This line is your problem:

litersOfPetrol = Float.parseFloat(df.format(litersOfPetrol));

There you formatted your float to string as you wanted, but but then that string got transformed again to a float, and then what you printed in stdout was your float that got a standard formatting. Take a look at this code

import java.text.DecimalFormat;

String stringLitersOfPetrol = "123.00";
System.out.println("string liters of petrol putting in preferences is "+stringLitersOfPetrol);
Float litersOfPetrol=Float.parseFloat(stringLitersOfPetrol);
DecimalFormat df = new DecimalFormat("0.00");
df.setMaximumFractionDigits(2);
stringLitersOfPetrol = df.format(litersOfPetrol);
System.out.println("liters of petrol before putting in editor : "+stringLitersOfPetrol);

And by the way, when you want to use decimals, forget the existence of double and float as others suggested and just use BigDecimal object, it will save you a lot of headache.

You can use Double.parseDouble() to convert a String to a double:

String text = "12.34"; // example String
double value = Double.parseDouble(text);

For your case it looks like you want:

double total = Double.parseDouble(jlbTotal.getText());
double price = Double.parseDouble(jlbPrice.getText());

Use Integer.parseInt (see javadoc), that converts your String to int using base two:

int decimalValue = Integer.parseInt(c, 2);

IsNumeric() method works for only Integer value not for decimal value

like this

String str = '10';
system.debug('===strToDec=='+str.isNumeric());

This will return true.

String str = '10.25';

Decimal strToDec = decimal.valueOf(str);

system.debug('===strToDec=='+strToDec);

String decToStr = String.valueOf(strToDec);

system.debug('===decToStr=='+decToStr);

Run this code in developer console ad check output.

In you case do something like. First convert string to numeric then check and again convert string into decimal for future use

String  str = '10.25';
Integer intCheck = Integer.ValueOf(str);
Decimal decVal = Decimal.ValueOf(str);
if(String.ValueOf(intCheck ).isNumeric())
//use decVal here

Don't understand why you need this.

Use DecimalFormat

String formattedNumber = new DecimalFormat("0.00").format(x);