrayList.add method
.

How to calculate only integer types in array?

import java.util.*;
class Distance {
private String name;
private int dist;
public Distance(String name, int dist) {
this.name = name;
this.dist = dist;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getDist() {
return dist;
}
public void setDist(int dist) {
this.dist = dist;
}
public String toString() {
return "Distance [name=" + name + ", school street=" + dist + "]";
}
}
class DistanceComp {
public static Distance longdistance(Distance[] dim) {
Distance max = dim[0];
for (int i = 1; i < dim.length; i++) {
if (max.getDist() < dim[i].getDist())
max = dim[i];
}
return max;
}
public static Distance shortdistance(Distance[] dim) {
Distance min = dim[0];
for (int i = 1; i < dim.length; i++) {
if (min.getDist() > dim[i].getDist())
min = dim[i];
}
return min;
}
}
public class week03_01 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
Distance[] dist = new Distance[3];
System.out.print(">> how many students? : ");
int num = in.nextInt();
for (int i = 0; i < num; i++) {
System.out.print(">> name and distance : ");
dist[i] = new Distance(in.next(), in.nextInt());
}
System.out.println("\na student with the longest commute to school : " + DistanceComp.longdistance(dist));
System.out.println("a student with the shortest commute to school : " + DistanceComp.shortdistance(dist));
System.out.println("school distance difference is " + );
}
}
I also want to print the "shool distance differecnce".
but it doesn't calculate. i think it has String types.
I think calculate only integer types in an array, but i don't know the code.
Or is s there any other way? Ask for advice.

In your DistanceComp class, create a method to subtract two Distances similar like you did for longdistance and shortdistance:
public static int subtractDistance(Distance dist1, Distance dist2) {
int difference = Math.abs(dist1.getDist() - dist2.getDist());
return difference;
}
Then, use that in your System.out:
System.out.println("school distance difference is " + DistanceComp.subtractDistance(DistanceComp.longdistance(dist), DistanceComp.shortdistance(dist)));
Some notes fyi:
Your code currently only works with 3 students.
Instead of using long names, assign them to a shorter-named variable. This helps with code readability.

Mean, Median, and Mode - Newb - Java

We had a lab in Comsci I couldn't figure out. I did a lot of research on this site and others for help but they were over my head. What threw me off were the arrays. Anyway, thanks in advance. I already got my grade, just want to know how to do this :D
PS: I got mean, I just couldn't find the even numbered median and by mode I just gave up.
import java.util.Arrays;
import java.util.Random;
public class TextLab06st
{
public static void main(String args[])
{
System.out.println("\nTextLab06\n");
System.out.print("Enter the quantity of random numbers ===>> ");
int listSize = Expo.enterInt();
System.out.println();
Statistics intList = new Statistics(listSize);
intList.randomize();
intList.computeMean();
intList.computeMedian();
intList.computeMode();
intList.displayStats();
System.out.println();
}
}
class Statistics
{
private int list[]; // the actual array of integers
private int size; // user-entered number of integers in the array
private double mean;
private double median;
private int mode;
public Statistics(int s)
{
size = s;
list = new int[size];
mean = median = mode = 0;
}
public void randomize()
{
Random rand = new Random(12345);
for (int k = 0; k < size; k++)
list[k] = rand.nextInt(31) + 1; // range of 1..31
}
public void computeMean()
{
double total=0;
for (int f = 0; f < size; f++)
{
total = total + list[f];
}
mean = total / size;
}
public void computeMedian()
{
int total2 = 0;
Arrays.sort(list);
if (size / 2 == 1)
{
// total2 =
}
else
{
total2 = size / 2;
median = list[total2];
}
}
public void computeMode()
{
// precondition: The list array has exactly 1 mode.
}
public void displayStats()
{
System.out.println(Arrays.toString(list));
System.out.println();
System.out.println("Mean: " + mean);
System.out.println("Median: " + median);
System.out.println("Mode: " + mode);
}
}

Here are two implementations for your median() and mode() methods:
public void computeMedian() {
Arrays.sort(list);
if ( (list.size & 1) == 0 ) {
// even: take the average of the two middle elements
median = (list[(size/2)-1] + list[(size/2)]) / 2;
} else {
// odd: take the middle element
median = list[size/2];
}
}
public void computeMode() {
// precondition: The list array has exactly 1 mode.
Map<Integer, Integer> values = new HashMap<Integer, Integer>();
for (int i=0; i < list.size; ++i) {
if (values.get(list[i]) == null) {
values.put(list[i], 1);
} else {
values.put(list[i], values.get(list[i])+1);
}
}
int greatestTotal = 0;
// iterate over the Map and find element with greatest occurrence
Iterator it = values.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pair = (Map.Entry)it.next();
if (pair.getValue() > greatestTotal) {
mode = pair.getKey();
greatestTotal = pair.getValue();
}
it.remove();
}
}

