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Subgoals

Section 14.6 Writing-ArrayLists-WE3-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 14.6.1

Exercises Exercises

1.
Q7: What does the following code do?
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ArrayList<Integer> alpha = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50));
ArrayList<Integer> beta = new ArrayList<>(alpha);
beta.add(42);
alpha = beta;
  • Inserts a new value into the first position of the list
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  • Inserts a new value into the middle position of the list
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  • Inserts a new value into the last position of the list
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  • Deletes a value from a specific position of the list
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  • Finds and deletes a value from the list
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2.
Q8: What does the following code do?
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int target = /* some value */ ;
ArrayList<Integer> delta = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50, 60, 70, 80, 90));
boolean found = false;
int i;
for (i = 0; i < delta.size() && !found; i++) {
    if (delta.get(i) == target) {
        found = true;
    }
}
if (found) {
    delta.remove(i - 1);
    delta.add(-999);
}
  • Inserts a new value into the first position of the list
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  • Inserts a new value into the middle position of the list
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  • Inserts a new value into the last position of the list
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  • Deletes a value from a specific position of the list
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  • Finds and deletes a value from the list
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3.
Q9: What does the following code do?
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ArrayList<Integer> alpha = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50));
ArrayList<Integer> beta = new ArrayList<>();
beta.add(99);
beta.addAll(alpha);
alpha = beta;
  • Inserts a new value into the first position of the list
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  • Inserts a new value into the middle position of the list
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  • Inserts a new value into the last position of the list
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  • Deletes a value from a specific position of the list
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  • Finds and deletes a value from the list
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4.
Q10: What does the following code do?
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int pos = /* some value */ ;
ArrayList<Integer> rho = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50, 60, 70, 80, 90));
rho.remove(pos);
rho.add(-999);
  • Inserts a new value into the first position of the list
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  • Inserts a new value into the middle position of the list
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  • Inserts a new value into the last position of the list
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  • Deletes a value from a specific position of the list
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  • Finds and deletes a value from the list
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5.
Q11: What does the following code do?
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ArrayList<Integer> alpha = new ArrayList<>(Arrays.asList(10, 20, 30, 40, 50));
ArrayList<Integer> gamma = new ArrayList<>(alpha);
gamma.add(gamma.size() / 2, 11);
alpha = gamma;
  • Inserts a new value into the first position of the list
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  • Inserts a new value into the middle position of the list
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  • Inserts a new value into the last position of the list
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  • Deletes a value from a specific position of the list
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  • Finds and deletes a value from the list
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