2008 AP CSA FRQ 4: Checker
Topic: Interfaces & Polymorphism
Skills: Interface implementation, boolean logic, composite patterns, NOT logic
Unit: Unit 3 (Class Creation) (2025-2026 AP CSA)
Points: 9 | Time: ~22 minutes
Study Guide: Unit 3: Class Creation
Part (a)
Write the complete SubstringChecker class that implements Checker.
Try First! Attempt before viewing solution.
Solution
public class SubstringChecker implements Checker
{
private String target;
public SubstringChecker(String target)
{
this.target = target;
}
public boolean accept(String text)
{
return text.indexOf(target) >= 0;
}
}Part (b)
Write the complete NotChecker class that negates another Checker.
Solution
public class NotChecker implements Checker
{
private Checker checker;
public NotChecker(Checker checker)
{
this.checker = checker;
}
public boolean accept(String text)
{
return !checker.accept(text);
}
}Key Concepts
✓ implements keyword for interfaces
✓ Composition: wrapping one Checker in another
✓ indexOf returns -1 if not found, >= 0 if found
✓ Decorator pattern with NotChecker
Common Mistakes
✗ Forgetting implements Checker
✗ Using == instead of >= for indexOf check
✗ Not storing the Checker reference in NotChecker
✗ Forgetting to negate the result in NotChecker
Official Resources
Author: Tanner Crow - AP CS Teacher, 11+ years