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Understanding Arrays and For Loops in Java Programming

This guide covers fixed-size collections and the usage of arrays in Java programming. Arrays are a special fixed-size collection type, allowing for storage of objects and primitive types. It explains the syntax for declaring and initializing arrays, including array literals and accessing elements using square brackets. The guide also discusses for loops, their syntax, and practical examples of iteration. Learn how to analyze web server records with a LogAnalyzer project using arrays and for loops to enhance your programming skills effectively.

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Understanding Arrays and For Loops in Java Programming

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  1. Grouping objects Arrays and for loops

  2. Fixed-size collections • Sometimes the maximum collection size can be pre-determined. • Programming languages usually offer a special fixed-size collection type: an array. • Java arrays can store objects or primitive-type values. • Arrays use a special syntax.

  3. The weblog-analyzer project • Web server records details of each access. • Supports webmaster’s tasks. • Most popular pages. • Busiest periods. • How much data is being delivered. • Broken references. • Analyze accesses by hour.

  4. Creating an array object public class LogAnalyzer { private int[] hourCounts; private LogfileReader reader; public LogAnalyzer() { hourCounts = new int[24]; reader = new LogfileReader(); } ... } Array variable declaration Array object creation

  5. The hourCounts array

  6. Using an array • Square-bracket notation is used to access an array element: hourCounts[...] • Elements are used like ordinary variables. • On the left of an assignment: hourCounts[hour] = ...; • In an expression: adjusted = hourCounts[hour] – 3; hourCounts[hour]++;

  7. Standard array use private int[] hourCounts; private String[] names; ... hourCounts = new int[24]; ... hourcounts[i] = 0; hourcounts[i]++; System.out.println(hourcounts[i]); declaration creation use

  8. Array literals declaration and initialization • Array literals can only be used in initializations. private int[] numbers = { 3, 15, 4, 5 }; System.out.println(numbers[i]);

  9. Array literals • Arrays can be created and assigned values in one statement: double[] prices = {14.95, 12.95, 11.95, 9.95}; int[] values = {3, 5, 7, 9}; boolean[] responses = {true, false, true, true, false}; String[] bookCodes = {"warp", "mbdk", "citr"}; Book[] books = {new Book("warp"), new Book("mbdk")}; String[] suits = {"Spades","Hearts","Clubs","Diamonds"};

  10. Array length • Note: ‘length’ is not a method!! private int[] numbers = { 3, 15, 4, 5 }; int n = numbers.length; no parenthesis

  11. The for loop • There are two variations of the for loop, for-each and for. • The for loop is often used to iterate a fixed number of times. • Often used with a variable that changes a fixed amount on each iteration.

  12. For loop pseudo-code General form of a for loop for(initialization; condition; post-body action) { statements to be repeated } Equivalent in while-loop form initialization; while(condition) { statements to be repeated post-body action }

  13. A Java example for loop version for(int hour = 0; hour < hourCounts.length; hour++) { System.out.println(hour + ": " + hourCounts[hour]); } while loop version int hour = 0; while(hour < hourCounts.length) { System.out.println(hour + ": " + hourCounts[hour]); hour++; }

  14. Practice • Given an array of numbers, print out all the numbers in the array, using a for loop. int[] numbers = { 4, 1, 22, 9, 14, 3, 9}; for ...

  15. for loop with bigger step // Print multiples of 3 that are below 40. for(int num = 3; num < 40; num = num + 3) { System.out.println(num); }

  16. Review • Arrays are appropriate where a fixed-size collection is required. • Arrays use special syntax. • For loops offer an alternative to while loops when the number of repetitions is known. • For loops are used when an index variable is required.

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