FRQ 2 - Class Writing and Design - 2018 Exam Q3 - CS112 - Interfaces

Solution

Here is the complete implementation of the CodeWordChecker class that satisfies all points on the rubric.

Complete Code

public class CodeWordChecker implements StringChecker {
    private int minLength;
    private int maxLength;
    private String notAllowed;
    
    public CodeWordChecker(int minLen, int maxLen, String symbol) {
        minLength = minLen;
        maxLength = maxLen;
        notAllowed = symbol;
    }
    
    public CodeWordChecker(String symbol) {
        minLength = 6;
        maxLength = 20;
        notAllowed = symbol;
    }
    
    public boolean isValid(String str) {
        return str.length() >= minLength && 
               str.length() <= maxLength && 
               str.indexOf(notAllowed) == -1;
    }
}

Part-by-part Explanation

Class Declaration (1 point)

public class CodeWordChecker implements StringChecker

• Must include the implements StringChecker clause
• This makes it a requirement to implement the isValid() method

Instance Variables (1 point)

private int minLength;
private int maxLength;
private String notAllowed;

• All three instance variables must be declared as private
• minLength and maxLength are int types
• notAllowed is a String

Three-Parameter Constructor (3 points)

public CodeWordChecker(int minLen, int maxLen, String symbol) {
    minLength = minLen;
    maxLength = maxLen;
    notAllowed = symbol;
}

• 1 point: Correct header with three parameters
• 1 point: Initializes minLength using first parameter
• 1 point: Initializes maxLength and notAllowed using remaining parameters

One-Parameter Constructor (1 point)

public CodeWordChecker(String symbol) {
    minLength = 6;
    maxLength = 20;
    notAllowed = symbol;
}

• Uses the parameter to set notAllowed
• Uses default literal values 6 and 20 for min and max
• Alternative: Could call this(6, 20, symbol);

isValid() Method (4 points)

public boolean isValid(String str) {
    return str.length() >= minLength && 
           str.length() <= maxLength && 
           str.indexOf(notAllowed) == -1;
}

• 1 point: Correct header: public boolean isValid(String __)
• 1 point: Checks if length is between min and max (inclusive)
• 1 point: Checks if string does NOT contain notAllowed using indexOf()
• 1 point: Returns true only if ALL conditions are met, false otherwise

How It Works

1. The class implements an interface, so it must provide the isValid() method.
2. Two constructors provide flexibility: one with custom values, one with defaults.
3. The isValid() method performs three checks using logical AND (&&) to ensure all conditions are met.
4. The indexOf() method returns -1 if the substring is not found, which means the string passes that check.

Scoring Guidelines

Points Breakdown (10 points total)

Class Declaration and Instance Variables (2 points)

• 1 point: Declares header: public class CodeWordChecker implements StringChecker
• 1 point: Declares all appropriate private instance variables (minLength, maxLength, notAllowed)

Constructors (4 points)

• 1 point: Declares headers for both constructors:
  • public CodeWordChecker(int __, int __, String __)
  • public CodeWordChecker(String __)
• 1 point: Uses all three parameters to initialize instance variables in 3-parameter constructor
• 1 point: Initializes minLength using first parameter in 3-parameter constructor
• 1 point: Uses parameter and default values (6 and 20) to initialize instance variables in 1-parameter constructor

isValid() Method (4 points)

• 1 point: Declares header: public boolean isValid(String __)
• 1 point: Checks for length between min and max inclusive (uses >= and <=)
• 1 point: Checks that the string does not contain the unwanted string (uses indexOf() or contains())
• 1 point: Returns true if length is between min and max AND does not contain the unwanted string, false otherwise

Common Mistakes to Avoid

• Forgetting to include implements StringChecker in the class header
• Using == instead of .equals() when comparing strings
• Using str.indexOf(notAllowed) == 0 (only checks if it starts with the unwanted string)
• Using > or < instead of >= and <= for length checks (the bounds are inclusive)
• Forgetting to make instance variables private
• Not initializing all instance variables in the constructors
• Using wrong default values in the 1-parameter constructor (must be 6 and 20)
• Using OR (||) instead of AND (&&) in the isValid() method
• Returning true when the unwanted string IS found (logic should be inverted)

Alternative Approaches

Using constructor chaining:

public CodeWordChecker(String symbol) {
    this(6, 20, symbol);
}

Using contains() instead of indexOf():

public boolean isValid(String str) {
    return str.length() >= minLength && 
           str.length() <= maxLength && 
           !str.contains(notAllowed);
}

Both of these alternatives are equally valid and will receive full credit.