Designing an ADT   The design of an ADT should evolve

Designing an ADT The design of an ADT should evolve naturally during the problem-solving process Questions to ask when designing an ADT What data does a problem require? What operations does a problem require? Examples: polynomial, appointment book ADT can suggest other ADTs

Appendix 1. Java Exceptions (review) Exception A mechanism for handling an error during execution A method indicates that an error has occurred by throwing an exception

Java Exceptions Catching exceptions try block A statement that might throw an exception is placed within a try block Syntax try { statement(s); } // end try

Java Exceptions Catching exceptions (Continued) catch block Used to catch an exception and deal with the error condition Syntax catch (exceptionClass identifier) { statement(s); } // end catch

Java Exceptions Types of exceptions Checked exceptions Instances of classes that are subclasses of the java.lang.Exception class Must be handled locally or explicitly thrown from the method Used in situations where the method has encountered a serious problem

Checked exceptions public class TestExceptionExample { public static void getInput(String fileName) { FileInputStream fis; fis new FileInputStream(fileName); // file processing code appears here } // end getInput public static void main(String[] args) { getInput("test.dat"); } // end main } // end TestExceptionExample

Java Exceptions Types of exceptions (Continued) Runtime exceptions Used in situations where the error is not considered as serious Can often be prevented by fail-safe programming Instances of classes that are subclasses of the RuntimeException class Are not required to be caught locally or explicitly thrown again by the method

Java Exceptions Throwing exceptions A throw statement is used to throw an exception throw new exceptionClass (stringArgument); Defining a new exception class A programmer can define a new exception class class MyException extends Exception { public MyException(String s) { super(s); } // end constructor } // end MyException

Implementing ADTs Choosing the data structure to represent the ADT’s data is a part of implementation Choice of a data structure depends on Details of the ADT’s operations Context in which the operations will be used Implementation details should be hidden behind a wall of ADT operations A program would only be able to access the data structure using the ADT operations

An Array-Based Implementation of the ADT List An array-based implementation A list’s items are stored in an array items A natural choice Both an array and a list identify their items by number A list’s kth item will be stored in items[k-1]

An Array-Based Implementation of the ADT List Figure 4-11 An array-based implementation of the ADT list

An Array-Based Implementation of the ADT List public class ListArrayBased implements ListInterface { private static final int MAX LIST 50; private Object items[]; // an array of list items private int numItems; // number of items in list

Appendix 2. Arrays in Java (review) Arrays are sequences of identically typed values Values are stored at specific numbered positions in the array The first value is stored at index 0, the second at index 1, the ith at index i-1, and so on The last item is stored at position n-1, assuming that n values are stored in the array Values are stored sequentially in main memory

Arrays in Java To declare an array follow the type with (empty) []s int[] grade; //or int grade[]; //both declare an int array In Java arrays are objects!

Objects in Java String s new String("cat"); s cat

Objects in Java (review) String s new String("cat"); s The variable s is a reference to the object cat

Objects in Java String s new String("cat"); S null; s Makes s not refer to the object any more cat

Objects in Java String s new String("cat"); S null; s Makes s not refer to the object any more cat The object gets deleted by Java’s automated garbage collection

Objects in Java String s new String("cat"); String t s; s cat t This makes another reference to the object --- but no new object is created!!

Arrays in Java To declare an array follow the type with (empty) []s int[] grade; //or int grade[]; //both declare an int array In Java arrays are objects so must be created with the new keyword To create an array of ten integers: int[] grade new int[10]; Note that the array size has to be specified, although it can be specified with a variable at run-time

Arrays in Java When the array is created memory is reserved for its contents Initialization lists can be used to specify the initial values of an array, in which case the new operator is not used int[] grade {87, 93, 35}; //array of 3 ints To find the length of an array use its .length variable int numGrades grade.length; //note: not .length()!!

Array Indexing int[] arr {3,7,6,8,1,7,2}; creates a new integer array with seven elements The elements are assigned values as given in the initialization list Individual elements can be accessed by referring to the array name and the appropriate index int x arr[3]; would assign the value of the fourth array element (8) to x arr[5] 11; would change the sixth element of the array from 7 to 11 arr[7] 3; would result in an error because the index is out of bounds index value 0 3 1 7 2 6 3 8 4 1 5 7 11 6 2 error!

Arrays and Main Memory int[] grade; Declares an array variable main memory is depicted below grade In Java arrays are objects, so this is a reference variable “null pointer”

Arrays and Main Memory int[] grade; grade new int[4]; Creates a new array of size 4 main memory is depicted below grade 0 0 points to the newly created array object 0 0 Stores 4 ints consecutively. The ints are initialized to 0

Arrays and Main Memory int[] grade; grade new int[4]; grade[2] 23; Assigns 23 to the third item in the array main memory is depicted below grade 0 0 023 0 But how does the system “know” where grade[2] is?

Offset Calculations Given something like grade[2] 23; how do we find a particular element in the array? We know the address of the first element in the array Because we know the type of the values stored in the array, we know the size of each element in the array 4 bytes in the case of an int We know which element we want to access We can therefore calculate the address of the desired element as being: address of first element index * size of stored type

Passing Arrays to Methods Array variables are reference variables When an array variable is passed as an argument to a method the method is being given the address of an array object Not a new copy of the array object Any changes made to the array in the method are therefore made to the original (and only) array object If this is not desired, a copy of the array should be made within the method

Arrays are Static Data Structures The size of an array must be specified when it is created with new and cannot be changed If the array is full new items can’t be added to it There are, time consuming, ways around this To avoid this problem make arrays much larger than they are needed However this wastes space