Answer to Questions and Exercises: Generics

1. Write a generic method to count the number of elements in a collection that have a specific property (for example, odd integers, prime numbers, palindromes).
   
   Answer:

   public final class Algorithm {
       public static <T> int countIf(Collection<T> c, UnaryPredicate<T> p) {
   
           int count = 0;
           for (T elem : c)
               if (p.test(elem))
                   ++count;
           return count;
       }
   }
   

   where the generic UnaryPredicate interface is defined as follows:

   public interface UnaryPredicate<T> {
       public boolean test(T obj);
   }
   

   For example, the following program counts the number of odd integers in an integer list:

   import java.util.*;
   
   class OddPredicate implements UnaryPredicate<Integer> {
       public boolean test(Integer i) { return i % 2 != 0; }
   }
   
   public class Test {
       public static void main(String[] args) {
           Collection<Integer> ci = Arrays.asList(1, 2, 3, 4);
           int count = Algorithm.countIf(ci, new OddPredicate());
           System.out.println("Number of odd integers = " + count);
       }
   }
   

   The program prints:

   Number of odd integers = 2
   

2. Will the following class compile? If not, why?

   public final class Algorithm {
       public static T max(T x, T y) {
           return x > y ? x : y;
       }
   }
   

   Answer: No. The greater than (>) operator applies only to primitive numeric types.
   
   
3. Write a generic method to exchange the positions of two different elements in an array.
   
   Answer:

   public final class Algorithm {
       public static <T> void swap(T[] a, int i, int j) {
           T temp = a[i];
           a[i] = a[j];
           a[j] = temp;
       }
   }
   

4. If the compiler erases all type parameters at compile time, why should you use generics?
   
   Answer: You should use generics because:
   • The Java compiler enforces tighter type checks on generic code at compile time.
   • Generics support programming types as parameters.
   • Generics enable you to implement generic algorithms.
   
   
   
5. What is the following class converted to after type erasure?

   public class Pair<K, V> {
   
       public Pair(K key, V value) {
           this.key = key;
           this.value = value;
       }
   
       public K getKey(); { return key; }
       public V getValue(); { return value; }
   
       public void setKey(K key)     { this.key = key; }
       public void setValue(V value) { this.value = value; }
   
       private K key;
       private V value;
   }
   

   Answer:

   public class Pair {
   
       public Pair(Object key, Object value) {
           this.key = key;
           this.value = value;
       }
   
       public Object getKey()   { return key; }
       public Object getValue() { return value; }
   
       public void setKey(Object key)     { this.key = key; }
       public void setValue(Object value) { this.value = value; }
   
       private Object key;
       private Object value;
   }
   

6. What is the following method converted to after type erasure?

   public static <T extends Comparable<T>>
       int findFirstGreaterThan(T[] at, T elem) {
       // ...
   }
   

   Answer:

   public static Comparable
       int findFirstGreaterThan(Comparable[] at, Comparable elem) {
       // ...
       }
   

7. Will the following method compile? If not, why?

   public static void print(List<? super Number> list) {
       for (Number n : list)
           System.out.print(n + " ");
       System.out.println();
   }
   

   Answer: Yes.
   
   
8. Write a generic method to find the maximal element in the range [begin, end) of a list.
   
   Answer:

   import java.util.*;
   
   public final class Algorithm {
       public static <T extends Object & Comparable<? super T>>
           T max(List<? extends T> list, int begin, int end) {
   
           T maxElem = list.get(begin);
   
           for (++begin; begin < end; ++begin)
               if (maxElem.compareTo(list.get(begin)) < 0)
                   maxElem = list.get(begin);
           return maxElem;
       }
   }
   

9. Will the following class compile? If not, why?

   public class Singleton<T> {
   
       public static T getInstance() {
           if (instance == null)
               instance = new Singleton<T>();
   
           return instance;
       }
   
       private static T instance = null;
   }
   

   Answer: No. You cannot create a static field of the type parameter T.
   
   
10. Given the following classes:

    class Shape { /* ... */ }
    class Circle extends Shape { /* ... */ }
    class Rectangle extends Shape { /* ... */ }
    
    class Node<T> { /* ... */ }
    

    Will the following code compile? If not, why?

    Node<Circle> nc = new Node<>();
    Node<Shape>  ns = nc;
    

    Answer: No. Because Node<Circle> is not a subtype of Node<Shape>.
    
    
11. Consider this class:

    class Node<T> implements Comparable<T> {
        public int compareTo(T obj) { /* ... */ }
        // ...
    }
    

    Will the following code compile? If not, why?
    
    Answer: Yes.

    Node<String> node = new Node<>();
    Comparable<String> comp = node;
    

12. How do you invoke the following method to find the first integer in a list that is relatively prime to a list of specified integers?

    public static <T>
        int findFirst(List<T> list, int begin, int end, UnaryPredicate<T> p)
    

    Note that two integers a and b are relatively prime if gcd(a, b) = 1, where gcd is short for greatest common divisor.
    
    Answer:

    import java.util.*;
    
    public final class Algorithm {
    
        public static <T>
            int findFirst(List<T> list, int begin, int end, UnaryPredicate<T> p) {
    
            for (; begin < end; ++begin)
                if (p.test(list.get(begin)))
                    return begin;
            return -1;
        }
    
        // x > 0 and y > 0
        public static int gcd(int x, int y) {
            for (int r; (r = x % y) != 0; x = y, y = r) { }
                return y;
        }
    }
    

    The generic UnaryPredicate interface is defined as follows:

    public interface UnaryPredicate<T> {
        public boolean test(T obj);
    }
    

    The following program tests the findFirst method:

    import java.util.*;
    
    class RelativelyPrimePredicate implements UnaryPredicate<Integer> {
        public RelativelyPrimePredicate(Collection<Integer> c) {
            this.c = c;
        }
    
        public boolean test(Integer x) {
            for (Integer i : c)
                if (Algorithm.gcd(x, i) != 1)
                    return false;
    
            return c.size() > 0;
        }
    
        private Collection<Integer> c;
    }
    
    public class Test {
        public static void main(String[] args) throws Exception {
    
            List<Integer> li = Arrays.asList(3, 4, 6, 8, 11, 15, 28, 32);
            Collection<Integer> c = Arrays.asList(7, 18, 19, 25);
            UnaryPredicate<Integer> p = new RelativelyPrimePredicate(c);
    
            int i = ALgorithm.findFirst(li, 0, li.size(), p);
    
            if (i != -1) {
                System.out.print(li.get(i) + " is relatively prime to ");
                for (Integer k : c)
                    System.out.print(k + " ");
                System.out.println();
            }
        }
    }
    

    The program prints:

    11 is relatively prime to 7 18 19 25