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Formatting returned Strings?

I'm really new to coding and just got assigned my first coding homework involving methods and returns. I managed to struggle through and end up with this, which I'm pretty proud of, but I'm not quite sure it's right. Along with that, my return statements are all on the same lines instead of formatted how my teacher says they should be ("n is a perfect number", then the line below says "factors: x y z", repeated for each perfect number. Below are the exact instructions plus what it outputs. Anything will help!
Write a method (also known as functions in C++) named isPerfect that takes in one parameter named number, and return a String containing the factors for the number that totals up to the number if the number is a perfect number. If the number is not a perfect number, have the method return a null string (do this with a simple: return null; statement).
Utilize this isPerfect method in a program that prompts the user for a maximum integer, so the program can display all perfect numbers from 2 to the maximum integer
286 is perfect.Factors: 1 2 3 1 2 4 7 14
It should be
6 is perfect
Factors: 1 2 3
28 is perfect
Factors: 1 2 4 7 14
public class NewClass {
public static void main(String[] args) {
Scanner input = new Scanner(System.in) ;
System.out.print("Enter max number: ") ;
int max = input.nextInt() ;
String result = isPerfect(max) ;
System.out.print(result) ;
}
public static String isPerfect(int number) {
String factors = "Factors: " ;
String perfect = " is perfect." ;
for (int test = 1; number >= test; test++) {
int sum = 0 ;
for (int counter = 1; counter <= test/2; counter++) {
if (test % counter == 0) {
sum += counter ;
}
}
if (sum == test) {
perfect = test + perfect ;
for (int counter = 1; counter <= test/2; counter++) {
if (test % counter == 0) {
factors += counter + " " ;
}
}
}
}
return perfect + factors ;
}
}

Couple of things you could do:
Firstly, you do not need two loops to do this. You can run one loop till number and keep checking if it's divisible by the iterating variable. If it is, then add it to a variable called sum.
Example:
.
factors = []; //this can be a new array or string, choice is yours
sum=0;
for(int i=1; i<number; i++){
if(number % i == 0){
sum += i;
add the value i to factors variable.
}
}
after this loop completes, check if sum == number, the if block to return the output with factors, and else block to return the output without factors or factors = null(like in the problem statement)
In your return answer add a newline character between perfect and the factors to make it look like the teacher's output.

You can try the solution below:
public String isPerfect(int number) {
StringBuilder factors = new StringBuilder("Factors: ");
StringBuilder perfect = new StringBuilder(" is perfect.");
int sum = 0;
for (int i = 1; i < number; i++) {
if (number % i == 0) {
sum += i;
factors.append(" " + i);
}
}
if (sum == number) {
return number + "" + perfect.append(" \n" + factors);
}
return number + " is not perfect";
}

Keep separate variables for your template bits for the output and the actual output that you are constructing. So I suggest that you don’t alter factors and perfect and instead declare one more variable:
String result = "";
Now when you’ve found a perfect number, add to the result like this:
result += test + perfect + '\n' + factors;
for (int counter = 1; counter <= test/2; counter++) {
if (test % counter == 0) {
result += counter + " ";
}
}
result += '\n';
I have also inserted some line breaks, '\n'. Then of course return the result from your method:
return result;
With these changes your method returns:
6 is perfect.
Factors: 1 2 3
28 is perfect.
Factors: 1 2 4 7 14
Other tips
While your program gives the correct output, your method doesn’t follow the specs in the assignment. It was supposed to check only one number for perfectness. Only your main program should iterate over numbers to find all perfect numbers up to the max.
You’ve got your condition turned in an unusual way here, which makes it hard for me to read:
for (int test = 1; number >= test; test++) {
Prefer
for (int test = 1; test <= number; test++) {
For building strings piecewise learn to use a StringBuffer or StringBuilder.
Link
Java StringBuilder class on Javapoint Tutorials, with examples.

