Double value with specific precision in java

add this instance of DecimalFormat to the top of your method:

DecimalFormat four = new DecimalFormat("#0.0000"); // will round and display the number to four decimal places. No more, no less.

// the four zeros after the decimal point above specify how many decimal places to be accurate to.
// the zero to the left of the decimal place above makes it so that numbers that start with "0." will display "0.____" vs just ".____" If you don't want the "0.", replace that 0 to the left of the decimal point with "#"

then, call the instance "four" and pass your double value when displaying:

double value = 0;
System.out.print(four.format(value) + " kg/n"); // displays 0.0000

I suggest you to use the BigDecimal class for calculating with floating point values. You will be able to control the precision of the floating point arithmetic. But back to the topic :)

You could use the following:

static void test(String stringVal) {
    final BigDecimal value = new BigDecimal(stringVal).multiply(new BigDecimal("2.2046"));
    DecimalFormat df = new DecimalFormat();
    df.setMaximumFractionDigits(4);
    df.setMinimumFractionDigits(4);
    System.out.println(df.format(value) + " kg\n");
}

public static void main(String[] args) {
    test("0");
    test("1");
    test("3.1");
}

will give you the following output:

0,0000 kg

2,2046 kg

6,8343 kg

DecimalFormat will allow you to define how many digits you want to display. A '0' will force an output of digits even if the value is zero, whereas a '#' will omit zeros.

System.out.print(new DecimalFormat("#0.0000").format(value)+" kg\n"); should to the trick.

See the documentation

Note: if used frequently, for performance reasons you should instantiate the formatter only once and store the reference: final DecimalFormat df = new DecimalFormat("#0.0000");. Then use df.format(value).

System.out.format("%.4f kg\n", 0.0d) prints '0.0000 kg'