What would be the most performant (also safe) way to replace the last digit of a long?

What would be the most performan开发者_运维问答t (also safe) way to replace the last digit(Least significant digit) of a long(that was actually generated as timestamp by System.currentTimeInMillis()) by some other digit?

Or is there a better way to attach any fixed attachment to the end of it, by making use of bitwise operations?

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In your comments you say that both binary digits or decimal digits would be fine. Since Andrew posted the decimal version, i post the binary version in which you want to replace the 2 ls-bits:

The following program goes through the 4 possibilities with which you can replace the 2 ls-bits and produces the output:

9999999999999999
9999999999999996
9999999999999997
9999999999999998
9999999999999999

code:

public class A {
public static void main(String[] args) {

    long aLong = 9999999999999999L;
    System.out.println(aLong);

    long aLong2 = aLong & ~3 + 0;
    System.out.println(aLong2);
    aLong2 = aLong & ~3 + 1;
    System.out.println(aLong2);
    aLong2 = aLong & ~3 + 2;
    System.out.println(aLong2);
    aLong2 = aLong & ~3 + 3;
    System.out.println(aLong2);


}
}

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If this is not a hypothetical question, ie to find a fast algorithm for the heck of it, then please ignore this answer. The correct way (as has been mentioned) is to do (somelong/10)*10 + newvalue

A faster (hypothetical) way is probably to have some two dimension array of adjustment values.

int[][] adjustment = new int[16][10];

where the first array index represents what the current value is anded with 0x0F (the last 4 bits) the second array index would be what you want the new number to be

the value is the adjustment to the variable

so the code would be

newLong = somelong + adjustment[somelong&0x0F][what_you_want_the_new_digit];

so no multiplication or division

as an example, let's say the input number is 22, and you want it to be 26

26 is 011010 so 26 & 0x0F is the bottom 4 bits 1010 which is 10

adjustment[10][6] = 4 (you have precalculated that it is 4)

so you'd have 22 + 4 = 26

obviously index 10 is the same as index 0, index 11, is the same as index 1, etc, etc.