How to add a long and preserve precision when printing-CodePudding

How do I preserve precision in my sum output. I think that what is happening is that the precision is exactly the same, but one option chooses to round.

public class Main {
    public static void main(String[] args) {
        final long seconds = 1637208584L;
        final long micro_of_second = 795307;
        final long sum = seconds   micro_of_second;
        System.out.println("seconds   micro_of_second: "   seconds   micro_of_second);
        System.out.println("sum:                       "   sum);
        System.out.println("equal?:                    "   (sum == seconds   micro_of_second));
    }
}

Output

seconds   micro_of_second: 1637208584795307
sum:                       1638003891
equal?:                    true

CodePudding user response：

The issue here isn't with precision its with the line

System.out.println("seconds   micro_of_second: "   seconds   micro_of_second);

this line implicitly converts seconds and micro_of_second to a string and concatenates them together.

so the result you are seeing is actually:

"seconds micro_of_second: " "1637208584" "795307"

what you want to do is evaluate the integer addition before adding it to the string. you can accomplish this by changing that line to

System.out.println("seconds   micro_of_second: "   (seconds   micro_of_second));

CodePudding user response：

In Java, expressions are evaluated left to right. Thus

"seconds   micro_of_second: "   seconds   micro_of_second

Is semantically identical to

("seconds   micro_of_second: "   seconds)   micro_of_second

The expression

"seconds   micro_of_second: "   seconds

evaluates to a String, where the (base 10) String-representation of seconds ("1637208584") is appended to the String "seconds micro_of_second: ". resulting in the String "seconds micro_of_second: 1637208584".
Then, the (base 10) String-representation of micro_of_second (795307) is appended to the String "seconds micro_of_second: 1637208584", resulting in the String seconds micro_of_second: 1637208584795307.

By contrast, seconds micro_of_second adds both values (1637208584 795307), evaluating to the long-value 1638003891.

If we want to first add both longs, we can enforce the evaluation order by introducing parentheses:

"seconds   micro_of_second: "   (seconds   micro_of_second)

This will evaluate to the String "seconds micro_of_second: 1638003891".

Alternatively, if we want to get the total amount of microseconds, we need to multiply the value of seconds with 1_000_000L before adding micro_of_second (since there are 1_000_000 microseconds in a second):

"micros: "   (seconds * 1_000_000L   micro_of_second)

Ideone demo

As an aside: In Java, variable names should be written in camelCase instead of snake_case (micro_of_second -> microOfSecond)