10 - 02/11/09 23:25:02 CS 61B: Lecture 10 Wednesday,...

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02/11/09 23:25:02 1 10 CS 61B: Lecture 10 Wednesday, February 11, 2009 Today’s reading: equals() ======== Every class has an equals() method. If you don’t define one explictly, then "r1.equals(r2)" returns the same boolean value as "r1 == r2", where r1 and r2 are references. However, many classes redefine equals() to compare the _content_ of two objects. String is such a class. In the following example, "s1 == s2" is false, but "s1.equals(s2)" is true. "s2 == s3" and "s2.equals(s3)" are both true. --- --------- --- --------- --- s1 |.+--->| "Yes" | s2 |.+--->| "Yes" |<---+.| s3 --- --------- --- --------- --- For Homework 3, you use the equals() method to compare Integer objects. Integer is a class of the java.lang library; an Integer object contains one primitive int field. Two distinct Integer objects are equals() if they contain the same int. There are at least four different degrees of equality. (1) Reference equality, ==. (2) Shallow structural equality: two objects are "equals" if all their fields are ==. For example, two SLists whose "size" fields are equal and whose "head" fields point to the same SListNode. (3) Deep structural equality: two objects are "equals" if all their fields are "equals". For example, two SLists that represent the same sequence of items (though the SListNodes may be different). (4) Logical equality. Two examples: (a) Two "Set" objects are "equals" if they contain the same elements, even if the underlying lists store the elements in different orders. (b) The Fractions 1/3 and 2/6 are "equals", even though their numerators and denominators are all different. The "equals" method for a particular class may test any of these four levels of equality, depending on what seems appropriate. Let’s write an equals() method for SLists that tests for deep structural equality. The following method returns true only if the two lists represent identical sequences of items. public class SList { public boolean equals(SList other) { if (size != other.size) { return false; } SListNode n1 = head; SListNode n2 = other.head; while (n1 != null) { if (!n1.item.equals(n2.item)) { return false; } n1 = n1.next; n2 = n2.next; } return true; } } Note that this implementation may fail if the SList invariants have been corrupted. TESTING ======= Complex software, like Project 1, is easier to debug if you write test code to make sure that the methods you write are working correctly. We’ll consider three types of testing: (1) Modular testing: testing each method and each class separately. (2)
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This note was uploaded on 02/21/2010 for the course CS 61B taught by Professor Canny during the Spring '01 term at University of California, Berkeley.

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10 - 02/11/09 23:25:02 CS 61B: Lecture 10 Wednesday,...

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