Taking the Steve Yegge Challenge (Part 1: Bits and Bytes)
A few months ago I read Steve Yegge’s old post about phone screen tips, which included five categories that at a minimum should be covered during a phone screen. I found that I wasn’t as up to snuff as I thought I was in a couple of those categories, including “Bits and Bytes”. I took this as a challenge.
In particular I discovered I couldn’t even name the size in memory of each of the Java primitives. About a month ago I looked them all up, as I was preparing to interview for a job at Terracotta. The sad thing is, I’ve already forgotten them all again. So I am hoping now, once and for all, to burn them into my brain.
| primitive | size (bits) | min value | max value | comments |
|---|---|---|---|---|
| byte | 8 | -27 = -128 | 27-1 = 127 | signed 2's complement |
| short | 16 | -215 = -32768 | 215-1 = 32767 | signed 2's complement |
| int | 32 | -231 = -2,147,483,648 | 231-1 = 2,147,483,647 | signed 2's complement |
| long | 64 | -263 = -9,223,372,036,854,775,808 | 263-1 = 9,223,372,036,854,775,807 | signed 2's complement |
| float | 32 | 32-bit IEEE 754 floating point | ||
| double | 64 | 64-bit IEEE 754 floating point | ||
| char | 16 | '\u0000' (or 0) | '\uffff' (or 65,535) | Unicode |
| boolean | size isn't precisely defined in JLS |
Java bitwise and bit shift operators (legal on Java integral types)
| operator | explanation | comment |
|---|---|---|
| ~ | bitwise complement | inverts a bit pattern |
| << | signed left shift | keeps sign (high bit) of first operand |
| >> | signed right shift | keeps sign (high bit) of first operand |
| >>> | unsigned right shift | shifts a zero into high (leftmost) bit |
| & | bitwise and | see a clever real-world use of this |
| | | bitwise or | |
| ^ | bitwise exclusive or |
Notes on 2’s complement:
- high (leftmost) bit indicates sign (0 indicates positive, 1 indicates negative)
- largest value for n bits is 2(n-1)-1, smallest value is -2(n-1)
- processors can treat (add) all numbers uniformly (no need to check for sign and/or subtract)
- To negate a two’s-complement number, invert all the bits (bitwise NOT) then add 1 to the result.
- More conceptually, 2’s compliment of positive number obtained by subtracting that from 2n. For example, for 8 bits, “-3″ is gotten by subtracting “3″ (00000011) from 28 (100000000) to get (11111101).
- One interesting side effect of 2’s complement is that the smallest negative number is a weird number - it violates the aforementioned algorithm to flip the bit, which all other numbers adhere to. This leads to an exciting “feature” of
Math.abs(), where it’s possible for that method to return a negative number. For example, using the smallest negativeint,Math.abs(-2147483648)returns-2147483648!
