This question includes a topic, interfaces, that is no longer part of the AP Java subset and will not be directly tested on current exams. Inheritance is still a tested topic, and this problem is a good example of working with inheritance.
Question #2 on the 2017 AP Computer Science exam was testing your ability to use an arbitrary Java interface called StudyPractice. You were to implement this interface to create a class called MultPractice.
The StudyPractice interface, defined below, specifies two methods that you must implement; getProblem and nextProblem.
public interface StudyPractice {
/** Returns the current practice problem */
String getProblem();
/** Changes to the next practice problem */
void nextProblem();
}
Our implementation of MultPractice must implement both of these methods.
public class MultPractic implements StudyPractice {
private int numOne;
private int numTwo;
public MultPractice(int a, int b) {
numOne = a;
numTwo = b;
}
public String getProblem() {
return numOne + " TIMES " + numTwo;
}
public void nextProblem() {
numTwo++;
}
Let’s start with the first line. We’re creating a class called MultPractice that implements the interface StudyPractice.
For this class we’ll need two int instance variables. Names don’t really matter, but they do both need to be private. That’s a College Board preference that all instance variables should be private and they typically will take off points if you do not specify instance variable as private.
The constructor takes two int parameters. We know this because of the examples in the problem. There are several example calls similar to StudyPractice p1 = new MultPractice(7, 4). This tells us that the constructor must take two int parameters.
Then we implement the getProblem and nextProblem methods. getProblem returns a string in a specific format, starting with the first number, followed by " TIMES ", followed by the second number. nextProblem increments the second number so that the next time getProblem is called it will return a different string.
