Please help with decimal format

Make sure that you keep your DecimalFormat objects and your decimalPlaces variables in sync. For example:

public class Tester {
private int decimalPlaces;
private DecimalFormat decimalFormat;

public void setNumberOfDigits(int nod) {
decimalPlaces = nod;
String format = "#.";
while( nod-- > 0 ) {
format = format + "0";
}
decimalFormat = new DecimalFormat(format);
// after you call this function, make sure you update the numbers that you are printing.
}
}

Hope this helps.

Sorry, the "-- >" isn't an "arrow" it is "nod--" is greater than 0. The "--" is shorthand for decrement variable after returning. For example:

int i = 0; // Before After Value
i++; // 0 1 0
++i; // 1 2 2
i--; // 2 1 2
--i; // 1 0 0

The variable "format" is a temporary that is used to create the pattern for displaying numbers. It should be declared inside the function as shown in the example. "decimalFormat" should be declared as a class member.

Hope this helps.

OK, I have created a new class to just test decimal format.

I must be doing something wrong.

Thanks,
Eric

import java.awt.*;
import java.awt.event.*;
import java.text.DecimalFormat;
import java.lang.*;
import javax.swing.*;

public class DecimalFormatTester extends JFrame
{
private int decimalPlaces;
private DecimalFormat decimalFormat;
private String userDigits = "";
private JTextField decimalFormatField;
private String output = "";
JTextArea outputArea;

public DecimalFormatTester()
{
super( "Using Decimal Format" );

Container container = getContentPane();
container.setLayout( new FlowLayout() );

outputArea = new JTextArea();

JLabel decimalFormatLabel = new JLabel( " Decimal Points" );
decimalFormatField = new JTextField( 10 );

decimalFormatField.addActionListener
(
new ActionListener() // anonymous inner class
{
// read value from user
public void actionPerformed( ActionEvent event )
{
setNumberOfDigits( Integer.parseInt( decimalFormatField.getText() ) );
String decimalPlacesArray[] = new String[ decimalPlaces ];
for( int counter = 0; counter < decimalPlacesArray.length; counter++ )
{
decimalPlacesArray[ counter ] = "0";
}
String decimalString = "";
for( int counter = 0; counter < decimalPlacesArray.length; counter++ )
{
decimalString += decimalPlacesArray[ counter ];
}
userDigits = "." + decimalString;
setNumberOfDigits( decimalPlaces );
}
} // end anonymous inner class
); // end call to addActionListener


container.add( decimalFormatLabel );
container.add( decimalFormatField );
container.add( outputArea );


setSize(420, 125 );
setVisible( true );
}

public void setNumberOfDigits(int nod)
{
decimalPlaces = nod; String format = "#.";
while( nod-- > 0 )
{
format = format + "0";
}
decimalFormat = new DecimalFormat(format);
// after you call this function, make sure you update the numbers that you are printing.
}

private void displayResult( String operation )
{
output += "uD1 " + decimalPlaces;
displayOutput();
}

private void displayOutput()
{
outputArea.setText( output );
}

public static void main( String args[] )
{
DecimalFormatTester dft = new DecimalFormatTester();
dft.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}
}

I couldn't figure out what some of the code was trying to do. But it didn't seem like you were actually using the DecimalFormat object. Here's the code that I changed:

import java.awt.*;
import java.awt.event.*;
import java.text.DecimalFormat;
import java.lang.*;
import javax.swing.*;

public class DecimalFormatTester extends JFrame
{
private int decimalPlaces;
private DecimalFormat decimalFormat;
private String userDigits = "";
private JTextField decimalFormatField;
private String output = "";
JTextArea outputArea;

public DecimalFormatTester()
{
super( "Using Decimal Format" );

Container container = getContentPane();
container.setLayout( new FlowLayout() );

outputArea = new JTextArea();

JLabel decimalFormatLabel = new JLabel( " Decimal Points" );
decimalFormatField = new JTextField( 10 );

// Text fields don't post action events, so try binding on key events.
decimalFormatField.addKeyListener(new KeyListener() {

public void keyTyped(KeyEvent e) {
}

public void keyPressed(KeyEvent e) {
}

public void keyReleased(KeyEvent e) {
if( e.getKeyCode() == KeyEvent.VK_ENTER ) {
System.out.println("Here");
setNumberOfDigits( Integer.parseInt( decimalFormatField.getText() ) );

output = decimalFormat.format(Math.PI);
displayOutput();

// I don't know what you are trying to do here
/*
String decimalPlacesArray[] = new String[ decimalPlaces ];
for( int counter = 0; counter < decimalPlacesArray.length; counter++ )
{
decimalPlacesArray[ counter ] = "0";
}
String decimalString = "";
for( int counter = 0; counter < decimalPlacesArray.length; counter++ )
{
decimalString += decimalPlacesArray[ counter ];
}
userDigits = "." + decimalString;
setNumberOfDigits( decimalPlaces );*/
}
}
});

container.add( decimalFormatLabel );
container.add( decimalFormatField );
container.add( outputArea );

setSize(420, 125 );
setVisible( true );
}

public void setNumberOfDigits(int nod)
{
decimalPlaces = nod; String format = "#.";
while( nod-- > 0 )
{
format = format + "0";
}
decimalFormat = new DecimalFormat(format);
// after you call this function, make sure you update the numbers that you are printing.
}

private void displayResult( String operation )
{
output += "uD1 " + decimalPlaces;
displayOutput();
}

private void displayOutput()
{
outputArea.setText( output );
}

public static void main( String args[] )
{
DecimalFormatTester dft = new DecimalFormatTester();
dft.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}
}

If you have any questions, just post them.

Hope this helps.

In reference to the code that you posted, you are using Math.PI, so method displayResult( String operation ) never gets invoked.

In order for me to use the input from the user, I would have to use decimalPlaces in place of Math.PI.

Am I reading this right?

Thanks,
Eric

OK, please let me know if I am understanding this correctly.

DecimalFormat uses patterns.

In the code that you posted, the pattern is "#.".

So the # is the number that the user enters.

Is that right?

How do I use this to apply it to a sum of two fractions or something similar?
I am wanting to be able to let a user input two rational numbers (two numerators and two denominator), and enter the number of decimal places they prefer to see. Then the user can add, subtract, multiply, or divide the two fractions. I have everything working except for this part. I just want to understand how this works. Sorry for all the questions.

Thanks,
Eric

Thanks,
Eric