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Adding iterations together in Java

I'm trying to add together the sum of the iterations of a for loop. This is what I have so far.
import java.util.Scanner;
public class Pennies
{
public static void main (String [] args)
{
double amount; //To hold the amount of pennies per day
double days; //To hold the days user saved.
double total;
System.out.println("I will display a table of salary if you're paid every day in pennies and it doubles every day.");
Scanner keyboard = new Scanner(System.in);
System.out.print("How many days do you wish to save for?");
days = keyboard.nextDouble();
//Display the table
System.out.print("Day \t Salary \n");
for (amount = 1; amount <= days; amount++)
{
total = amount * .01 * amount;
System.out.println(amount + "\t \t $" + total);
}
}
}
Any help would be greatly appreciated!

In order to keep on adding the salary for each day and keeping the track of total for each day (as I get it from your statement), you can change:-
total = amount * .01 * amount;
to
total += amount * .01 * amount; // total = total + (amount*0.01*amount)
which(when not printing each day information separately) can be simplified as:-
total = amount * .01 * amount * days;

I compiled your code and noticed that your numbers were off. Assuming that you want the first day's pay to be one penny, and for it to double every following day, here's what I came up with. It's hard to tell if this is exactly what you want since you didn't actually ask a question, so let me know if this is what you're looking for.
public static void main(String[] args) {
System.out
.println("I will display a table of salary if you're paid every day in pennies and it doubles every day.");
Scanner keyboard = new Scanner(System.in);
System.out.print("How many days do you wish to save for?");
double days = keyboard.nextDouble();
// Display the table
System.out.print("Day \t Salary \n");
double pay = 0.01;
double totalPay = 0.0;
for (int i = 1; i <= days; i++) {
System.out.println(i + "\t \t $" + pay);
totalPay += pay;
pay *= 2;
}
System.out.println("Total Pay \t $" + totalPay);
keyboard.close();
}
You're originally doing total = amount*0.1*amount, which won't give you what you want. You're confusing squaring amount with simply doubling it.
Edit: Also consider changing days to an int. I don't see any reason why it should be a double

Operators can't be applied to java string

I am writing a program that takes the loan amount, interest rate, and total paid from a text file. It is supposed to update the interest rate to total interest and the same for total paid. Then it is supposed to calculate the monthly payments.
I keep getting the error operators can't be applied to java string for the calculations including loan. I'm guessing this is because you can't use strings in calculations? Maybe i'm wrong. I am stumped.
Example input:
56750.00 .065 72.00
43675.00 .075 48.00
64950.00 .045 36.00
24799.00 .085 48.00
My code
import java.io.*;
import java.util.*;
import java.text.*;
public class DB4
{
static Scanner keyboard = new Scanner(System.in);
public static void main(String[] args) throws FileNotFoundException
{
String loan;
int Count = 0;
double interest = 0;
double Numberofmonths = 0;
double totint;
double totpay;
double monthly;
Scanner inFile= new Scanner(new FileReader("Project4InData.txt"));
PrintWriter outFile = new PrintWriter("Project4.out");
while (inFile.hasNext())
{
loan = inFile.next();
interest = inFile.nextDouble();
Numberofmonths = inFile.nextDouble();
// calcs
totint = interest * loan;
totpay = totint + loan;
monthly = loan / 12;
outFile.print("Loan Amount: " + loan);
outFile.print(" ");
outFile.println("Interest: " + totint);
outFile.print(" ");
outFile.println("Total paid: " + totpay);
outFile.print(" ");
outFile.println("Monthly payment: " + monthly);
}
inFile.close();
outFile.close();
}
}

loan is a string, so interest * loan doesn't make sense.
To realize why it doesn't make sense to apply * to a string, consider what "abc" * 2 or "def" * "ghi" would mean. (Absolutely nothing, which is why Java doesn't allow you to perform those operations on string objects).
You can apply + to strings, but it doesn't do addition, it does concatenation.
Same logic applies to the / operator.
Make loan a double.

How do I call a method into my main method so it can execute the program?

