Java: Math.round function not working with integer

Question

Looks like there is issue with java's Math.round function, when passing integer value to it. I ran it for couple of inputs but giving suprisingly wrong results.

Sample Code:

    public static void main(String[] args) {
        System.out.println("roundOff1: " + Math.round(1669053278));
        System.out.println("roundOff2: " + Math.round(1669053304));
        System.out.println("roundOff3: " + Math.round(1669053314));
        System.out.println("roundOff4: " + Math.round(1669053339));
    }

Stdout:

roundOff1: 1669053312
roundOff2: 1669053312
roundOff3: 1669053312
roundOff4: 1669053312

My use case was to round of the System.currentTimeMillis()/1000 but end in getting wrong result. Did I really found a bug in Java or missing something here?

Answer 1

int values dont have decimal places so they cant be rounded so u need to add an extra parametre to round method eg to round to nearest 10 you can use

 Math. round(num/10.0) * 10;

Answer 2

public static void main(String[] args) {

    System.out.println("roundOff1: " + Math.round(1669053278d));
    System.out.println("roundOff2: " + Math.round(1669053304d));
    System.out.println("roundOff3: " + Math.round(1669053314d));
    System.out.println("roundOff4: " + Math.round(1669053339d));
}

You need to use double value as shown above. Math.round(double) return long. But Math.round(float) returns int. By default, your code was using Math.round(float) & the result value was out of int range. That was the issue

Answer 3

The reason why it does not work in your case is because the rounding function can only optimally work if your data is a decimal! Your numerical data are whole numbers (numbers without a decimal). As a result, the Math.round function will not work! So, to use the Math.round function correctly, you have to have decimal numericals as your data.

To illustrate this, I have written your code and added decimals to showcase you the optimal output of the Math.round() on Java:

import java.lang.Math; 
class RoundingNumbers{
public static void main(String[] args){
double first_parameter = 1669053278.3;
System.out.println("roundOff1: " + Math.round(first_parameter));
double second_parameter = 1669053304.8;
System.out.println("roundOff2: " + Math.round(second_parameter));
double third_parameter = 1669053314.8;
System.out.println("roundOff3: " + Math.round(third_parameter));
double fourth_parameter = 1669053339.5;
System.out.println("roundOff4: " + Math.round(fourth_parameter));
}

Answer 4

Try using java.time.Instant to accomplish your rounding of system time as in:

// the getEpochSecond() truncates - so add 500 millis first
long roundedSecs = Instant.now().plusMillis(500).getEpochSecond();

And a demo program:

// use a loop iterating every 250millis to demonstrate rounding

public static void main(String[] args) {
    try {
    for (int i = 0; i < 10; i++) {
        Instant nowTime = Instant.now();
        long b4 = nowTime.getEpochSecond();
        
        nowTime = nowTime.plusMillis(500);

       long result = nowTime.getEpochSecond();
       System.out.println("b4: "+b4+" after:"+result);
       Thread.sleep(250);
    }
    } catch (Exception e) {}
}

Prints:

b4: 1669056744 after:1669056744
b4: 1669056744 after:1669056745
b4: 1669056745 after:1669056745
b4: 1669056745 after:1669056746
b4: 1669056745 after:1669056746
b4: 1669056746 after:1669056746
b4: 1669056746 after:1669056746
b4: 1669056746 after:1669056747
b4: 1669056746 after:1669056747
b4: 1669056747 after:1669056747