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.

Use DecimalFormat

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

Yes you can do it with String.format:

String result = String.format("%.2f", 10.0 / 3.0);
// result:  "3.33"

result = String.format("%.3f", 2.5);
// result:  "2.500"

double total = 44;
String total2 = String.valueOf(total);

This will convert double to String

You can use java's Double.valueOf() defined here instead; then you can convert the resulting double to an int.

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.

String s = "0.01";
double d = Double.parseDouble(s);
int i = (int) d;

The reason for the exception is that an integer does not hold rational numbers (= basically fractions). So, trying to parse 0.3 to a int is nonsense. A double or a float datatype can hold rational numbers.

The way Java casts a double to an int is done by removing the part after the decimal separator by rounding towards zero.

int i = (int) 0.9999;

i will be zero.