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Java - public static double not returning

I'm trying to learning Java from zero. I have an exercise that after reading it all over again I can't find why doesn't work. Researching on Google and StackOverflow returned to zero results...
Main objective is to translate dollars to pesetas by just multiplying by a number. I have to use two functions and call them on "main".
My problem is that "convertToPesetas" isn't taking the returned double of "askDollars". Can someone hand me a rope?
import java.util.Scanner;
public class Converter
{
public static void main(String[] args){
askDollars();
convertToPesetas();
}
public static double askDollars(){
System.out.println("Type the quantity of dollars:");
Scanner keyboard= new Scanner(System.in);
double dollars= keyboard.nextDouble();
System.out.println("Dollars: "+dollars);
return dollars;
}
public static double convertToPesetas(double dollars){
double pesetas = pesetas*166.386;
System.out.println(dollars+ "€ equals to: "+pesetas+" pesetas");
return pesetas;
}
}

Because you're not storing or supplying that value:
askDollars();
convertToPesetas();
Save the returned value in a variable and pass that variable to the next method:
double dollars = askDollars();
convertToPesetas(dollars);
Note: convertToPesetas also returns a value. You don't seem to need it to do that. But, you could use that if you take your design in a different direction. As an academic exercise for your next step, consider three methods:
One which asks for the user input.
One which converts the dollars value to the pesetas value. This has no input or output, just a method argument and a return value.
One which prints the output.
Each method would do exactly one, simple thing. And when you have this, you'll find that the second method is free to easily be moved to other objects, etc. because it's entirely independent and not coupled to the user interface in any way.

Method returns 0

I've made a program that generates a random number but every time it gives back 0.0
Program:
import java.util.*;
public class RandomNumber {
public static void main(String args[]){
double QuantityColors = 5;
double Mastermind = 0;
Random(QuantityColors, Mastermind);
System.out.println(Mastermind);
}
public static double Random(double QuantityColors, double Mastermind){
Mastermind = Math.random();
Mastermind = Mastermind * QuantityColors;
Mastermind = (int) Mastermind;
return Mastermind ;
}
}
I've been searching where the problem is, but the problem is in the return.

a) you are doing nothing with the result of "Random".
b) you can not modify Java argument. See change a functions argument's values?

First of all, you can use a builtin function to generate a next integer with a certain upper bound: Random.nextInt(int). For instance:
Random rand = new Random();
int masterMind = rand.nextInt(QuantityColors);
Instead of writing a Random method yourself.
It is nearly always better to use builtins since these have been tested extensively, are implemented to be rather fast, etc.
Next you seem to assume that Java uses pass-by-reference. If you perform the following call:
Random(QuantityColors, Mastermind);
Java will make a copy of the value of MasterMind. Setting the parameter in a method has no use. The only way to set a value - not encapsulated in an object - is by returning value. So:
MasterMind = Random(QuantityColors, Mastermind);
To make a long story short: the method does not return 0, you simply don't do anything useful with it.
A better solution would thus be to drop the Random method and use:
import java.util.*;
public class RandomNumber {
public static void main(String args[]){
int quantityColors = 5;
Random rand = new Random();
int mastermind = rand.nextInt(QuantityColors);
System.out.println(mastermind);
}
}
Further remarks
In your random method:
public static double Random(double QuantityColors, double Mastermind){
the MasterMind parameter is rather useless since you immediately set it with another value, so you better remove it and use a local variable instead.
Furthermore Java standards say that the name of classes, interfaces, etc. start with an uppercase; the names of methods and variables with lowercase.
Finally it is unclear why you use doubles since all the values you calculate are clearly integral.

It looks like your code would work if you wrote
Mastermind = Random(QuantityColors, Mastermind);
...because Java is pass by value, so calling a function will not change the variable you passed in.

