Chapter 8: Part 3

Collections and Two-dimensional arrays

Objectives

• • One-Dimensional Arrays Array Initialization • • The Arrays Class: Searching and Sorting Arrays as Arguments • • The Collections Framework: ArrayLists Two-Dimensional Arrays • Common Programming Errors 2

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The Collections Framework: ArrayLists

• • Array – Data structure of choice for fixed-length collections of data that are related Many programming applications require variable length lists – Java provides a set of classes referred to as the collections framework – Provides seven different types of generic data structures 4

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The Collections Framework

• The Collections class: – Supports container classes – Provides functions for: • Searching • Sorting • Random shuffling • Reverse-ordering 6

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The Iterator Class

• Similar to an array’s index • Generalized index that keeps track of object’s position within a container • For some classes it provides the primary means of accessing individual elements • Obtaining an iterator: – Iterator iter = x.iterator(); 10

Parallel Arrays as Records

• Parallel arrays: – Corresponding data in a record resides in the same position in more than one array – Required in earlier programming languages that only supported array data structures – Can combine parallel elements in an object • Store objects in a one-dimensional array 11

Two-Dimensional Arrays

• • Consist of both rows and columns of elements Sometimes called tables • Example declaration: – int val[][]; • Example allocation: – val = new int[3][4]; • Elements are identified by position in an array 12

Two-Dimensional Arrays (continued)

• • • Can be initialized from within declaration statements: – int val[][] = {{8,16,9,52}, {3,15,27,6}, {7,25,2,10}}; Number of columns need not be the same for each row May be displayed by: – – Individual element notation Using loops • Usually nested loops 13

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Two-Dimensional Array Length

• val.length – Provides the number of rows in the array referenced by val • val[i].length – Provides the number of columns in the ith row of val array 15

Passing Two-Dimensional Arrays

• Identical to passing a one-dimensional array – The called method receives access to the entire array 16

Advanced Dimensioning Capabilities

• Can create two-dimensional arrays where each row has a different number of columns • To create: – Initialize a list that explicitly lists values for each row – Or use the new operator and place values into the newly created array 17

Larger Dimensional Arrays

• Can have any number of array dimensions • Similar to creating and allocating two-dimensional arrays • Declaration: int val[][][]; val = new int[3][2][2]; 18

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Common Programming Errors

• • • • Forgetting the empty bracket pairs when declaring an array’s name Declaring an array reference variable using explicit dimension sizes Using a subscript that references a nonexistent array element Not using a large enough counter value in a for loop counter to cycle through all array elements 20

Summary

• One-dimensional array: – Data structure – Stores list of values of same data type • Array elements: – Stored in contiguous locations in memory – Referenced using the array name and a subscript • Such as num[22] 21

Summary (continued)

• Two-dimensional array is declared by providing: – Data type – Reference variable name – Two sets of empty bracket pairs after the array’s name • Arrays may be initialized when they are declared • Collections framework – Set of classes providing generic data structures 22