I want to know what is the best and fastest implementation of hash algorithms for Java especially MD5 and SHA-2 512 (SHA512) or 256. I want a function to get a string as an argument and return the hash as the result. Thak you.
Edit: This is for getting mapping each URL to a unique hash. Since MD5 is not that reliable in this area, I'm more interested in finding the best & fastest implementation for SHA-2 algorithms. Note that I know even SHA-2 might produce the same hash for some URLs but I can live with that.
First things first: speed is overrated. You should make measures before declaring that a given algorithm is "too slow". Most of the time, hash function speed makes no noticeable difference anyway. If you have qualms about security, then first select a hash function which is secure enough, and then only worry about performance.
Moreover, you want to hash "strings". A Java String is, internally, a chunk from an array of char values which represent Unicode code points (actually, Unicode 16-bit code units which encode the code points using UTF-16). A hash function takes as input a sequence of bits or bytes. So you will have to make a conversion step, e.g. str.getBytes("UTF-8"), to obtain your string as a bunch of bytes. It is likely that the conversion step will have a non-negligible cost when compared to the hashing itself.
Note: beware of URL-encoding ! In a URL, some bytes can be replaced with sequences beginning with a '%' sign; this is meant to support non-printable characters, but it can be used on "standard" characters as well (e.g., replacing 'a' with '%61'). This means that two strings which are distinct (in the String.equals() sense) may actually represent the same URL (as far as URL processing is concerned). Depending on your situation, this may or may not be an issue.
You should first try to use Java's MessageDigest API with the standard (already installed) JCE provider (i.e. you call MessageDigest.getInstance("SHA-256")), and bench the result. Theoretically, the JCE may map the call to an implementation with "native" code (written in C or assembly), which will be faster than what you can get with Java.
That being said...
sphlib is an opensource implementation of many cryptographic hash functions, in C and in Java. The code has been optimized for speed, and, in practice, the Java version turns out to be faster than what the standard JRE from Sun/Oracle offers. Use this link in case the previous link fails (the main host server is sometimes down for maintenance, as seems to be the case right now)(warning: 10 MB download). The archive also contains a report (which was presented at the second SHA-3 candidate conference in 2010) which gives some measured performance figures on several platforms, for SHA-2 and the 14 "second round" candidates for the upcoming SHA-3.
But you really should make in-situation benchmarks. For instance, effects on L1 cache can have a drastic effect on performance, and cannot be accurately predicted by taking the function code and running it in isolation.