Fix NullPointerException? [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 8 years ago.
How could I get the program to output all the information? IT currently returns a NullPointException error. Thanks.
I am supposed to use the delete methods just as they are, I cannot change them, but I am sure there must be something I can do.
public class TestCandidate7
{
public static int getTotal(Candidate[] election)
{
int total = 0;
for(Candidate candidate : election )
{
total += candidate.numVotes;
}
return total;
}
public static void printResults(Candidate[] election)
{
double percent;
System.out.println("Candidate Votes Received % of Total Votes");
for (int x = 0; x < election.length; x++)
{
percent = (double) (election[x].votes()) / getTotal(election) * 100;
System.out.printf("%-15s %10d %20.0f", election[x].getName(), election[x].votes(), percent);
System.out.println();
}
}
public static void deleteByLoc(Candidate[] election,
int location)
{
if ((location > 0) && (location < election.length))
{
//move items up in the array -
for(int index = location; index < election.length -1; index++)
election[index] = election[index + 1];
election[election.length-1] = null;
}
}
public static void deleteByName(Candidate[] election,
String find)
{
int location = 0;
int index;
// find location of item you want to delete
for(index = 0; index < election.length; index++)
if ((election[index] != null) && (election[index].getName().equals(find)))
{
location = index;
break;
}
else if (election[index] == null)
{
location = -1;
break;
}
if ((index != election.length) && (location >= 0))
{ //move items up in the array
for(index = location; index < election.length -1; index++)
election[index] = election[index + 1];
election[election.length-1] = null;
}
}
public static void main(String[] args)
{
Candidate[] election = new Candidate[10];
// create election
election[0] = new Candidate("John Smith", 5000);
election[1] = new Candidate("Mary Miller", 4000);
election[2] = new Candidate("Michael Duffy", 6000);
election[3] = new Candidate("Tim Robinson", 2500);
election[4] = new Candidate("Joe Ashtony", 1800);
election[5] = new Candidate("Mickey Jones", 3000);
election[6] = new Candidate("Rebecca Morgan", 2000);
election[7] = new Candidate("Kathleen Turner", 8000);
election[8] = new Candidate("Tory Parker", 500);
election[9] = new Candidate("Ashton Davis", 10000);
System.out.println("Original results:");
System.out.println();
printResults(election);
System.out.println();
System.out.println("Total of votes in election: " + getTotal(election) );
System.out.println();
deleteByLoc(election, 6);
System.out.println("Deleted location 6:");
System.out.println();
printResults(election);
System.out.println();
System.out.println("Total of votes in election: " + getTotal(election) );
System.out.println();
deleteByName(election, "Kathleen Turner");
System.out.println("Deleted Kathleen Turner:");
System.out.println();
printResults(election);
System.out.println();
System.out.println("Total of votes in election: " + getTotal(election) );
System.out.println();
}
}
Candidate
public class Candidate
{
// instance variables
int numVotes;
String name;
/**
* Constructor for objects of class InventoryItem
*/
public Candidate(String n, int v)
{
// initialise instance variables
name = n;
numVotes = v;
}
public int votes()
{
return numVotes;
}
public void setVotes(int num)
{
numVotes = num;
}
public String getName()
{
return name;
}
public void setName(String n)
{
name = n;
}
public String toString()
{
return name + " received " + numVotes + " votes.";
}
}

When you "delete" array elements, after the shift you assign null to the most right element of the array.
In your getTotal() you traverse the entire array and retrieve the value of numVotes for each element. When you reach the null element you are getting the exception since null does not have any fields..

Having trouble calling another class that reads a txt file into an array that is sorted in the program(band operand error)

