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Answer to Questions and Exercises: Generics

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

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.

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;
    }
}

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.

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;
}

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

Will the following method compile? If not, why?

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

Answer: Yes.

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;
    }
}

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.

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>.

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;

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