1. Chapter 11 Arrays Continued
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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.
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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.
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4. Vocabulary Array list Binary search Bubble sort Immutable object
Insertion sort
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5. Vocabulary (cont.) Linear search Selection sort Substring
Wrapper class
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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
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7. Advanced Operations on Strings (cont.)
Table 11-1: Some commonly used String methods
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8. Advanced Operations on Strings (cont.)
Table 11-1: Some commonly used String methods (cont.)
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9. Advanced Operations on Strings (cont.)
Table 11-1: Some commonly used String methods (cont.)
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10. Advanced Operations on Strings (cont.)
Example 11.2: Count the words and compute the average word length in a sentence.
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11. Advanced Operations on Strings (cont.)
Example 11.2: Count the words and compute the average word length in a sentence (cont.).
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12. Searching
Linear search: Search a data structure (such as an array) from beginning to end
Searching an array of objects:
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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.
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14. Searching (cont.)
Binary search code:
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15. Figure 11-1: Trace of a binary search of an array
Searching (cont.)
Figure 11-1: Trace of a binary search of an array
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16. Table 11-2: Behavior of the method compareTo
Searching (cont.)
To compare objects, best if the class implements the Comparable interface
compareTo method
Table 11-2: Behavior of the method compareTo
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17. Searching (cont.)
Binary search for objects:
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18. Searching (cont.) Implementing a Comparable class example:
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19. Figure 11-2: Array before and after 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
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20. Sorting: Selection Sort
Basic idea:
Table 11-3: Trace of data during a selection sort
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21. Sorting: Selection Sort (cont.)
Must be able to find smallest number in an array and swap items in an array
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22. Table 11-4: Trace of data during one pass of a bubble sort
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
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23. Sorting: Bubble Sort (cont.)
Pseudocode:
Fewer data exchanges than selection sort
Sort can stop early if array already sorted
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24. Sorting: Insertion Sort
After kth pass of sorting loop (k starting at 1), first k items should be in sorted order.
Table 11-4: Trace of data during an insertion sort
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25. Sorting: Insertion Sort (cont.)
Pseduocode:
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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:
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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.
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28. Insertions and Removals (cont.)
Figure 11-3: Inserting an item into an array
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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.
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30. Insertions and Removals (cont.)
Figure 11-4: Removing an item from an array
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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
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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
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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.
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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:
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35. The Class java.util.ArrayList (cont.)
Table 11-6: Some commonly used ArrayList methods
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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
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37. The Class java.util.ArrayList (cont.)
ArrayList objects automatically “box” and “unbox” primitive values when used with ArrayList methods.
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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
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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)
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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
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41. Graphics and GUIs: Menus (cont.)
Example 11.6: TestScoresView class (with menus)
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42. Graphics and GUIs: Menus (Cont.)
Example 11.6: TestScoresView class (with menus, cont.)
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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.
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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.
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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.
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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.
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