Ok, so I need to enhance the functionality of my existing Loan class to be able to calculate any of the 4 missing elements in a Loan, whether it is the Principal, APR, Term, or Payment. I already have the necessary calculations to get the monthly Payment, but I need help with the other 3.

I know finding APR requires a binary search.

The non-java formula for principal is something like this

Principal = Payment
APR/12 * ( 1 + 1/(1 + APR/12)exponent Term - 1)


Term = log ( 1 / (Payment/(APR/12 * Principal - 1)) + 1
log ( 1 + APR/12)

They numbers on top are divided by the bottom. Here is my current code.

.

public class Loan {
private double principal;
private double APR;
private int term;
private double monthlyPayment;
static DecimalFormat df = new DecimalFormat("0.##");

public static void main(String[] args) throws Exception {
Loan a = new Loan(30000, .04, 60, 0); //Principal, APR, Term. Calculate: Payment
Loan b = new Loan(30000, .04, 0, 522.5); //Principal, APR, Payment. Calculate: Term
Loan c = new Loan(0, .04, 60, 522.5); //APR, Payment, Term. Calculate: Principal
Loan d = new Loan(30000, 0, 60, 522.5); //Principal, Term, Payment. Calculate: APR
System.out.println(a.getMonthlyPayment());
}

private Loan() { }
/*
public Loan(double a, double b, int c) {
setPrincipal(a);
setAPR(b);
setTerm(c);
setMonthlyPayment(calculatePayment(a, b, c));
}
*/

// Principal APR Term Payment
public Loan(double a, double b, int c, double d) {
if (a == 0) {
//Calculate: Principal
}
else if (b == 0) {
//Calculate: APR
}
else if (c == 0) {
//Calculate: Term
}
else if (d == 0) {
//Calculate: Payment
setPrincipal(a);
setAPR(b);
setTerm(c);
setMonthlyPayment(calculatePayment(a, b, c));
}
else {
//Verify input
}
}

public void setPrincipal(double a) {
principal = a;
}

public void setAPR(double b) {
APR = b;
}

public void setTerm(int c) {
term = c;
}

public void setMonthlyPayment(double mp) {
monthlyPayment = mp;
}

public double getPrincipal() {
return principal;
}

public double getAPR() {
return APR;
}

public int getTerm() {
return term;
}

public double getMonthlyPayment() {
return monthlyPayment;
}

public static double calculatePayment(double principal, double APR, int term) {
double monthlyInterestRate = APR / 12;
double monthlyPayment = (monthlyInterestRate * principal) * (1 + (1 / (Math.pow(1 + monthlyInterestRate, term) - 1)));
return monthlyPayment;
}

public static void amortize(double principal, double APR, int term, double monthlyPayment) {
double monthlyInterestRate = APR / 12;
double totalPayment = monthlyPayment * term;
int payment = 1;
double balance = principal;

//print information
System.out.println("Loan Amount: $" + principal);
System.out.println("Number of Payments: " + term);
System.out.println("Interest Rate: " + APR + "%");
System.out.println("Monthly Payment: $" + df.format(monthlyPayment));
System.out.println("Total Payment: $" + df.format(totalPayment));
System.out.println();
System.out.println("Payment#\tInterest\tPrincipal\tBalance");

do {
double monthlyInterest = monthlyInterestRate * balance;
principal = monthlyPayment - monthlyInterest;
balance -= principal;

//Show Amortization Schedule
System.out.print(df.format(payment));
System.out.print("\t ");
System.out.print(df.format(monthlyInterest));
System.out.print("\t ");
System.out.print(df.format(principal));
System.out.print("\t ");
System.out.print(df.format(balance));
System.out.println();
payment++;
}
while (payment <= term);
}
}