Today. Reading. Homework. Lecture Notes CPSC 224 (Spring 2012) hashcode() method. Collections class. Ch 9: hw 7 out (due in a week)

Transcription

1 Today hashcode() method Collections class Reading Ch 9: Homework hw 7 out (due in a week) S. Bowers 1 of 9

2 The Object hashcode() function The signature: public int hashcode() What it does: returns an integer representation of an object When it s used (overriden): rarely, if ever used by the default implementation of tostring() used in various collection types (more later) must be overriden if we override equals() technically, this is part of the hashcode() contract Default implementation (in Object): derived from the object s memory address not technically required by Java... (but typically done) S. Bowers 2 of 9

3 Small hashcode() test... public class HashCodeTest { public static void main(string[] args) { Object o1 = new Object(); System.out.println("o1 = " + o1.hashcode()); Object o2 = new Object(); System.out.println("o2 = " + o2.hashcode()); A few runs on my Mac: java HashCodeTest o1 = o2 = java HashCodeTest o1 = o2 = java HashCodeTest o1 = o2 = S. Bowers 3 of 9

4 The hashcode() contract: consistent return same value for the same object (during program execution) equality if two objects are equal via equals() return the same hashcode value for both (why we have to implement if we override equals()) inequality (not required) it is OK to return the same value for non-equal objects may give better hash table performance if you don t though! (chaining) Equality causes the most issues... otherwise we could just use the default implementation but won t work since we have 2 different objects (2 memory locations) Note: it s trivial to write a hashcode() method that satisfies the contract public int hashcode() { return 43; but this generally degrades hash tables to linked lists! S. Bowers 4 of 9

5 Q: What is the hashcode() method in String? public int hashcode() { int hash = 0; for(int i = 0; i < length(); i++) hash = 31 * hash + charat(i); return hash; For example, for "abc": computes: 31*(31*(31*0 + charat( a )) + charat( b )) + charat( c ) is also: 31 2 *charat( a ) *chartat( b ) + charat( c ) in general is: 31 n 1 s n 2 s s n 1 In general you want to do something similar... assign each field a hash code and give it a weight goal is to evenly distribute the values doesn t need to be a perfect hash function S. Bowers 5 of 9

6 Q: How could we override hashcode() for Point and ColorPoint? public class Point { private int x; private int y;... public class ColorPoint { private Point point; private Color color;... note that equals() is overriden in Color // in Point: public int hashcode() { return 31*x + y; // in ColorPoint: public int hashcode() { return 31*color.hashCode() + point.hashcode(); or something similar to this... S. Bowers 6 of 9

7 The Collections class Part of the larger Collections Framework in Java we ll go over this more later Contains static helper methods for working with collections you should look over these (we will look at a few) Some methods rely on the Comparable interface: public interface Comparable<T> { int compareto(t o); The T in Comparable<T> is a type parameter (can be any class type) If x is current object, x.compareto(y) returns: < 0... if x < y 0... if x == y > 0... if x > y Defines the natural ordering of class objects The contract (where sign(e) is -1, 0, or 1 if e negative, zero, or positive) sign(x.compareto(y)) == -sign(y.compareto(x)) x.compareto(y) == 0 sign(x.compareto(z)) == sign(y.compareto(z)) x.compareto(y) > 0 && y.compareto(z) > 0 x.compareto(z) > 0 Recommended: (x.compareto(y) == 0) == x.equals(y) S. Bowers 7 of 9

8 Some methods use the Comparator interface: public interface Comparator<T> { int compare(t o1, T o2); public boolean equals(object obj); The unnatural order of class objects Q: What is meant by equals() here? testing equality of comparators not needed... why? since alread in Object included as a reminder not required to implement this... what happens if you don t? compare(x, y) returns: < 0... if x < y 0... if x == y > 0... if x > y same contract as Comparable you can pass comparator objects into some methods! S. Bowers 8 of 9

9 Methods in Collections to change the order of collection elements Note: these are not the fully type-parameterized definitions void reverse(list list) void shuffle(list list) void sort(list list) void sort(list list, Comparator c) void swap(list list, int i, int j) // reverse order // randomly permute // must be list of comparables // comparator for sorting // swap elems i and j Some examples: List<String> list = Arrays.asList("a", "ab", "aab"); // sort alphabetically (using compareto in String) Collections.sort(list); assert list.tostring().equals("[a, aab, ab]"); // reverse the sorted list Collections.reverse(list); assert list.tostring().equals("[ab, aab, a]"); // sort from largest to smallest link (an "unnatural" ordering) Collections.sort(list new Comparator<String>() { public int compare(string x, String y) { if(x.length() > y.length()) return -1; if(y.length() < x.length()) return 1; return 0; ); assert list.tostring().equals("[aab, ab, a]"); return y.length() - x.length() also works but has overflow issues S. Bowers 9 of 9