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Java Forum / General / September 2006hashcode
public int hashCode() Returns a hash code for this string. The hash code for a String object is computed as s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] How this value is calculated ? say i have String s="abc" so , s[0]= a s[1]=b s[2]=c but how do i get the power of a character ? say the first term ... a*31^2 = whats this ? whats the neumerial value of it ? gk schreef: > public int hashCode() > [quoted text clipped - 17 lines] > say the first term ... > a*31^2 = whats this ? whats the neumerial value of it ? chars are converted to ints if arithmetic is done with them. H. - -- Hendrik Maryns http://tcl.sfs.uni-tuebingen.de/~hendrik/ ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 [quoted text clipped - 18 lines] >> >> but how do i get the power of a character ? The calculation does not call for any powers of characters, only powers of 31. Exponentiation, which is what is meant in this context by "^", has higher precedence than multiplication. >> say the first term ... >> a*31^2 = whats this ? whats the neumerial value of it ? > > chars are converted to ints if arithmetic is done with them. Also, it is not normal, or efficient, to evaluate a polynomial by explicitly calculating e.g. x^(n-1) etc. Usually, it is done by alternately adding a coefficient, starting with the one for the highest power, and then multiplying by x (in this case, by 31). So start with s[0]. While the last character to have been used is not the last character of the string, multiply the sum so far by 31 and add the next character. Patricia >> -----BEGIN PGP SIGNED MESSAGE----- >> Hash: SHA1 [quoted text clipped - 21 lines] > of 31. Exponentiation, which is what is meant in this context by "^", > has higher precedence than multiplication. This isn't Pascal, so that's XOR, which has a lower precedence than multiplication. Dan.  SignatureDaniel Dyer http://www.dandyer.co.uk >>> -----BEGIN PGP SIGNED MESSAGE----- >>> Hash: SHA1 [quoted text clipped - 24 lines] > This isn't Pascal, so that's XOR, which has a lower precedence than > multiplication. You seem to have been confused by missing context. The quote is neither Pascal, nor Java, nor any other programming language, but a mathematical expression represented as text in the String hashCode documentation. In context: "Returns a hash code for this string. The hash code for a String object is computed as s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] using int arithmetic, where s[i] is the ith character of the string, n is the length of the string, and ^ indicates exponentiation. (The hash value of the empty string is zero.)" [http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#hashCode()] Patricia >>> The calculation does not call for any powers of characters, only powers >>> of 31. Exponentiation, which is what is meant in this context by "^", [quoted text clipped - 18 lines] > > [http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#hashCode()] Oops, sorry. I've never thought to look at how String implements hashCode. Dan. SignatureDaniel Dyer http://www.dandyer.co.uk Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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