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A Java program with a loop that allows the user to enter a series of integers, then displays the smallest and largest numbers + the average

I've got an assignment that requires me to use a loop in a program that asks the user to enter a series of integers, then displays the smallest and largest numbers AND gives an average. I'm able to write the code that allows the user to enter however many integers they like, then displays the smallest and largest number entered. What stumps me is calculating the average based on their input. Can anyone help? I'm sorry if my code is a little janky. This is my first CS course and I'm by no means an expert.
import javax.swing.JOptionPane;
import java.io.*;
public class LargestSmallest
{
public static void main(String[] args)
{
int number, largestNumber, smallestNumber, amountOfNumbers;
double sum, average;
String inputString;
inputString = JOptionPane.showInputDialog("Enter an integer, or enter -99 to stop.");
number = Integer.parseInt(inputString);
largestNumber = number;
smallestNumber = number;
sum = 0;
for (amountOfNumbers = 1; number != -99; amountOfNumbers++)
{
inputString = JOptionPane.showInputDialog("Enter an integer, or enter -99 to stop.");
number = Integer.parseInt(inputString);
if (number == -99)
break;
if (number > largestNumber)
largestNumber = number;
if (number < smallestNumber)
smallestNumber = number;
sum += number;
}
average = sum / amountOfNumbers;
JOptionPane.showMessageDialog(null, "The smallest number is: " + smallestNumber + ".");
JOptionPane.showMessageDialog(null, "The largest number is: " + largestNumber + ".");
JOptionPane.showMessageDialog(null, "The average off all numbers is: " + average + ".");
}
}

The problem is that you do an extra
inputString = JOptionPane.showInputDialog("Enter an integer, or enter -99 to stop.");
number = Integer.parseInt(inputString);
at the beginning. You don't count that in a sum. That's why you get unexpected results.
The fix would be:
replace the declarations line with:
int number = 0, largestNumber, smallestNumber, amountOfNumbers;
Remove
inputString = JOptionPane.showInputDialog("Enter an integer, or enter -99 to stop.");
number = Integer.parseInt(inputString);
That go before the loop
Replace for (amountOfNumbers = 0 with for (amountOfNumbers = 1

This is my first CS course
Then allow me to show you a different way to do your assignment.
Don't use JOptionPane to get input from the user. Use a Scanner instead.
Rather than use a for loop, use a do-while loop.
Usually you declare variables when you need to use them so no need to declare all the variables at the start of the method. However, be aware of variable scope.
(Notes after the code.)
import java.util.Scanner;
public class LargestSmallest {
public static void main(String[] args) {
Scanner stdin = new Scanner(System.in);
int largestNumber = Integer.MIN_VALUE;
int smallestNumber = Integer.MAX_VALUE;
int number;
double sum = 0;
int amountOfNumbers = 0;
do {
System.out.print("Enter an integer, or enter -99 to stop: ");
number = stdin.nextInt();
if (number == -99) {
break;
}
if (number > largestNumber) {
largestNumber = number;
}
if (number < smallestNumber) {
smallestNumber = number;
}
sum += number;
amountOfNumbers++;
} while (number != -99);
if (amountOfNumbers > 0) {
double average = sum / amountOfNumbers;
System.out.printf("The smallest number is: %d.%n", smallestNumber);
System.out.printf("The largest number is: %d.%n", largestNumber);
System.out.printf("The average of all numbers is: %.4f.%n", average);
}
}
}
largestNumber is initialized to the smallest possible number so that it will be assigned the first entered number which must be larger than largestNumber.
Similarly, smallestNumber is initialized to the largest possible number.
If the first value entered is -99 then amountOfNumbers is zero and dividing by zero throws ArithmeticException (but maybe you haven't learned about exceptions yet). Hence, after the do-while loop, there is a check to see whether at least one number (that isn't -99) was entered.
You don't need to use printf to display the results. I'm just showing you that option.