Java 1 student totally lost. Recursion program

Hi very first Java class and it seems to be going a mile a minute. We learn the basics on a topic and we are asked to produce code for more advanced programs than what helped us get introduced to the topic.
Write a recursive program which takes an integer number as Input. The program takes each digit in the number and add them all together, repeating with the new sum until the result is a single digit.
Your Output should look like exactly this :
################### output example 1
Enter a number : 96374
I am calculating.....
Step 1 : 9 + 6 + 3 + 7 + 4 = 29
Step 2 : 2 + 9 = 11
Step 3 : 1 + 1 =2
Finally Single digit in 3 steps !!!!!
Your answer is 2.
I understand the math java uses to produce the output I want. I can do that much after learning the basics on recursion. But with just setting up the layout and format of the code I am lost. I get errors that make sense but have trouble correcting with my inexperience.
package numout;
import java.util.Scanner;
public class NumOut {
public static void main(String[] args) {
System.out.print("Enter number: ");
Scanner scan = new Scanner(System.in);
int n = scan.nextInt();
System.out.println(n);
}
public int sumDigit(int n){
int sum = n % 9;
if(sum == 0){
if(n > 0)
return 9;
}
return sum;
}
}
The output understandably duplicates the code given by the input from the user.
I had trouble calling the second class when I tried to split it up into two. I also know I am not soprln n, or the sum. So I try to make it into one and I can visibly see the problem but am unaware how to find the solution.

Think of recursion as solving a problem by breaking it into similar problems which are smaller. You also need to have a case where the problem is so small that the solution is obvious, or at least easily computed. For example, with your exercise to sum the digits of a number, you need to add the ones digit to the sum of all the other digits. Notice that sum of all the other digits describes a smaller version of the same problem. In this case, the smallest problem will be one with only a single digit.
What this all means, is that you need to write a method sumDigits(int num) that takes the ones digit of num and adds it to the sum of the other digits by recursively calling sumDigits() with a smaller number.

This is how you need to do : basically you are not using any recursion in your code. Recursion is basically function calling itself. Don't be daunted by the language, you will going to enjoy problem solving once you start doing it regularly.
public static void main(String []args){
Scanner scan = new Scanner(System.in);
int n = scan.nextInt();
printSingleDightSum(n);
}
public static void printSingleDightSum(int N) {
int sum = 0;
int num = N;
while(num !=0 ){
int a = num%10;
sum + = a;
num = num/10;
}
if(sum < 10) {
System.out.println('single digit sum is '+sum);
return;
} else {
printSingleDightSum(sum);
}
}

Here is the code, I will add comments and an explanation later but for now here is the code:
package numout;
import java.util.Scanner;
public class NumOut {
public static void main(String[] args) {
System.out.println("################### output example 1");
System.out.print("Enter number: ");
final int n = new Scanner(System.in).nextInt();
System.out.print("\nI am Calculating.....");
sumSums(n, 1);
}
public static int sumSums(int n, int step) {
System.out.print("\n\nStep " + step + " : ");
final int num = sumDigit(n);
System.out.print("= " + num);
if(num > 9) {
sumSums(num, step+1);
}
return num;
}
public static int sumDigit(int n) {
int modulo = n % 10;
if(n == 0) return 0;
final int num = sumDigit(n / 10);
if(n / 10 != 0)
System.out.print("+ " + modulo + " ");
else
System.out.print(modulo + " ");
return modulo + num;
}
}

