Java unexpected console output

Question

How can I tabulate my output so that the calculations line up with Celsius and Fahrenheit?

Also, how do I display a number like 183.20000000000002 more naturally?

public static void main(String[] args) {
    double cel = 0;
    double fahrenheit =cel * 1.8+ 32;

    int i;
    System.out.println();// blank line
    System.out.print("Hello ");// output line
    System.out.println();
    System.out.println("This Program will show temperature conversions from 0-100\nThen in reverse \nCelsius              Fahrenheit");

    for (i = 0; i <25; i++){    
        cel =cel+ 4;
        fahrenheit =cel * 1.8+ 32;
        System.out.println(+ cel + "                  " + fahrenheit);
    }
}

Answer 1

You can use either String#format or System.out.printf to generate formatted output, for example

public static void main(String[] args) {
    double cel = 0;
    double fahrenheit = cel * 1.8 + 32;

    int i;
    System.out.println();// blank line
    System.out.print("Hello ");// output line
    System.out.println();
    System.out.println("This Program will show temperature conversions from 0-100\nThen in reverse");
    System.out.printf("%s | %s%n", "Celsius", "Fahrenheit");

    for (i = 0; i < 25; i++) {
        cel = cel + 4;
        fahrenheit = cel * 1.8 + 32;
        System.out.printf(" %6.2f | %6.2f%n", cel, fahrenheit);
    }
}

Which outputs...

Hello 
This Program will show temperature conversions from 0-100
Then in reverse
Celsius | Fahrenheit
   4.00 |  39.20
   8.00 |  46.40
  12.00 |  53.60
  16.00 |  60.80
  20.00 |  68.00
  24.00 |  75.20
  28.00 |  82.40
  32.00 |  89.60
  36.00 |  96.80
  40.00 | 104.00
  44.00 | 111.20
  48.00 | 118.40
  52.00 | 125.60
  56.00 | 132.80
  60.00 | 140.00
  64.00 | 147.20
  68.00 | 154.40
  72.00 | 161.60
  76.00 | 168.80
  80.00 | 176.00
  84.00 | 183.20
  88.00 | 190.40
  92.00 | 197.60
  96.00 | 204.80
 100.00 | 212.00

Check out this for more details about the available formatting options

Answer 2

Use the DecimalFormat class to ensure that only the right number of decimals show up. Floating Points (including doubles, which are just "double wide" floating points) use a special binary representation and some decimals don't always round correctly. Using Formatting classes ensures that you don't output a number that is longer than a user expected (eg telling a user a price is $1.0000001 wouldn't be good practice--its part of the reason why floating points are poor choices for things like money).

Example:

double data = 0.08d+0.02d;
//The # means "optional digit", the zeros mean mandatory
//e.g. 4 formatted with "#.##" would be "4", but with 
//"#.00" would be "4.00"
DecimalFormat fmt = new DecimalFormat("#.00");
System.out.println(fmt.format(data));

To address the second part of your question (although that should be in a separate question altogether), I recommend you review printf() . Here is a good guide on its use.