How to fix getting an average from user input

The instructions are the following:
Write a method called inputThenPrintSumAndAverage that does not have any parameters.
The method should not return anything (void) and it needs to keep reading int numbers from the keyboard.
When the user enters something that is not an int then it needs to print a message in the format "SUM = XX AVG = YY". XX represents the sum of all entered numbers of type int.
YY represents the calculated average of all numbers of type long.
I've coded the following method but I keep getting the incorrect average. What can I change to get the correct average?
public static void inputThenPrintSumAndAverage(){
Scanner scanner = new Scanner(System.in);
int sum = 0, counter = 0;
long average = 0L;
while(true) {
boolean number = scanner.hasNextInt();
if(!number)
{
counter++;
break;
}
else {
int digit = scanner.nextInt();
sum += digit;
counter++;
}
}
average = sum / counter;
System.out.println("SUM = " + sum + " AVG = " + average);
}

The following should do:
if(!number)
break;
Your average is wrong because you are incrementing "counter" more than you should.
Of course in the end you are going to have to add one more if statement to make sure you are not attempting to divide by counter if the counter is zero. In that case, an average is undefined.
Also, as others have pointed out in the comments, it is entirely unclear to us what you mean when you say "I keep getting the incorrect average", and we generally frown upon questions worded so vaguely. But if by any chance a "correct average" for you means an average with decimals, then you should use a double instead of a long for your average, and you should cast your counter to double before dividing, so as to force a double division instead of a long division.

Use scanner.hasNextInt() to read util a non-integer value is supplied as input:
public static void inputThenPrintSumAndAverage() {
Scanner scanner = new Scanner(System.in);
int number, sum = 0, counter = 0;
long average = 0L;
while (scanner.hasNextInt()) {
number = scanner.nextInt();
sum += number;
counter++;
}
if(counter != 0){
average = sum / counter;
}
System.out.println("SUM = " + sum + " AVG = " + average);
}
Note that you should check the value of counter to avoid a division by zero.
If you want the average to be in decimals, change the type of average to double: double average; and cast the division to double: average = (double) sum / counter;.

how do I display the correct output when no positve numbers are entered

I'm new to coding. Assignment is to calculate the average of all the positive numbers input and exit when a zero is input. If no positive numbers are input display a message average not possible.
The following is what I have so far. I am stuck on the part about printing out the message "cannot calculate the average" when only a zero or negative numbers are input.
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int numbers = 0;
int sumOfNumbers = 0;
double averagePositive = 0;
while (true) {
System.out.println("Give a number: ");
int number = Integer.valueOf(scanner.nextLine());
if (number == 0)
break;
if (number > 0)
sumOfNumbers = number + sumOfNumbers;
if (number > 0)
numbers = numbers + 1;
if (number > 0)
averagePositive = (double)sumOfNumbers / (double)numbers;
}
System.out.println(averagePositive);
}

Try it as follows...
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Give a number: ");
int num=input.nextInt();
int tot=0; //total
int count=0; // counting the positive numbers
if(num>0){
while(num!=0){
tot+=num;
count++;
System.out.print("Give a number: ");
num=input.nextInt();
if(num<0){
System.out.print("Not possible");
return;
}
}
double avg =(double)tot/n;
System.out.print("Average: "+avg);
}else{
System.out.println("Cannot calculate the average.");
}
}

I'd probably do it like this to keep it simple. Also in general, try not to cramp code together. Most formal project demand a certain degree of styling and usually spaces between operators and braces, etc... is required. In the long run it makes the code more readable and easier to maintain.
In your code there was no need to repeat the same if test for number > 0 multiple times, they could have all been bundled together. If the program was bigger and more complex I may have named the variable names with more qualification but for a short program like this, brief names were sufficient for clarity.
continue and break are important keywords to control loop behavior and can be used to increase brevity and clarity. continue goes back to the top of the loop immediately and break exits the innermost loop immediately. Dividing a double by an int yields a double so I was able to eliminate a cast. And the += operator makes it a little easier to read the line.
Also in Java and C any if() or else clause that contains one line doesn't require braces and unless a program is nested in such a way that adding the braces anyway adds to the clarity, it is often clearer to omit the braces in that case. The if statement illustrates both ways in a single statement.
import java.util.Scanner;
public class avg
{
static int count = 0;
static double sum = 0;
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("\nEnter a sequence of positive numbers (0 to calculate average):\n");
while (true) {
System.out.print("Number? ");
int n = scanner.nextInt();
if (n < 0) {
System.out.println("Negative numbers not allowed.");
continue;
} else if (n == 0)
break;
sum += (double)n;
++count;
}
System.out.println("Average of " + count + " numbers = " +
(double)(sum / count) + "\n");
System.exit(1);
}
}
Sample output:
$ java avg
Enter a sequence of positive numbers (0 to calculate average)
Number? 1
Number? 2
Number? 3
Number? 4
Number? 5
Number? -6
Negative numbers not allowed.
Number? 0
Average of 5 numbers = 3.0