Processing numbers program

Firstly, I'm taking AP Computer Science this year, and this question is related to an exercise we were assigned in class. I have written the code, and verified that it meets the requirements to my knowledge, so this is not a topic searching for homework answers.
What I'm looking for is to see if there's a much simpler way to do this, or if there's anything I could improve on in writing my code. Any tips would be greatly appreciated, specific questions asked below the code.
The exercise is as follows: Write a program called ProcessingNumbers that does:
Accepts a user input as a string of numbers
Prints the smallest and largest of all the numbers supplied by the user
Print the sum of all the even numbers the user typed, along with the largest even number typed.
Here is the code:
import java.util.*;
public class ProcessingNumbers {
public static void main(String[] args) {
// Initialize variables and objects
Scanner sc = new Scanner(System.in);
ArrayList<Integer> al = new ArrayList();
int sumOfEven = 0;
// Initial input
System.out.print("Please input 10 integers, separated by spaces.");
// Stores 10 values from the scanner in the ArrayList
for(int i = 0; i < 10; i++) {
al.add(sc.nextInt());
}
// Sorts in ascending order
Collections.sort(al);
// Smallest and largest values section
int smallest = al.get(0);
int largest = al.get(al.size() - 1);
System.out.println("Your smallest value is " + smallest + " and your largest value is " + largest);
// Sum of Even numbers
int arrayLength = al.size();
for (int i = 0; i < al.size(); i++) {
if (al.get(i) % 2 == 0) {
sumOfEven += al.get(i);
}
}
System.out.println("The sum of all even numbers is " + sumOfEven);
// Last section, greatest even number
if (al.get(arrayLength - 1) % 2 == 0) {
System.out.println("The greatest even number typed is " + al.get(arrayLength - 1));
} else {
System.out.println("The greatest even number typed is " + al.get(arrayLength - 2));
}
sc.close();
}
}
Here are specific questions I'd like answered, if possible:
Did I overthink this? Was there a much simpler, more streamlined way to solve the problem?
Was the use of an ArrayList mostly necessary? We haven't learned about them yet, I did get approval from my teacher to use them though.
How could I possibly code it so that there is no 10 integer limit?
This is my first time on Stackoverflow in quite some time, so let me know if anything's out of order.
Any advice is appreciated. Thanks!

Usage of the ArrayList wasn't necessary, however it does make it much simpler due to Collections.sort().
To remove the 10 integer limit you can ask the user how many numbers they want to enter:
int numbersToEnter = sc.nextInt();
for(int i = 0; i < numbersToEnter; i++) {
al.add(sc.nextInt());
}
Another note is that your last if-else to get the highest even integer doesn't work, you want to use a for loop, something like this:
for (int i = al.size() - 1; i >= 0; i--) {
if (al.get(i) % 2 == 0) {
System.out.println("The greatest even number typed is " + al.get(i));
break;
}

I wouldn't say so. Your code is pretty straightforward and simple. You could break it up into separate methods to make it cleaner and more organized, though that isn't necessary unless you have sections of code that have to be run repeatedly or if you have long sections of code cluttering up your main method. You also could have just used al.size() instead of creating arrayLength.
It wasn't entirely necessary, though it is convenient. Now, regarding your next question, you definitely do want to use an ArrayList rather than a regular array if you want it to have a variable size, since arrays are created with a fixed size which can't be changed.
Here's an example:
int number;
System.out.print("Please input some integers, separated by spaces, followed by -1.");
number = sc.nextInt();
while (number != -1) {
al.add(number);
number = sc.nextInt();
}