Program That Teaches Someone Multiplication with Random Numbers

I am writing a program (game) that teaches someone multiplication. In this program, random pair of numbers are to be generated and inserted into the question: "What is x * y = z?" If the person answers correctly, then the system will print out "Very Good!" If the person does not answer the question correctly, then the system will print out, "No. Please try again." (Which, in return, the program will continue to ask the question until the person answers the question correctly.) As the person answers the question correctly, a new method will generate another question for the person to answer.
My code is breaking at the variable "answer" and the if statement "(guess != answer)."
Here is my code:
public class Exercise_535 {
public class Multiply{
SecureRandom randomNumbers = new SecureRandom();
int answer;
}
public void Quiz() {
Scanner input = new Scanner(System.in);
int guess;
System.out.println("Enter your answer: ");
guess = input.nextInt();
while(guess != -1)
{
checkResponse(guess);
System.out.println("Enter your answer: ");
guess = input.nextInt();
}
}
public void createQuestion(){
SecureRandom randomNumbers = new SecureRandom();
int digit1 = randomNumbers.nextInt();
int digit2 = randomNumbers.nextInt();
answer = digit1 * digit2;
System.out.printf("How much is %d times %d\n", digit1, digit2);
}
public void checkResponse(){
if (guess != answer)
System.out.println("No. Please try again.");
else{
System.out.print("Very Good!");
createQuestion();
}
}
}
Is there anyone that is able to help, or at least point me in the correct direction?
Thanks.

It is important to understand the scope of variables. In java if you create a variable (where you say int guess or int answer) that variable only lives within whatever curly braces you put it in -> { }. So if you need that variable in another method, you need to pass it that variable in the parentheses. checkResponse() doesn't know what guess or answer are, because they aren't declared in that scope, and you don't pass them in at the start of the function (you could have checkResponse(int guess, int answer) and then pass those in when you call it, for example).
You have an inner class Multiply, is there a reason you created a class within a class? There are reasons to do that, but it doesn't seem like you have any reason to do that here.
Also I don't see a main function, which is the entry point to a Java program, and all other functions need to be called from there (so Main() could then call Quiz() in your case, which would then call your other two functions). Computers read programs one line at a time, and when you call a function/method (like Quiz()) it jumps to that part, and then returns when that function calls "return".
I know this is a lot of information, but it doesn't seem like you understand how Java programs flow. What are you using to study Java? If you are reading a book or doing a course, I recommend reviewing some of the earlier lessons, to understand the flow of the program better. It is difficult for people to answer your question because the way your code is set up doesn't have a logical flow (which is why it isn't working). Hope this helps a little.

How do I pass these integers to the method below it?

I'm confusing myself here. My goal was to make a simple program that took the number of values the user wants to average, store them in an array (while adding them together) and finally giving the average of these numbers.
My thing is, I am trying to understand the concept of multiple classes and methods as I am new so I tried using another class just do do all the work, while the Main class would just create the object from the other class, and then run their methods. Maybe I am asking something impossible. Take a look at my code.
This is my Main class:
public class Main
{
public static void main(String[] args)
{
System.out.println("Please enter numbers to average together");
OtherClass averages = new OtherClass();
averages.collectNumbers();
averages.AverageNumbers();
}
}
Now I am not sure if anything goes in those parameters, or if I can even use "averages.AverageNumbers();" without creating another object with "OtherClass" called something else? I am pretty sure it's legal though.
Here is my other class for this project entitled "OtherClass"
import java.util.Scanner;
public class OtherClass // using this to name obj
{
public void collectNumbers() //name of our method that does things
{
Scanner sc = new Scanner(System.in);
System.out.println("how many integers would you like to average? ");
int givenNum = sc.nextInt();
System.out.println("Alright, I will average " + givenNum + " values. \nPress enter after each:");
int[] numCollect = new int[givenNum];
int sum = 0;
for (int i = 0; i < numCollect.length; i++)
{
numCollect[i] = sc.nextInt();
sum = sum + numCollect[i];
}
System.out.println(sum);
}
public int AverageNumbers(int givenNum, int sum)
{
int average = sum / givenNum;
System.out.println(average);
return average;
}
}
So when I run this now with the the method AverageNumbers, it does not work. I am suspecting that maybe I am passing in the integers wrong? I have been toying with it for about an hour now, so I am asking for help. How do I make this work?

This will work if you declare sum and givenNum as fields of your OtherClass, instead of as local variables. So, before the collectNumbers method, write
private int sum;
private int givenNum;
and remove the declarations of these two variables inside collectNumbers. So, for example, instead of
int givenNum = sc.getInt();
you'll just have
givenNum = sc.getInt();
because the variable already exists. Also change the declaration of the averageNumbers method to
public int averageNumbers()
because you no longer need to pass those two values in to this method.
This is the archetypical example of using the objects of a class to carry a small amount of data around, instead of just using a class as a way to group methods together. The two methods of this class work with sum and givenNum, so it makes sense to store these in each object of this class.
Lastly, in your averageNumbers method, you have an integer division, which will automatically round down. You probably want a floating point division instead, so you could write
double average = (double) sum / givenNum;
which converts sum to a double-precision floating point number before the division, and therefore does the division in floating point, instead of just using integers. Of course, if you make this change, you'll need to change the return type of this method to double too.

