A Practical Introduction to Data Structures and Algorithm Analysis

146 Chap. 4 Lists, Stacks, and Queues}public int size() // Return list size{ return list.length(); }Figure 4.31 (continued)find the record prior to removal, we would still need to shift down the remainingrecords in the list to fill the gap left by the remove operation.Given two keys, we have not properly addressed the issue of how to comparethem. One possibility would be to simply use the basic ==, = operatorsbuilt into Java. This is the approach taken by our implementations for dictionariesshown in Figure 4.31. If the key type is int, for example, this will workfine. However, if the key is a pointer to a string or any other type of object, thenthis will not give the desired result. When we compare two strings we probablywant to know which comes first in alphabetical order, but what we will get fromthe standard comparison operators is simply which object appears first in memory.Unfortunately, the code will compile fine, but the answers probably will not be fine.In a language like C++ that supports operator overloading, we could requirethat the user of the dictionary overload the ==, = operators for the givenkey type. This requirement then becomes an obligation on the user of the dictionaryclass. Unfortunately, this obligation is hidden within the code of the dictionary (andpossibly in the user’s manual) rather than exposed in the dictionary’s interface. Asa result, some users of the dictionary might neglect to implement the overloading,with unexpected results. Again, the compiler will not catch this problem.The Java Comparable interface provides an approach to solving this problem.In a key-value pair implementation, the keys can be required to implementthe Comparable interface. In other applications, the records might be requiredto implement ComparableThe most general solution is to have users supply their own definition for comparingkeys. The concept of a class that does comparison (called a comparator)is quite important. By making these operations be generic parameters, the requirementto supply the comparator class becomes part of the interface. This design isan example of the Strategy design pattern, because the “strategies” for comparingand getting keys from records are provided by the client. In many cases, it is alsopossible for the comparator class to also extract the key from the record type, as analternative to storing key-value pairs.We will use the Comparable interface in Section 5.5 to implement comparisonin heaps, and in Chapter 7 to implement comparison in sorting algorithms.