2020 AP CSA Sample Q1 - Gizmo (ArrayList of Objects)

Question

The Gizmo class represents gadgets that people purchase. Some Gizmo objects are electronic and others are not.

Provided Gizmo Class

public class Gizmo
{
    /** Returns the name of the manufacturer of this Gizmo. */
    public String getMaker()
    { /* implementation not shown */ }

    /** Returns true if this Gizmo is electronic, and false otherwise. */
    public boolean isElectronic()
    { /* implementation not shown */ }

    /** Returns true if this Gizmo is equivalent to the Gizmo object
     *  represented by the parameter, and false otherwise.
     */
    public boolean equals(Object other)
    { /* implementation not shown */ }

    // There may be instance variables, constructors, and methods not shown.
}

OnlinePurchaseManager Class

public class OnlinePurchaseManager
{
    /** An ArrayList of purchased Gizmo objects, instantiated in the constructor. */
    private ArrayList purchases;

    /** Returns the number of purchased Gizmo objects that are electronic and are
     *  manufactured by maker, as described in part (a).
     */
    public int countElectronicsByMaker(String maker)
    { /* to be implemented in part (a) */ }

    /** Returns true if any pair of adjacent purchased Gizmo objects are equivalent, and
     *  false otherwise, as described in part (b).
     */
    public boolean hasAdjacentEqualPair()
    { /* to be implemented in part (b) */ }

    // There may be instance variables, constructors, and methods not shown.
}

Part (a): Write countElectronicsByMaker

Write the countElectronicsByMaker method. The method examines the ArrayList instance variable purchases to determine how many Gizmo objects purchased are electronic AND are manufactured by maker.

Example Data

Part (b): Write hasAdjacentEqualPair

When purchasing items online, users occasionally purchase two identical items in rapid succession without intending to (e.g., by clicking a purchase button twice). A vendor may want to check a user's purchase history to detect such occurrences.

Write the hasAdjacentEqualPair method. The method detects whether two adjacent Gizmo objects in purchases are equivalent, using the equals method of the Gizmo class.

• If an adjacent equivalent pair is found, return true
• If no such pair is found, OR if purchases has fewer than two elements, return false

Part (c): Design Discussion

A programmer would like to add a method getCheapestGizmoByMaker, which returns the least expensive Gizmo purchased by an individual from a given maker.

Write a description of how you would change the Gizmo and OnlinePurchaseManager classes to support this modification. (No code required)

Part (a) Solution: countElectronicsByMaker

Solution Code

public int countElectronicsByMaker(String maker)
{
    int count = 0;
    
    for (int i = 0; i < purchases.size(); i++)
    {
        Gizmo g = purchases.get(i);
        
        if (g.isElectronic() && g.getMaker().equals(maker))
        {
            count++;
        }
    }
    
    return count;
}

Alternative using enhanced for loop:

public int countElectronicsByMaker(String maker)
{
    int count = 0;
    
    for (Gizmo g : purchases)
    {
        if (g.isElectronic() && g.getMaker().equals(maker))
        {
            count++;
        }
    }
    
    return count;
}

Explanation

1. Initialize counter: Start with count = 0.
2. Iterate through purchases: Use either indexed or enhanced for loop.
3. Check both conditions: Must be electronic AND from the specified maker.
4. Increment if match: Only count if both conditions are true.
5. Return final count.

Part (b) Solution: hasAdjacentEqualPair

Solution Code

public boolean hasAdjacentEqualPair()
{
    // Need at least 2 elements to have an adjacent pair
    if (purchases.size() < 2)
    {
        return false;
    }
    
    for (int i = 0; i < purchases.size() - 1; i++)
    {
        if (purchases.get(i).equals(purchases.get(i + 1)))
        {
            return true;
        }
    }
    
    return false;
}

Alternative (size check built into loop):

public boolean hasAdjacentEqualPair()
{
    for (int i = 0; i < purchases.size() - 1; i++)
    {
        if (purchases.get(i).equals(purchases.get(i + 1)))
        {
            return true;
        }
    }
    return false;
}

Explanation

1. Check minimum size: Need at least 2 elements for adjacent pairs (optional but clear).
2. Loop through adjacent pairs: Stop at size() - 1 because we compare i with i + 1.
3. Use equals method: The problem specifies using Gizmo's equals method.
4. Return true immediately: As soon as we find a match, return true.
5. Return false if no match: Only after checking all pairs.

Part (c): Design Discussion

This part asks you to describe (not code) how to add getCheapestGizmoByMaker.

Common Mistakes to Avoid

Part (a) Common Errors:

• Using == instead of .equals(): For String comparison, always use .equals()
• Only checking one condition: Must check electronic AND maker, not just one
• Using || instead of &&: Both conditions must be true
• Forgetting to return: Must return the count at the end

Part (b) Common Errors:

• Loop bound error: Using purchases.size() instead of purchases.size() - 1
• Comparing non-adjacent elements: Must compare i with i + 1
• Using == instead of .equals(): For object comparison, use .equals()
• Not handling empty/single-element list: Should return false for lists with fewer than 2 elements
• Returning too late: Should return true immediately when a pair is found