Checkpoint 6.27.1.
What does the following code do? Guess before you run it. Then, add another enhanced for each loop that computes the product of all the elements in myList by multiplying them. Print out the product after the new loop.
Section 6.27 Traversing ArrayLists with Loops
While loops, for loops, and enhanced for each loops can all be used to traverse an ArrayList just like an array.
Subsection 6.27.1 Enhanced For Each Loop
You can use a enhanced for-each loop to traverse through all of the items in a list, just like you do with an array as shown in the main method below.
Practice
Subsection 6.27.2 For Loop
You can also use a
while or for loop to process list elements using the index. The ArrayList index starts at 0 just like arrays, but instead of using the square brackets [] to access elements, you use the get(index) to get the value at the index and set(index,value) to set the element at an index to a new value. If you try to use an index that is outside of the range of 0 to the number of elements − 1 in an ArrayList, your code will throw an ArrayIndexOutOfBoundsException, just like in arrays.
Checkpoint 6.27.2.
The following code will throw an ArrayIndexOutOfBoundsException. Can you fix it?
Subsection 6.27.3 While Loop
The example below demonstrates a while loop and an object-oriented approach where the list is a field of the current object and you use an object method rather than a class (static) method to loop through the list.
Checkpoint 6.27.3.
The following code removes a name from a list. Set the found variable to the appropriate true or false values at line 13 and line 20 to make the code work.
Be careful when you remove items from a list as you loop through it. Remember that removing an item from a list will shift the remaining items to the left. Notice that the method above only increments the current index if an item was not removed from the list. If you increment the index in all cases you will miss checking some of the elements since the rest of the items shift left when you remove one.
Do not use the enhanced for each loop if you want to add or remove elements when traversing a list because it will throw a ConcurrentModificationException error. Since for each loops do not use an index, you cannot do this special case of incrementing only if it is changed. So if you are going to add or remove items or you need the index, use a regular for-loop or a while loop.
Checkpoint 6.27.4.
7-3-4: Assume that
nums has been created as an ArrayList object and it initially contains the following Integer values [0, 0, 4, 2, 5, 0, 3, 0]. What will nums contain as a result of executing numQuest?
ArrayList<Integer> list1 = new ArrayList<Integer>();
private ArrayList<Integer> nums;
// precondition: nums.size() > 0;
// nums contains Integer objects
public void numQuest()
{
int k = 0;
Integer zero = new Integer(0);
while (k < nums.size())
{
if (nums.get(k).equals(zero))
nums.remove(k);
k++;
}
}
- [0, 4, 2, 5, 3]
- Incrementing the index each time through the loop will miss when there are two zeros in a row.
- [3, 5, 2, 4, 0, 0, 0, 0]
- This would be true if the code moved the zeros to the end, but that is not what it does.
- [0, 0, 0, 0, 4, 2, 5, 3]
- This would be true if the code moved the zeros to the font, but that is not what it does.
- [4, 2, 5, 3]
- This would be correct if k was only incremented when an item was not removed from the list.
Checkpoint 6.27.5.
The following has the correct code for the method getScore plus at least one extra unneeded code statement. This method will calculate and return the score for a word game. The code should loop through all of the elements in wordList and if the length of the current word is 3 it should add one to the score, if the length of the word is 4 it should add 2 to the score, and if the length is greater than 4 it should add 3 to the score. The method should return the score. Drag the needed blocks from the left into the correct order on the right. Check your solution by clicking on the Check button. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks. There is one extra block that is not needed in a correct solution.
Checkpoint 6.27.6.
The following has the correct code for a method called insertInOrder plus at least one extra unneeded code statement. This method should add the passed name in alphabetic order to a private list field called nameList. Drag the needed blocks from the left into the correct order on the right. Check your solution by clicking on the Check button. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks. There is one extra block that is not needed in a correct solution.
Checkpoint 6.27.7.
Subsection 6.27.4 Summary
-
ArrayLists can be traversed with an enhanced for each loop, or a while or for loop using an index.
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Deleting elements during a traversal of an ArrayList requires using special techniques to avoid skipping elements, since remove moves all the elements down.
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Since the indices for an ArrayList start at 0 and end at the number of elements − 1, accessing an index value outside of this range will result in an ArrayIndexOutOfBoundsException being thrown.
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Changing the size of an ArrayList while traversing it using an enhanced for loop can result in a ConcurrentModificationException being thrown. Therefore, when using an enhanced for loop to traverse an ArrayList, you should not add or remove elements.
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