![ Square-bracket notation is used to access an array element:
hourCounts[...]
Elements are used like ordinary variables.
The target of an assignment:
hourCounts[hour] = ...;
datatype[] arrayRefVar = new datatype[arraySize];
double[] myArray = new double[20];
datatype arrayRefVar[] = new datatype[arraySize];
double myArray[] = new double[20];
23](https://image.slidesharecdn.com/lecture7-iteratorandforloopoverarrays-160913065815/85/Lecture-7-Iterator-and-for-loop-over-arrays-23-320.jpg)
![ The following example shows the declaration, creation, and
initialization of an array:
double[] myArray = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the following statements:
double[] myArray = new double[4];
myArray[0] = 1.9;
myArray[1] = 2.9;
myArray[2] = 3.4;
myArray[3] = 3.5;
26
declaration,
creation and
initialization](https://image.slidesharecdn.com/lecture7-iteratorandforloopoverarrays-160913065815/85/Lecture-7-Iterator-and-for-loop-over-arrays-26-320.jpg)
Lecture 7- Iterator and for loop over arrays
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- 2. To understand the iterator concept and its types. To differentiate between index access and Iterators To present which loop structure is used to remove elements from the collection. To implement a music-organizer project. To implement an auction system example. To describe why arrays are necessary in programming. To illustrate array reference variables and create arrays. To initialize the values in an array. To access array elements using indexed variables. To declare, create, and initialize an array using an array initializer. To illustrate how for loop is working. 2
- 3. An iterator is an object that provides functionality to iterate over all elements of a collection. Iteration is a vital tool in almost every programming project Iterator (note the uppercase I) is a Java type, but we will also encounter a method called iterator (lowercase i). Collections; like ArrayList, have a iterator() method. The iterator method of ArrayList returns an Iterator object. 3
- 4. Iterator is also defined in the java.util package, so we must add a second import statement to the class file to use it: import java.util.ArrayList; import java.util.Iterator; An Iterator provides just three methods, and two of these are used to iterate over a collection: hasNext and next. Neither takes a parameter, but both have non-void return types, so they are used in expressions. 4
- 5. The way we usually use an Iterator can be described in pseudo- code as follows: 5
- 6. One of the keys to understanding how Iterator works is that the call to Next causes the Iterator to return the next item in the collection and then move past that item. Therefore, successive calls to next on an Iterator will always return distinct items. The Iterator reaches the end of the collection and then returns false from a call to hasNext. Once hasNext has returned false, it would be an error to try to call next on that particular Iterator object. 6
- 7. We have at least three different ways in which we can iterate over an ArrayList. We can use: ◦ a for-each loop ◦ the get method with an integer index variable, while loop ◦ an Iterator object All approaches seem about equal in quality. Iteration is an important programming pattern. 7
- 8. For-each loop is: ◦ slightly easier to understand, but the least flexible. ◦ used if all elements of a collection are to be processed (i.e., definite iteration). While loop is: ◦ iteration can more easily be stopped in the middle of processing (indefinite iteration), ◦ preferable when processing only a part of the collection. ◦ used for repetition that doesn't involve a collection. Iterator object: ◦ Iteration can more easily be stopped in the middle of processing (indefinite iteration), ◦ Preferable when processing only a part of the collection. ◦ Often used with collections where indexed access is not very efficient, or impossible. ◦ Used to remove from a collection. 8
- 9. We need to think which looping structure to use when we have to remove elements from the collection while iterating. The proper solution to removing while iterating is to use an Iterator. Its third method (in addition to hasNext and next) is remove. It takes no parameter and has a void return type. Calling remove will remove the item that was returned by the most recent call to next. 9
- 10. Here is some sample code. Note that we do not use the tracks collection variable in the body of the loop. While both ArrayList and Iterator have remove methods, we must use the Iterator’s remove method, not the ArrayList’s. 10
- 11. In music-organizer Project we have seen: ◦ we can use an ArrayList object, created from a class out of the class library, to store an arbitrary number of objects in a collection. ◦ we can use a loop to iterate over all elements in the collection. ◦ With an ArrayList, we can access elements either by index or we can iterate over all elements using an Iterator object. 11