Close list with variables with the fixed number -1

I am very new to coding and Java. I have the following assignment: Write a program that reads a couple of positive numbers from the input and computes and prints the average, with 3 decimals precision. The input list closes with the number -1.
So I have a working program, however I have no clue how to integrate the condition 'print the average with 3 decimals precision'. Do you have any idea how to fix this? Many thanks!
See my code below:
import java.util.Scanner;
public class Parta {
public static void main(String[] args){
Scanner numInput = new Scanner(System.in);
double avg = 0.0;
double count = 0.0;
double sum = 0.0;
System.out.println("Enter a series of numbers. Enter -1 to quit.");
while (numInput.hasNextDouble())
{
double negNum = numInput.nextDouble();
if (negNum == -1)
{
System.out.println("You entered " + count + " numbers averaging " + avg + ".");
break;
}
else
{
sum += negNum;
count++;
avg = sum/count;
}
}
}
}

You just have to break out of the loop for your -1 condition.
while(1) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
if(n == -1)
break;
}

Change
for(int i=0; i < numbers.length + 1= -1 ; i++)
to
for(int i=0; i < n ; i++)
The
%n
is out of place in the print statement also. I'd remove that.
To implement your -1 condition, check for a == -1 in the for loop:
if (a == -1) {break;}

The input list closes with the number -1.
I assume this means that -1 is the final number you are looking for and when read then all inputs are then completed? You just need a condition to check if the number you are looking at is -1, if it is then stop reading.

Your code does not meet your requirements.
The first requirement is that you have to calculate fractions. But you stick to int as type of your variables. As written by #nbokmans your variables should be of type double or float.
The other problem is that your code takes the first number given as the count of the numbers to follow. But you're told to use any number for calculation until input is -1. You cannot do this with a for loop, you need a while loop for this.
An the easiest way to accomplish your task is to calculate the result on the fly while getting the input:
pseudo code:
declare sum as double initially 0.0;
while(input is not -1)
sum = (sum + input) / 2;
output sum:

How to Write a Summation of Fives in Java

I need to write a program in Java that can take the multiples of five up to a value given by the user, and then add all of the multiples together. I need to write it with a while loop.
Here's what I have so far:
import java.util.Scanner;
public class SummationOfFives {
public static void main(String[] args){
//variables
double limit;
int fives = 0;
//Scanner
System.out.println("Please input a positive integer as the end value: ");
#SuppressWarnings("resource")
Scanner input = new Scanner(System.in);
limit = input.nextDouble();
//While Loop
while ((fives+5)<=limit)
{
fives = fives+5;
System.out.println("The summation is: "+fives);
}
}
}
When I run this program however, all it gives me is the multiples:
Please input a positive integer as the end value:
11
The summation is: 5
The summation is: 10

You're nearly there! Think about what your output is telling you. In your while loop, fives is the next multiple of 5 on each iteration. You're not adding it to a total variable anywhere.
So - define a total before the loop e.g.
int total = 0;
keep adding to it in the loop (where your System.out.println is now) e.g.
total = total + fives;
output the total after the loop e.g.
System.out.println(total);

I added a total variable into your loop that will accumulate the value of all of the summations.
int counter =1;
int total = 0;
//While Loop
while ((fives+5)<=limit)
{
total = counter*5;
counter++;
fives = fives+5;
System.out.println("The summation is: "+fives);
System.out.println("The total is: "+total);
}

The summation you do in fives is wrong. You need another variable multiple initialised to 0 that you will increment by 5 at each step of the loop. The stop condition in the while is (multiple < limit). Then fives are the sum of the multiples.
double limit;
int fives = 0;
int multiple = 0
//While Loop
while (multiple<=limit)
{
multiple += 5;
fives = fives + multiple;
System.out.println("So far, the summation is: "+fives);
}