Obviously, I'm not looking for all the answers but some direction would be much appreciated.
So I have this program that prompts the user to enter their annual interest rate and savings amount. The program calculates the compound value for 6 months.
I'm having issues calling the new method into the main method to execute the calculations. The main method asks the users for their annual interest rate and the savings amount. It calls the new method, and if all are positive numbers it executes the program. If any are negative it comes up with an error.
I thought I called the method in the last line with calling it but obviously I am wrong. I have tried googling and am having trouble understanding this calling aspect. Any information would be appreciated. I'm new so I'm still working on this programming thing! Not sure why that last curly bracket isn't in the code. Here's the code:
import java.text.DecimalFormat;
import javax.swing.JOptionPane;
public class FifthAssignment {
static double savingAmount = 0.0; // the 2 given static doubles
static double annualInterestRate = 0.0;
int sixthMonth;
public static double compoundValueMethod(double savingAmount, double annualInterestRate, int sixthMonth) {
return sixthMonth; // sixMonth is the compound value
}
{
DecimalFormat formatter = new DecimalFormat(".00");
double monthlyInterestRate = annualInterestRate / 12;
if (savingAmount < 0)
if (annualInterestRate < 0) {
JOptionPane.showMessageDialog(null, "Error. Both of your values are negative!");
System.exit(0);
}
if (savingAmount < 0) {
JOptionPane.showMessageDialog(null, "Error. Your savings amount is negative!");
}
else if (annualInterestRate < 0) {
JOptionPane.showMessageDialog(null, "Error. Your annual interest rate is negative!");
}
else {
for (int i = 1; i <= 6; i++) {
sixthMonth = (int) ((savingAmount + sixthMonth) * (1 + monthlyInterestRate));
}
}
}
public static void main(String[] args) {
DecimalFormat formatter = new DecimalFormat(".00"); // bring in the
// decimal
// formatting for
// program
String SA = JOptionPane.showInputDialog("What is your savings amount? "); // Window
// pops
// up
// asking
// for
// your
// savings
// amount.
// As
// a
// string?
savingAmount = Double.parseDouble(SA); // Returns a double given to
// value in string above
String AIR = JOptionPane.showInputDialog("What is the annual interest rate? "); // Window
// pops
// up
// asking
// for
// annual
// interest
// rate.
// String
// as
// above.
annualInterestRate = Double.parseDouble(AIR); // Returns the same as
// savings amount but
// for annual interest
// rate
{
JOptionPane.showMessageDialog(null, "Your total compounded value after 6 months is "
+ compoundValueMethod(savingAmount, annualInterestRate, 6));
}
}
}

Function Call should have parenthesis
Try This:
JOptionPane.showMessageDialog(null, "Your total compounded value after 6 months is " + compoundValueMethod(savingAmount,annualInterestRate,6));
Your function should return the computed value, but in your case you are returning 'void', so it should be like this
public static int compoundValueMethod(double savingAmount, double annualInterestRate, int sixthMonth) {
int res = 0;
DecimalFormat formatter = new DecimalFormat(".00");
sixthMonth = 6;
double monthlyInterestRate = annualInterestRate / 12;
if (savingAmount < 0)
if (annualInterestRate < 0) {
JOptionPane.showMessageDialog(null, "Error. Both of your values are negative!");
System.exit(0);
}
if (savingAmount < 0) {
JOptionPane.showMessageDialog(null, "Error. Your savings amount is negative!");
}
else if (annualInterestRate < 0) {
JOptionPane.showMessageDialog(null, "Error. Your annual interest rate is negative!");
}
else {
for (int i = 1; i <= 6; i++) {
res = (int) ((savingAmount + res ) * (1 + monthlyInterestRate));
}
}
return res;
}

Based on the current code you have written, the function is not called properly and should look like :
JOptionPane.showMessageDialog(null, "Your total compounded value after 6 months is " + compoundValueMethod(savingAmount,annualInterestRate,6));
If you always want the value of 6 to be passed to your function, then instead of having it as a parameter to the function, you should be better off declaring a static final class variable like this:
private final static int SIXTH_MONTH = 6;
If you want to read the number of months, then yes your function should have that parameter, you should read that parameter just like the other 2 params you are reading and pass it along. And you should remove the line that is assigning the value of 6 to your method parameter.
sixthMonth = 6;
and instead use the value passed in to the function from the main() function

Java loop, need suggestion for code simplification..

I am just practicing Java as a beginner. So here is the question:
Suppose you save $100 each month into a savings account with the annual interest rate 5%. Thus, the monthly interest rate is 0.00417. After the first month, the value in the account becomes 100 * (1 + 0.00417) = 100.417 and the second month will be (100 + firstMonthValue) * 1.00417, and then goes on like so every month. So here is my code:
import javax.swing.JOptionPane;
public class vinalcialApplication {
public static void main(String args[]){
String monthlySaving = JOptionPane.showInputDialog("Enter the monthly savings");
double monthsaving = Double.parseDouble(monthlySaving);
//define monthly rate
double monthlyrate = 1.00417;
double totalrate = monthlyrate + 0.00417;
double firstMonthValue = monthsaving * (totalrate);
double secondMonthValue = (firstMonthValue + 100)*(monthlyrate);
double thridMonthValue = (secondMonthValue + 100) * (monthlyrate);
.........
System.out.print("After the sixth month, the account value is " sixthMonthValue);
}
}
I mean the code works but it is too much code to write.. I am sure I can use a loop or if statement to do this but haven't figured a way to do it yet.. can you please help?
Thank you.