Here is a solution that:
Doesn't use Scanner (it's a heavyweight when all you need is a line of text)
Doesn't have a strict limit to the number of numbers
Doesn't need to ask how many numbers
Doesn't waste space/time on a List
Handles the case when no numbers are entered
Handles the case when no even numbers are entered
Fails with NumberFormatException if non-integer is entered
Moved actual logic to separate method, so it can be mass tested
public static void main(String[] args) throws Exception {
System.out.println("Enter numbers, separated by spaces:");
processNumbers(new BufferedReader(new InputStreamReader(System.in)).readLine());
}
public static void processNumbers(String numbers) {
int min = 0, max = 0, sumOfEven = 0, maxEven = 1, count = 0;
if (! numbers.trim().isEmpty())
for (String value : numbers.trim().split("\\s+")) {
int number = Integer.parseInt(value);
if (count++ == 0)
min = max = number;
else if (number < min)
min = number;
else if (number > max)
max = number;
if ((number & 1) == 0) {
sumOfEven += number;
if (maxEven == 1 || number > maxEven)
maxEven = number;
}
}
if (count == 0)
System.out.println("No numbers entered");
else {
System.out.println("Smallest number: " + min);
System.out.println("Largest number: " + max);
if (maxEven == 1)
System.out.println("No even numbers entered");
else {
System.out.println("Sum of even numbers: " + sumOfEven);
System.out.println("Largest even number: " + maxEven);
}
}
}
Tests
Enter numbers, separated by spaces:
1 2 3 4 5 6 7 8 9 9
Smallest number: 1
Largest number: 9
Sum of even numbers: 20
Largest even number: 8
Enter numbers, separated by spaces:
1 3 5 7 9
Smallest number: 1
Largest number: 9
No even numbers entered
Enter numbers, separated by spaces:
-9 -8 -7 -6 -5 -4
Smallest number: -9
Largest number: -4
Sum of even numbers: -18
Largest even number: -4
Enter numbers, separated by spaces:
No numbers entered

recursively add integers from 1^2 to n^2

i'm having some trouble recursively adding integers in java from 1^2 to n^2.
I want to be able to recursively do this in the recurvMath method but all i'm getting is an infinite loop.
import java.util.Scanner;
public class Lab9Math {
int count = 0;
static double squareSum = 0;
public static void main(String[] args){
int n = 0;
Scanner scan = new Scanner(System.in);
System.out.println("Please enter the value you want n to be: ");
n = scan.nextInt();
Lab9Math est = new Lab9Math();
squareSum = est.recurvMath(n);
System.out.println("Sum is: "+squareSum);
}
public int recurvMath(int n){
System.out.println("N:" +n);
if(n == 0){
return 0;
}//end if
if (n == 1){
return 1;
}//end if
if (n > 1){
return (recurvMath((int) ((int) n+Math.pow(n, 2))));
}//end if
return 0;
}//end method
}//end class
I'm not fully grasping the nature of defining this recursively, as i know that i can get to here:
return (int) (Math.pow(n, 2));
but i can't incorporate the calling of the recurvMath method correctly in order for it to work.
Any help would be appreciated. Thanks!

In general, when trying to solve recursive problems, it helps to try to work them out in your head before programming them.
You want to sum all integers from 12 to n2. The first thing we need to do is express this in a way that lends itself to recursion. Well, another way of stating this sum is:
The sum of all integers from 12 to (n-1)2, plus n2
That first step is usually the hardest because it's the most "obvious". For example, we know that "a + b + c" is the same as "a + b", plus "c", but we have to take a leap of faith of sorts and state it that way to get it into a recursive form.
So, now we have to take care of the special base case, 0:
When n is 0, the sum is 0.
So let's let recurvMath(n) be the sum of all integers from 12 to n2. Then, the above directly translates to:
recurvMath(n) = recurvMath(n-1) + n2
recurvMath(0) = 0
And this is pretty easy to implement:
public int recurvMath(int n){
System.out.println("N:" +n);
if(n == 0){
return 0;
} else {
return recurvMath(n-1) + (n * n);
}
}
Note I've chosen to go with n * n instead of Math.pow(). This is because Math.pow() operates on double, not on int.
By the way, you may also want to protect yourself against a user entering negative numbers as input, which could get you stuck. You could use if (n <= 0) instead of if (n == 0), or check for a negative input and throw e.g. IllegalArgumentException, or even use Math.abs() appropriately and give it the ability to work with negative numbers.
Also, for completeness, let's take a look at the problem in your original code. Your problem line is:
recurvMath((int) ((int) n+Math.pow(n, 2)))
Let's trace through this in our head. One of your int casts is unnecessary but ignoring that, when n == 3 this is recurvMath(3 + Math.pow(3, 2)) which is recurvMath(12). Your number gets larger each time. You never hit your base cases of 1 or 0, and so you never terminate. Eventually you either get an integer overflow with incorrect results, or a stack overflow.

