Question

Okay, I am still new to Java, and I am going on my ninth week of this stuff. I am going to post some source code from a school project – this is NOT the entire source – this chunk of code is my loop to amortize a loan. I am trying to amortize from a menu selection (menu prints to a text field) or from user input. The problem is, and I cannot figure out if it is math, or my loop, is that it is not amortizing the loan properly. I just want to know if anyone sees something obvious that I missed, and if so could you please point it out so I can get on the right track? Thanks in advance!

SOURCE:

private void amortizeButtonActionPerformed( java.awt.event.ActionEvent evt ) {
  // This module borrowed from Ryan Jones in George Griepp's PRG 420 class.

  NumberFormat nf = NumberFormat.getCurrencyInstance();
  int Monthly = 0;
  int monthcount = 0;
  String Output = "";

  int i = 0; // For first loop
  double loanamount = Double.parseDouble( tempTextField3.getText() );        //all loan amounts  are the same  
  double rate = Double.parseDouble( tempTextField1.getText() );     //Array for the rate
  double time = Double.parseDouble( tempTextField2.getText() );         //Array for the time
  double totalnumpayments = 0;         //Set for 
  double monthlypayment = 0; //Set for math calculation in first loop
  double interestPayment = 0;            //Set for math calculation in first loop
  double totaltime = 0; //Set for second loop to know how long to loop
  double loan = 0; //Set for second loop
  double interestPayment2 = 0; //Set for second loop
  double principlePayment = 0;  //Set for second loop


  for ( i = 0; i < time; i++ ) {//First loop This loops through the arrays and gives the first message listed below three times
    monthlypayment = ( loanamount * ( ( rate / 12 ) / ( 1 - Math.pow( ( 1 + ( rate / 12 ) ), -( time * 12 ) ) ) ) );
    interestPayment = loanamount * ( rate * 100 / 1200 );
    totaltime = ( time * 12 );


    jTextArea1.setText( "" );
    jTextArea1.setText( "This loan has an interest rate of " + ( rate * 100 ) + "%" + " and a starting loan amount of " + nf.format( loanamount ) );
    jTextArea1.setText( "Payment Number\t\t" + "Towards Principle\t\t" + "Towards Interest\t" + "Remaining on loan" );
    jTextArea1.setText( "" ); // Part of the first loop this will appear three times with the math listed above


    System.out.println( totaltime );
    Monthly++;

    Output += ( ( monthcount++ ) + "\t\t\t" + nf.format( principlePayment ) + "\t\t\t" + nf.format( interestPayment2 ) + "\t\t\t" + nf.format( loan - principlePayment ) + "\n" );


    loan = -principlePayment;// Changes the numbers as the loop goes
    interestPayment2 = loan * ( rate * 100 / 1200 );// Changes the numbers as the loop goes
    principlePayment = monthlypayment - interestPayment2;// Changes the numbers as the loop goes

  }
  jTextArea1.setText( Output );
}

Answer 1

There are a few things, which might be an issue or not (I am not familiar with the maths behind loans).

1. You calculated totaltime in your for-loop, without making use of the i variable. Hence it will always result in the same result, and can be calculated outside the loop
2. By calling setText on the jTextArea1 inside the loop, where your last call is with the empty String, you might as well clear it before entering the for-loop, as in the end the jTextArea1 will just be empty. Or just remove the code since after the for loop you set the text to Output
3. The loan = -principlePayment is a weird statement. So if the principlePayment is a positive number, loan will be a negative number, and interestPayment2 as well
4. You would better use a StringBuilder to construct your Output variable

I think number 3 is the most important for your calculations.