Documentation

The Java™ Tutorials

Questions and Exercises

- 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).
- 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; } }

- Write a generic method to exchange the positions of two different elements in an array.
- If the compiler erases all type parameters at compile time, why should you use generics?
- 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; }

- What is the following method converted to after type erasure?
public static <T extends Comparable<T>> int findFirstGreaterThan(T[] at, T 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(); }

- Write a generic method to find the maximal element in the range
`[begin, end)`of a list. - 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; }

- Given the following classes:
Will the following code compile? If not, why?
class Shape { /* ... */ } class Circle extends Shape { /* ... */ } class Rectangle extends Shape { /* ... */ } class Node<T> { /* ... */ }

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

- Consider this class:
Will the following code compile? If not, why?
class Node<T> implements Comparable<T> { public int compareTo(T obj) { /* ... */ } // ... }

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?
Note that two integers
public static <T> int findFirst(List<T> list, int begin, int end, UnaryPredicate<T> p)

*a*and*b*are relatively prime if gcd(*a, b*) = 1, where gcd is short for greatest common divisor.