- 12. The auction project models part of the operation of an online auction system. The idea is that an auction consists of a set of items offered for sale. These items are called “lots”, and each is assigned a unique lot number by the program. A person can try to buy a lot they want by bidding an amount of money for it. The class structure of the auction project contains the following classes: Auction, Bid, Lot, and Person. 12
- 13. If a data field of a reference type does not reference any object, the data field holds a special literal value, null. So null is a value for the field that makes it clear that there is currently “no object” being referred to by that variable. If a variable contains the null value, a method call should not be made on it. We sometimes have to use an if statement to test whether a variable contains null or not before calling a method on that variable. Example: if(highestBid == null) … 13
- 14. The Lot class stores a description of the lot, a lot number, and details of the highest bid received for it so far. The most complex part of the class is the bidFor method. 14
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- 18. Objects are often created and handed on elsewhere immediately. The following code is doing two things: ◦ We are creating a new Lot object. ◦ We are also passing this new object to the ArrayList’s add method. Lot furtherLot = new Lot(…); lots.add(furtherLot); Alternate way (We don’t really need furtherLot): lots.add(new Lot(nextLotNumber, description)); 18
- 19. Methods often return objects. We often immediately call a method on the returned object. Bid bid = lot.getHighestBid(); Person bidder = bid.getBidder(); Each method in the chain is called on the object returned from the previous method call in the chain. 19
- 20. We can use the anonymous object concept and chain method calls: lot.getHighestBid().getBidder() The call to getHighestBid returns an anonymous Bid object, and the getBidder method is then called on that object. 20
- 21. A special fixed-size collection type is available: an array. Array is a data structure that represents a collection of the same types of data. Array advantages over the flexible-size collection classes: ◦ Access to the items held in an array is often more efficient than access to the items in a comparable flexible-size collection. ◦ Arrays are able to store either objects or primitive-type values. Flexible- size collections can store only objects. Arrays use a special syntax. 21
- 22. Following picture represents array myList. Here, myList holds ten double values and the indices are from 0 to 9. 22
- 23. Square-bracket notation is used to access an array element: hourCounts[...] Elements are used like ordinary variables. The target of an assignment: hourCounts[hour] = ...; datatype[] arrayRefVar = new datatype[arraySize]; double[] myArray = new double[20]; datatype arrayRefVar[] = new datatype[arraySize]; double myArray[] = new double[20]; 23
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- 25. Once an array is created, its size is fixed. It cannot be changed. You can find its size using arrayRefVar.length For example, myArray.length returns 20 25
- 26. The following example shows the declaration, creation, and initialization of an array: double[] myArray = {1.9, 2.9, 3.4, 3.5}; This shorthand notation is equivalent to the following statements: double[] myArray = new double[4]; myArray[0] = 1.9; myArray[1] = 2.9; myArray[2] = 3.4; myArray[3] = 3.5; 26 declaration, creation and initialization
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- 28. How an array variable is associated with an array object ? An array of 24 integers presented in the following figure: 28
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- 30. Loop statements allow a block of statements to be repeated. The for-each loop allows iteration over a whole collection. The while loop allows the repetition to be controlled by a Boolean expression. All collection classes provide special Iterator objects that provide sequential access to a whole collection. We presented Flexible Collection like ArrayList. Classes such as ArrayList conveniently allow us to create collections containing an arbitrary number of objects. Arrays are appropriate where a fixed-size collection is required. Arrays use a special syntax. For loops are used when an index variable is required. For loops offer an alternative to while loops when the number of repetitions is known. Used with a regular step size. 30
- 31. Barnes, David J., and Kölling, Michael. 2012. Objects First with Java, A practical Introduction Using BlueJ (5th Edition). Boston: Preston. Liang, Y. Daniel. 2011. Introduction to Java Programming, Comprehensive (8th Ed.) Prentice Hall. http://www.tutorialspoint.com/java/java_decision_making.htm http://www.homeandlearn.co.uk/java/java.html 31