CodeWordChecker solution - AP 2018 FRQ - https://clsc.be/25

CodeWordChecker.java

public class CodeWordChecker implements StringChecker {
  private int min;
  private int max;
  private String no;
  
  public CodeWordChecker(int mi, int ma, String n) {
    min = mi;
    max = ma;
    no = n;
  }
  public CodeWordChecker(String n) {
    this(6, 20, n);    
  }
  
  public boolean isValid(String str) {
    return str.length() >= min && str.length() <= max && str.indexOf(no) == -1; 
  }
}

StringChecker.java

public interface StringChecker {
  /** Returns true if str is valid */
  public boolean isValid(String str);
}