instead of saying:
return (recurvMath((int) ((int) n+Math.pow(n, 2))));
i instead said:
return (int) ((Math.pow(n, 2)+recurvMath(n-1)));

Try this
import java.util.Scanner;
public class Lab9Math {
int count = 0;
static double squareSum = 0;
public static void main(String[] args){
int n = 0;
Scanner scan = new Scanner(System.in);
System.out.println("Please enter the value you want n to be: ");
n = scan.nextInt();
Lab9Math est = new Lab9Math();
squareSum = est.recurvMath(n);
System.out.println("Sum is: "+squareSum);
}
public int recurvMath(int n){
System.out.println("N:" +n);
if(n == 1){
return 1;
}//end if
// More simplified solution
return recurvMath(n-1) + (int) Math.pow(n, 2); // Here is made changes
}//end method
}//end class

Java - Finding even numbers and the percent of them within an array

This is the question that I need to figure out:
Write a method called percentEven that accepts an array of integers as a parameter and returns the percentage of even numbers in the array as a real number. For example, if the array stores the elements [6, 2, 9, 11, 3] then your method should return 40.0. If the array contains no even elements or no elements at all, return 0.0.
Here is what I have so far:
import java.util.*;
public class Change {
public static void main(String[] args) {
Scanner console = new Scanner(System.in);
System.out.println("Let's find the range.");
System.out.println("Enter five numbers to find the range.");
int num = console.nextInt();
int[] list = new int[num];
System.out.println("The numbers you entered are: " + list.length);
System.out.println();
percentEven(list);
}
public static void percentEven(int[] num){
int percent = 0;
int even = 0;
for(int i = 0; i < num.length; i++){
if(num[i] % 2 == 0){
even++;
}
}
percent = (even / num.length) *100;
System.out.println("The percent of even numbers is: " + percent);
}
}
When I run it, I get 100 as the percent.

Two issues here:
Cast one of them to a double or float.
percent = (even / (double) num.length) *100;
The other issue is that you never assign the numbers any value, so they are all 0. 0 % 2 is equal to 0, so the list is, by definition, 100% even.
You should also have a base case in the method when nums == {}, which would return 0.0 as the assignment states.

There are three major problems in your code:
You are reading only ONE integer, not five
You use this single integer to define the LENGTH of the array, not the content (so you don't put the integer into the array. So the array contains only zeroes, which means that all of them are even.
You are doing wrong integer arithmetic (as Obicere already stated in his answer). But this doesn't have any effect, as all elements of the array are even, so the result will be 100 in any case.

You are almost there. But you are initializing and storing the array wrong way. Do this
int num = console.nextInt();
int[] list = new int[num];
System.out.println("Enter " + num + " numbers");
for(int i = 0; i < num; i++) {
list[i] = console.nextInt();
}
System.out.println("The numbers you entered are: " + java.util.Arrays.toString(list));
System.out.println();
and also do as other suggested.

Write
percent = even * 100 / num.length;
Changing the order of the operations will make the integer division business work in your favour - you'll get a value rounded down to the next lowest percentage, rather than rounded down to zero.
Also fix the problem with all the numbers being zero by reading them from the keyboard, as in tintinmj's answer.

import java.util.*;
public class percentEvenClass
{
public static void main(String[] args){
int[] list = {6, 2, 9, 11, 3};
int percentEven_Result = percentEven(list);
System.out.println(percentEven_Result);
}
public static int percentEven(int[] list){
int count = 0;
int percent = 0;
for (int i=0; i<list.length; i++){
if (list[i] % 2 == 0){
count++;
}
percent = (count * 100)/ list.length;
}
return percent;
}
}