so I have a program that takes a txt file and is read in and uses the class of another java file to set up the array. I am trying use selection sort to sort the values placed into the array but it gives me bad operand types for the line: (if array[j] < array[min]). The text file used is:"Country" "Total CO2 2005 (million tonnes)" "Road CO2 (million tonnes)" "Road CO2 per person (tonnes)" "Cars per 1000 people"
10
USA 5951.13 1530.3 5.16 777
UK 2573.4 119.68 1.99 470
Italy 476.08 116.86 2 592
Germany 841.78 150.21 1.82 550
Canada 553.02 123.42 3.82 562
France 414.03 128.13 2.04 477
Russia 1575.44 114.69 0.8 178
Japan 1254.47 224.24 1.76 447
China 5100.6 228.02 0.3 17
India 1147.46 91.06 0.1 8
The program with the class being called carbonDioxide.java:
public class CarbonDioxideData {
private String country;
private double totalCO2;
private double roadCO2;
private double CO2PerPerson;
private int carsPerPerson;
public CarbonDioxideData() {
country = "blank_country";
totalCO2 = -1.0;
roadCO2 = -1.0;
CO2PerPerson = -1.0;
carsPerPerson = -1;
}
public String toString() {
String result = country;
result += " " + totalCO2;
result += " " + roadCO2;
result += " " + CO2PerPerson;
result += " " + carsPerPerson;
return result;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public double getTotalCO2() {
return totalCO2;
}
public void setTotalCO2(double totalCO2) {
this.totalCO2 = totalCO2;
}
public double getRoadCO2() {
return roadCO2;
}
public void setRoadCO2(double roadCO2) {
this.roadCO2 = roadCO2;
}
public double getCO2PerPerson() {
return CO2PerPerson;
}
public void setCO2PerPerson(double cO2PerPerson) {
CO2PerPerson = cO2PerPerson;
}
public int getCarsPerPerson() {
return carsPerPerson;
}
public void setCarsPerPerson(int carsPerPerson) {
this.carsPerPerson = carsPerPerson;
}
}
The program I am writing calling the two above, CarbonAnalysis.java:
import java.io.*;
import java.util.*;
public class CarbonAnalysis {
public static void main(String[]args){
//CarbonDioxideData c1 = new CarbonDioxideData();
//c1.setCountry("canada");
//System.out.println(c1);
Scanner userInput = new Scanner(System.in);
String fileName ="";
File inputFile = null;
Scanner fileReader = null;
while(fileReader==null){
try{
System.out.println("Enter input file name:");
fileName= userInput.next();
inputFile = new File(fileName);
fileReader = new Scanner(inputFile);
System.out.println("Successfully opening " + fileName);
}catch(IOException err){
System.out.println("Something went wrong");
System.out.println(err);
System.out.println("Please retry");
}
}
String testLine = fileReader.nextLine();
System.out.println(testLine);
int numberOfEntries = fileReader.nextInt();
System.out.println(numberOfEntries);
CarbonDioxideData[] array = new CarbonDioxideData[numberOfEntries];
for(int i =0;i<numberOfEntries;i++){
CarbonDioxideData c1 = new CarbonDioxideData();
String country = fileReader.next();
c1.setCountry(country);
double totalCO2 = fileReader.nextDouble();
c1.setTotalCO2(totalCO2);
double roadCO2 = fileReader.nextDouble();
c1.setRoadCO2(roadCO2);
double perPerson = fileReader.nextDouble();
c1.setCO2PerPerson(perPerson);
int cars = fileReader.nextInt();
c1.setCarsPerPerson(cars);
//System.out.println(c1);
array[i]=c1;
}
printArray(array);
emissionStats(array);
}
public static void printArray(CarbonDioxideData[] a){
for(int i=0; i<a.length;i++){
System.out.println(a[i]);
}
}
public static void emissionStats(CarbonDioxideData[] array){
for (int i = 0; i < array.length - 1; i++) {
// find index of smallest remaining value
int min = i;
for (int j = i + 1; j < array.length; j++) {
if (array[j] < array[min]) {
min = j;
}
}
// swap smallest value its proper place, a[i]
swap(array, i, min);
}
}
public static void swap(CarbonDioxideData[] a, int i, int j) {
if (i != j) {
CarbonDioxideData temp = a[i];
a[i] = a[j];
a[j] = temp;
}
}
}
The error I am receiving when compiling:
CarbonAnalysis.java:68: error: bad operand types for binary operator '<'
if array[j] < array[min]
first type: CarbonDioxideData
Second type: CarbondDioxidedata
I am at a loss I have no idea how to get it to work. Any help appreciated

< , > binary operators can be used with primitive types.
public class CarbonDioxideData implements Comparable<CarbonDioxideData> {
private String country;
private double totalCO2;
private double roadCO2;
private double CO2PerPerson;
private int carsPerPerson;
#Override
public int compareTo(CarbonDioxideData that) {
final int BEFORE = -1;
final int EQUAL = 0;
final int AFTER = 1;
if (this == that) return EQUAL;
//Compare function according to your logic
if (this.totalCO2 == that.totalCO2) return EQUAL;
if (this.totalCO2 > that.totalCO2)
return AFTER;
else
return BEFORE;
}
}
Your comparison should be as
if (array[j].compareTo(array[min]) < 0) {
min = j;
}