Alice in Action with Java Chapter 12 Arrays and Lists in Java

Objectives Understand Java’s array data structure Solve problems using Java’s LinkedList data structure Solve problems using Java’s ArrayList data structure Alice in Action with Java

Arrays and Lists in Java Features common to arrays and lists Used to define variables that store groups of items Provide access to a given item using an index Differences between arrays and lists Array’s size is fixed at runtime, a list’s size can change Array stores items using less memory than a list Array provides direct item access faster than a list Alice in Action with Java

Introductory Example: Air Pollution Reporting Review of AirPollutionIndex.java Read air pollution level readings from five points Compute and display the average reading (the index) Additional requirements of the enhanced program Display the five readings used to compute the average High-level algorithm for AirPollutionReport Build an array named readings with length = 5 Read the air pollution readings into readings Compute and display average of values in readings Display the individual values in readings Alice in Action with Java

Java Arrays General pattern for defining (declaring) an array Item[] anArray = new Item[N]; Item: specifies the items type Brackets tell compiler that anArray is an array handle new operator: allocates memory for the array N: an integer that specifies the array length (size) An example of an array definition: double[] readings = new double [NUM_READINGS]; NUM_READINGS is an integer constant = 5 readings is a handle to a 5 unit double type array Alice in Action with Java

Java Arrays (continued) Element: indexed unit variable in an array Item: value stored in an array element Items are initialized to default values for array’s type Example: default value for item in double type is 0.0 Creating an array parameter Place brackets between parameter’s type and its name General form: public ReturnType methodName( Item[] parameterName ){… Example: public static double average(double [] anArray) Alice in Action with Java

Java Arrays (continued) Alice in Action with Java

Java Arrays (continued) Components needed to access an array’s elements Array’s handle, item’s index, subscript operator ([]) Pattern for accessing an element: anArray[i] anArray is the handle to the array i: index value, which must be a non-negative integer An index value out of bounds throws an exception Example of an array access: readings[0] Accesses the first element of the readings array Note: index is off by one relative to item’s cardinal order length property: returns number of items in an array Alice in Action with Java

Java Arrays (continued) for loop provides convenient access to an array Example of an array traversal using a for loop for (int i = 0; i < arr.length; i++){ System.out.println("Reading #" + (i+1) + ": " + arr[i]);} for each loop Special loop used to read each item in an array Limitation: cannot be used to write to array’s elements Example of array traversal using a for each loop for (double item : anArray){sum += item;} Alice in Action with Java

Example 2: Month Names From Numbers Essential elements of user story Randomly generate a month number Query user for a month name matching the number Read the user’s response Display a message appropriate to the response Let the user keep playing as long as he or she wants Instance variables declared in the Month class An integer myNumber and a string myName Role of the Month()constructor Construct a Month object given a month number Alice in Action with Java

Example 2: Month Names From Numbers (continued) MONTHS will be used to store the month names The array is declared as a constant class variable Declaration also includes an initialization values list The array functions as a lookup table for the constructor Other Month members Accessors for the instance variables A toString()method Other classes used in MonthTester Random (to generate a random number) Scanner (to read in the month’s name) Alice in Action with Java

Example 2: Month Names From Numbers (continued) Alice in Action with Java

Arrays and Memory An array is a random access data structure Contiguous elements are accessed in constant time Subscript operation computes an element’s address How to compute the address of an array element Multiply the index i by the size of an item Add the resulting product to the starting address Example: anArray[4]= (anArray+4*itemSize) Insertion and removal are linear time operations Up to length–1 items are shifted in each operation Many of these operations can slow down a program Alice in Action with Java

Arrays and Memory (continued) Alice in Action with Java

Arrays and Memory (continued) Alice in Action with Java

Multidimensional Arrays Dimension: axis used to specify element’s location length determines space for one-dimensional array You can create arrays with multiple dimensions Example: two-dimensional array to model a table How to declare an N-dimensional array handle Use N pairs of brackets to declare Example of declaring a two-dimensional array private double [][] myTable = null; Alice in Action with Java

Multidimensional Arrays (continued) Defining a multidimensional array with default values Use the new operator and specify dimensions Ex: myTable = new double[rows][columns]; Defining multidimensional array with non-default values Use an initialization list Accessing a multidimensional array element Use N subscript operators for N dimensions Example: myTable[row][col] = item; Processing multidimensional arrays Use N for loops to traverse array with N dimensions Alice in Action with Java

Lists in Java Interface: a structure that only declares methods A class implementing an interface defines methods Example: String implements CharSequence Java’s List is an interface Two classes implementing List LinkedList: similar to Alice’s list data structure ArrayList: dynamic container offering fast access Alice in Action with Java

Lists in Java (continued) Alice in Action with Java

Lists in Java (continued) Alice in Action with Java

Using ArrayLists ArrayList implements the List interface Features of an ArrayList Reference type data structure Can increase in size during execution set()and get()access items in constant time Patterns for declaring and defining an ArrayList Insert items in an ArrayList using add() Alice in Action with Java

Using ArrayLists Rules for declaring an array list The type of an array list must be a reference type Use a wrapper class when a primitive type is needed Wrapper class contains primitive type plus operations Pattern for declaring an ArrayList ArrayList<ItemType> handleName = null; Example of a ArrayList declaration ArrayList<Double> listOfNumbers = null; Produces a handle called listOfNumbers listOfNumbers is capable of storing an arbitrary number of Double objects Alice in Action with Java

Using ArrayLists (continued) Alice in Action with Java

Using ArrayLists (continued) Alice in Action with Java

Summary Array: fixed-size container used to store items of the same type Items: values stored in an array’s elements Accessing an array element is a constant time operation Inserting and removing items in an array are linear time operations Arrays can have an arbitrary number of dimensions ArrayList includes the random access capability of arrays and the ability to change size Alice in Action with Java