If I understand correctly, this is called compound interest.
There is a mathematical formula to achieve what you wanted without looping.
Here is the formula from wikipedia
Where,
A = future value, P = principal amount (initial investment),r = annual nominal interest, rate, n = number of times the interest is compounded per year,t = number of years
Hope this helps you in solving what you wanted. I can give you sample code, but I think it is fairly easy to convert this formula to java statement. Let us know if you need any more info.
Source: http://en.wikipedia.org/wiki/Compound_interest

import javax.swing.JOptionPane;
public class vinalcialApplication {
public static void main(String args[]){
String monthlySaving = JOptionPane.showInputDialog("Enter the monthly savings");
double monthsaving = Double.parseDouble(monthlySaving);
//define monthly rate
double monthlyrate = 1.00417;
double totalrate = monthlyrate + 0.00417;
double value = monthsaving * (totalrate);
for(int i = 1; i<6;i++) {
value = (value + 100)*(monthlyrate);
}
System.out.print("After the sixth month, the account value is " value);
}

The maths is probably wrong, but the basic concept is sound.
public static void main(String[] args) {
double monthsaving = 100;
double monthlyrate = 1.00417;
double savings = 0;
// Loop for six months...
for (int index = 0; index < 6; index++) {
savings += monthsaving * monthlyrate;
System.out.println(index + ":" + savings);
}
System.out.println(savings);
}
Take a closer look at Control Flow Statements, paying attaching with while, do-while and for statements

Re-assigning the value to a double in a for loop

I'm currently working on a program that will calculate a simple interest rate and monthly payment taken from a loan. Although, I am running into one pretty big problem. I am trying to make it so that my principal value(see in code) is re-assigned the value of my new balance(see in code). Here is my code right now, I will explain in better detail under it:
import java.util.Scanner;
public class Payments {
public static double principal; //principal
public static double annualrate; //annual interest rate
public static double p; //monthly payment
public static double mr; //monthly interest rate
public static double nb; //new balance after monthly payments
public static double i; //interest (monthly)
public static String spaces = " "; //spaces for making clean columns
public static int months = 12;
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.println("Principal: $");
principal = input.nextDouble();
System.out.println("Anual Interest Rate: ");
annualrate = input.nextDouble();
System.out.println("Monthly Payment: $");
p = input.nextDouble();
calculate();
}
public static void calculate() {
mr = annualrate / 12;
i = mr * p;
nb = principal + i - p;
System.out.print("Month Principal Amt. Interest Payment New Balance");
System.out.println();
System.out.println();
for(int x = 1; nb > 0; nb = principal + i - p){
System.out.println(x + spaces + p + spaces + i + "%" + spaces + "$" + p + spaces + "$" + nb);
p = (Double)null;
p = nb;
}
}
}
So as you can most likely see by the comments in the code, all of the variables are shown. Now, disregard the null and me casting it to a double because that was the last thing that I tried to do before asking you guys :) anyways, my final question is, how can I go about re-assigning the value of principal to my new balance (nb)? Also, a side question, would a while-loop be better for this kind of program?

This has already been said in the comments, but the most immediate problem that I can see is in
for(int x = 1; nb > 0; x++){
your condition, nb > 0 will never ever change in that loop.
This means 1 of 2 things
your loop won't execute
your loop won't exit (infinite loop)
judging by how you're calculating nb(new balance) I'm assuming that it's going to be above 0 most of the time, and that your loop will never stop.
As things are right now, I'm not entirely sure what you actually want to do in that for loop or whether you even want a for loop at all, as that part is unclear.
what is the for loop supposed to do?
it almost looks like you're attempting to do something like
for (int x = 1; principal > 0; x++)
{
principal += principal * monthlyInterestRate;
principal -= payment;
System.out.println("insert output here");
}

nb = p + i - p; ???? looks like you need to double check your code.. the for loop doesnot change the condition at any place.. if the value is greater than 0, you will always lead it to an infinite loop, or it will never enter the loop at all( if np<1) . so check you code for runtime errors..