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Subgoals

Section 14.2 Writing-ArrayLists-WE1-P1

Subgoals for Evaluating ArrayLists.

  1. 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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  2. 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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  3. Instantiating an ArrayList object
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    1. Use the new keyword with the constructor to create a new ArrayList object
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    2. Assign to variable using: name = new ArrayList<>();
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    3. Optionally, initialize with values using: new ArrayList<>(List.of(value1, value2, ...)); (Java 9+)
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  4. 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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  5. 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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  6. Traversing an ArrayList
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    1. Decide whether accessing all elements, updating, or accessing a subset
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    2. If accessing only, 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 listName.get(i) to access or listName.set(i, newValue) to update
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  7. 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. Pass the ArrayList variable in the method call
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      3. Note: changes to the ArrayList in the method will persist (objects are passed by reference)
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    2. Reassigning an ArrayList
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      1. Determine that the reference to the list should point to a new ArrayList
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      2. Left-hand side is the variable name of the original list
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      3. Right-hand side is the new ArrayList, e.g., new ArrayList<>()
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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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Q2. Which of the following would it be easier to use an initializer list for versus a loop when creating an ArrayList? (Select all that are appropriate)
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  • Initialize an ArrayList of author names called FavoriteAuthors
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  • Initialize an ArrayList of numbers from 100 to 1000 called NineHundred
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  • Initialize an ArrayList of ribbon colors called RibbonColors
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  • Initialize an ArrayList with the top ten fastest times for your school’s track team in the 100 meter race named TopTen
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  • Initialize an ArrayList with the height (in inches) of all the children in a classroom called ClassroomHeights
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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);
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}

  • public void addOne(ArrayList<Integer> numbers) {
    for (int num : numbers) {
        num++;
    }
}
  • This does not modify the original list because it only changes a copy of each value.
  • public void addOne(ArrayList<Integer> numbers) {
    for (int num : numbers) {
        numbers.set(num, num + 1);
    }
}
  • This misuses num as both index and value.
  • public void addOne(ArrayList<Integer> numbers) {
    for (int i = 0; i < numbers.size(); i++) {
        numbers.set(i, i + 1);
    }
}
  • This sets each element to its index plus one, not to the original value plus one.
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