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Chapter 11 Arrays Continued

Chapter 11 Arrays Continued. Fundamentals of Java. Objectives. Use string methods appropriately. Write a method for searching an array. Understand why a sorted array can be searched more efficiently than an unsorted array. Write a method to sort an array. Objectives (cont.).

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Chapter 11 Arrays Continued

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  1. Chapter 11Arrays Continued Fundamentals of Java

  2. Objectives • Use string methods appropriately. • Write a method for searching an array. • Understand why a sorted array can be searched more efficiently than an unsorted array. • Write a method to sort an array. Fundamentals of Java

  3. Objectives (cont.) • Write methods to perform insertions and removals at given positions in an array. • Understand the issues involved when working with arrays of objects. • Perform simple operations with Java’s ArrayList class. Fundamentals of Java

  4. Vocabulary • Array list • Binary search • Bubble sort • Immutable object • Insertion sort Fundamentals of Java

  5. Vocabulary (cont.) • Linear search • Selection sort • Substring • Wrapper class Fundamentals of Java

  6. Advanced Operations on Strings • Most text-processing applications examine and manipulate the characters in strings. • Separating strings into segments • Searching for/replacing specific characters or substrings • Inserting text into a string • String objects are immutable. • No mutators in the String class Fundamentals of Java

  7. Advanced Operations on Strings (cont.) Table 11-1: Some commonly used String methods Fundamentals of Java

  8. Advanced Operations on Strings (cont.) Table 11-1: Some commonly used String methods (cont.) Fundamentals of Java

  9. Advanced Operations on Strings (cont.) Table 11-1: Some commonly used String methods (cont.) Fundamentals of Java

  10. Advanced Operations on Strings (cont.) Example 11.2: Count the words and compute the average word length in a sentence. Fundamentals of Java

  11. Advanced Operations on Strings (cont.) Example 11.2: Count the words and compute the average word length in a sentence (cont.). Fundamentals of Java

  12. Searching • Linear search: Search a data structure (such as an array) from beginning to end • Searching an array of objects: Fundamentals of Java

  13. Searching (cont.) • Binary search: An efficient search algorithm based on eliminating half of the data from the search at each iteration • Data must be sorted first. • Examine midpoint of data, then decide which half of the data to continue searching on. • Discard other half of data. Fundamentals of Java

  14. Searching (cont.) • Binary search code: Fundamentals of Java

  15. Searching (cont.) Figure 11-1: Trace of a binary search of an array Fundamentals of Java

  16. Searching (cont.) • To compare objects, best if the class implements the Comparable interface • compareTo method Table 11-2: Behavior of the method compareTo Fundamentals of Java

  17. Searching (cont.) • Binary search for objects: Fundamentals of Java

  18. Searching (cont.) • Implementing a Comparable class example: Fundamentals of Java

  19. Sorting • Arranging the elements of a collection of data (such as an array) in an ordered fashion Figure 11-2: Array before and after sorting Fundamentals of Java

  20. Sorting: Selection Sort • Basic idea: Table 11-3: Trace of data during a selection sort Fundamentals of Java

  21. Sorting: Selection Sort (cont.) • Must be able to find smallest number in an array and swap items in an array Fundamentals of Java

  22. Sorting: Bubble Sort • Pass through array comparing adjacent elements • If out of order, swap. Table 11-4: Trace of data during one pass of a bubble sort Fundamentals of Java

  23. Sorting: Bubble Sort (cont.) • Pseudocode: • Fewer data exchanges than selection sort • Sort can stop early if array already sorted Fundamentals of Java

  24. Sorting: Insertion Sort • After kth pass of sorting loop (kstarting at 1), first kitems should be in sorted order. Table 11-4: Trace of data during an insertion sort Fundamentals of Java

  25. Sorting: Insertion Sort (cont.) • Pseduocode: Fundamentals of Java

  26. Sorting (cont.) • Any of the search algorithms can be altered to support sorting of objects. • Object’s class(es) should implement Comparable • Have compareTo method • Example: Fundamentals of Java

