Writing-ArrayLists-WE1-P1

Section 14.2 Writing-ArrayLists-WE1-P1

Subgoals for Writing ArrayLists.

Importing the ArrayList class
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1. Before using ArrayList, import it from the java.util package:
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  import java.util.ArrayList;
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Declaring an ArrayList variable
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1. Determine the type of objects to be stored (use wrapper classes for primitives, e.g., Integer instead of int)
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2. Determine the name of the ArrayList variable
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3. Use syntax: ArrayList<DataType> name;
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Instantiating an ArrayList object
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1. Use the new keyword with the constructor to create a new ArrayList object (When ArrayLists are instantiated, they are empty and have a size of 0.)
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Adding elements to an ArrayList
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1. To add to the end of an ArrayList, use: listName.add(valueToBeAdded)
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2. To add an element at a specific location in an ArrayList, use: listName.add(index, valueToBeAdded) where index is within the bounds 0 to listName.size()
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Accessing an element in an ArrayList
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1. Determine the index of the element to be accessed
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2. Use: listName.get(index) to retrieve the element
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3. Ensure the index is within bounds: 0 to listName.size() - 1, otherwise an IndexOutOfBoundsException occurs
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Changing a value in an ArrayList
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1. Determine the index of the element to be changed
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2. Determine the new value or expression to assign
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3. Use: listName.set(index, newValue) to update the value
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4. Ensure the index is within bounds: 0 to listName.size() - 1, otherwise an IndexOutOfBoundsException occurs
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Traversing an ArrayList
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1. Decide whether accessing all elements, updating, or accessing a subset
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2. If only accessing elements, use an enhanced for (for-each) loop:
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  1. for (DataType item : listName) - iterates from first to last, storing a copy of each element in item
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3. If updating or using indices, use a traditional for loop:
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  1. Initialize loop control variable to 0 (or listName.size() - 1 for reverse)
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  2. Set condition: i < listName.size() (or i >= 0 for reverse)
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  3. Increment or decrement loop control variable appropriately
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4. Use Subgoals 5 or 6 to access or change values as appropriate
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Whole list actions
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1. Passing an ArrayList as an argument
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  1. Check if the method expects an ArrayList argument (check documentation or method signature)
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  2. When calling a method, pass a reference to an ArrayList (usually variable name) as an argument in the method call.
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  3. Note: changes to elements in the ArrayList that are done inside the method will persist
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Subsection 14.2.1

Exercises Exercises

1.

Q1: Put the code in the appropriate order so that an ArrayList of integers will store multiples of 5 (from 100 down to 0) in reverse order.

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import java.util.ArrayList;
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public ArrayList<Integer> storeReverse() {
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    ArrayList<Integer> reverseFives = new ArrayList<Integer>();
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    for (int i = 20; i >= 0; i--) {
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        reverseFives.add(i * 5);
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    }
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    return reverseFives;
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}

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2.

Q2: Which of the following should use an ArrayList rather than an array? (Select all that are appropriate)

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Store numbers from 100 to 1000
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Incorrect
Store a list of favorite authors
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Correct
Store the top ten fastest times for your school’s track team in the 100 meter race
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Incorrect
Store the incoming fast food restaurant orders
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Correct

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3.

Q3. Suppose you are writing a method that adds one to all of the elements of an ArrayList named numbers. Which of the following code snippets would be best for this task?

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public void addOne(ArrayList<Integer> numbers) {
    for (int i = 0; i < numbers.size(); i++) {
        numbers.set(i, numbers.get(i) + 1);
    }
}

Correct

public void addOne(ArrayList<Integer> numbers) {
    for (int num : numbers) {
        num++;
    }
}

Incorrect

public void addOne(ArrayList<Integer> numbers) {
    for (int num : numbers) {
        numbers.set(num, num + 1);
    }
}

Incorrect

public void addOne(ArrayList<Integer> numbers) {
    for (int i = 0; i < numbers.size(); i++) {
        numbers.set(i, i + 1);
    }
}

Incorrect

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