How to write a method to calculate compound interest by the year?

public class Balance {
public static void main(String[] args) {
System.out.printf("%.2f\n", balance(0.0, 0.0, 0.0));
}
/**
* #param principal
* #param rate
* #param years
* #return
*/
public static double balance(double principal, double rate, double years) {
double amount = 0;
Scanner sc = new Scanner(System.in);
System.out.print("Enter the initial investment amount: ");
principal = sc.nextDouble();
System.out.print("Enter the interest rate: ");
rate = sc.nextDouble();
System.out.print("Enter the number of years: ");
years = sc.nextDouble();
for (int i = 1; i < years; i++) {
amount = principal * Math.pow(1.0 + rate, years);
amount += principal;
}
return amount - principal;
}
}
My problem is with the printf line that I am using within the main method. Eclipse wants me to change the method balance from void to Object[]. When I do this I must return a value from balance. So I guess my question is, how would I return the proper value? Am I on the right track? Thank you for your time and constructive criticism. :)
EDIT - Thanks for the help everyone, much appreciated :) My math is off. I end up with 1000 more than I should have. hmmm.
So should I just take a 1000 from amount like so:
return amount - 1000;
Or this:
return amount - principal;
EDIT this is what I am going with since it is due tonight. Thanks to all for the assistance. :)

A few points:
balance() cannot be void, because you use its return value in S.out.printf(). Do you want balance to print to the screen, or do you want it to yield a number?
Your loop for (years = 0; years > 10; years++) won't run. Think about why. It might help to convert the for into a while, to visualize why.
You read in years as a double, but then use it as a counter in your loop. What type should it actually be?
Your balance() function takes three parameters, then immediately gets input and obliterates them. Do you want balance() to be provided these numbers, or do you want it to fetch them?
Otherwise, you seem to be on the right track.

The problem is that balance doesn't return anything, (it's a void function). Change it to:
public static double balance(double principal, double rate, double years) {
...
And inside that function, return the balance.
Java is telling you it wants an Object[] because printf is defined like this:
public static void printf(String format, Object ... params) {
// params is an Object[]
}
What this lets you do is pass parameters like this:
printf("some string", first, second, etc);
It lets you pass as many parameters as you want, and the function can handle them as if you passed an array.
It's exactly the same as if it was defined like this:
public static void printf(String format, Object[] params);
And you used it like this:
printf("some string", new Object[] { first, second, etc});
It's just easier to use.
EDIT:
The other option is to not print anything in main, but I would definitely advise returning the result and printing it in main. This follows the principle of making each function do as little as possible. balance should just calculate the balance. Printing it is unrelated.

Please consider a more drastic re-working of your code; as it is, your balance() function is doing all the work of your program (and your printf() line feels like an afterthought). If you break apart your code based on what the code does, I think you can do much better:
create a function that prompts the user and then reads in their input
create a function that calls the previous function three times for principal, rate, and years
create a function that computes and populates a payment schedule. Keep track of year, balance, payment, principal payment, and interest payment. (Or just the variables you're interested in -- but be aware that programs tend to grow new features, and these variables are often the second thing that users (or professors) ask to know when paying down a loan.)
create a function that prints the selected columns from your payment schedule.
create a function that orchestrates the previous functions.
When you re-write your program to use a GUI or webservice in three weeks, you'll definitely thank yourself for having written each function to do one task, and do it well. You'll also find it far easier to test smaller pieces of code that do only one task.
There is the risk of over engineering a too-generic solution -- what I'm really trying to suggest is a functional decomposition of your code into multiple smaller routines that do exactly what their name says. You might never move the program into a GUI or webservice or whatever, but you'll definitely be happier when someone reports that your amortization schedule is wrong, that you can control it via simpler programming, rather than having to re-type inputs all day long.

Yours is wrong, do this:
public static void main(String[] args) {
balance(1000.0, .05, 8.5);
}