  27. Insertions and Removals • Steps for insertion: • 1. Check for available space. • 2. Check validity of target index. • Between 0 and logical size • 3. Shift items from logical end of array to target index down by one position. • 4. Assign new item to cell at target index. • 5. Increment logical size by one. Fundamentals of Java

  28. Insertions and Removals (cont.) Figure 11-3: Inserting an item into an array Fundamentals of Java

  29. Insertions and Removals (cont.) • Steps for removal: • 1.Check validity of target index. • Between 0 and logical size • 2. Shift items from target index to logical end of array up by one position. • 3. Decrement logical size by one. Fundamentals of Java

  30. Insertions and Removals (cont.) Figure 11-4: Removing an item from an array Fundamentals of Java

  31. Working with Arrays of Objects • When array type is an interface type, abstract class, or superclass of 1+ other classes, array may contain different object types. • Might not all respond to common set of messages Fundamentals of Java

  32. Working with Arrays of Objects (cont.) • What if you want to perform an operation specific to one of the types in the array? • Can use the instanceOf operator to determine the specific type of element in the array • Most general arrays have type Object. • Can hold any type of object Fundamentals of Java

  33. The Class java.util.ArrayList • Contains sequence of elements ordered by position • Unlike an array in that: • It uses methods rather than [] to manipulate elements. • It tracks the logical size and physical size. • The logical size is 0 when created. • Size automatically adjusted as needed • The positions available for access range from 0 to the logical size minus 1. Fundamentals of Java

  34. The Class java.util.ArrayList (cont.) • Generic array list: Programmer must specify element type for the list • Raw array list:Can contain objects of any reference type • Declaring/instantiating a generic array list: Fundamentals of Java

  35. The Class java.util.ArrayList (cont.) Table 11-6: Some commonly used ArrayList methods Fundamentals of Java

  36. The Class java.util.ArrayList (cont.) • ArrayList objects cannot directly store primitive types. • Must use wrapper classes • Classes that contain the value of a primitive type • Boolean, Integer, Double, Character Fundamentals of Java

  37. The Class java.util.ArrayList (cont.) • ArrayList objects automatically “box” and “unbox” primitive values when used with ArrayList methods. Fundamentals of Java

  38. The Class java.util.ArrayList (cont.) • Advantages of ArrayList over arrays: • Includes many methods for tasks such as insertions, removals, and searches • Tracks own logical size and grows or shrinks automatically with the number of elements contained in it Fundamentals of Java

  39. Graphics and GUIs: Menus • A drop-down menu system consists of a menu bar, a number of menus, and several selections for each menu. • May have sub-menus • Menu item object for each menu selection (class JMenuItem) • Menu object for each menu (class JMenu) • Menu bar object in which all of the menu objects will appear (class JMenuBar) Fundamentals of Java

  40. Graphics and GUIs: Menus (cont.) • Listener objects are attached to menus. • When menu items are selected, events are fired and the listener objects respond. Figure 11-6: New user interface for the student test scores program Fundamentals of Java

  41. Graphics and GUIs: Menus (cont.) Example 11.6: TestScoresView class (with menus) Fundamentals of Java

  42. Graphics and GUIs: Menus (Cont.) Example 11.6: TestScoresView class (with menus, cont.) Fundamentals of Java

  43. Summary • Linear search: Simple search that works well for small- and medium-sized arrays • Binary search: Clever search that works well for large arrays but assumes that the elements are sorted • Comparisons of objects are accomplished by implementing the Comparable interface, which requires the compareTo method. Fundamentals of Java

  44. Summary (cont.) • Selection sort, bubble sort, and insertion sort are simple sort methods that work well for small- and medium-sized arrays. • Insertions and removals of elements at arbitrary positions are complex operations that require careful design and implementation. Fundamentals of Java

  45. Summary (cont.) • One can insert objects of any class into an array of Object. When retrieved from the array, objects must be cast down to their classes before sending them most messages. • The limitation of a fixed-size array can be overcome by using Java’s ArrayList class. Fundamentals of Java

  46. Summary (cont.) • An array list tracks and updates its logical size and provides many useful client methods. • Wrapper class, such as Integer, provides a way of packaging a value of a primitive type, such as int, in an object so that it can be stored in an array of Object or an array list. Fundamentals of Java

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