Some Java 8 example code

SomeMapExample.java

package cc.kevinlee.examples;

import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Optional;

import static cc.kevinlee.examples.KeyValPair.*;

/**
 * @author Lee, Seong Hyun (Kevin)
 * @since 2015-07-17
 */
public class SomeMapExample {
  public static void main(final String[] args) {
    final SomeMap<String, Integer> someMap = new SomeMap<>(new List[]{Arrays.asList(keyValPair("a", 1), keyValPair("b", 2), keyValPair("c", 3))});

    System.out.println("someMap.contains a? " + someMap.containsKey("a"));  // true
    System.out.println("someMap.contains b? " + someMap.containsKey("b"));  // true
    System.out.println("someMap.contains c? " + someMap.containsKey("c"));  // true
    System.out.println("someMap.contains d? " + someMap.containsKey("d"));  // false


    final SomeMapWithNullableBucket<String, Integer> someMapWithNullableBucket = new SomeMapWithNullableBucket();
    System.out.println(
        "someMapWithNullableBucket.contains a? " + someMapWithNullableBucket.containsKey("a")
    );   // false
  }
}

class KeyValPair<K, V> {
  final K key;
  final V value;

  public KeyValPair(final K key, final V value) {
    this.key = key;
    this.value = value;
  }

  public K getKey() {
    return key;
  }

  public V getValue() {
    return value;
  }

  public static <K, V> KeyValPair<K, V> keyValPair(final K key, final V value) {
    return new KeyValPair<>(key, value);
  }
}

class SomeMap<K, V> {

  private List<KeyValPair<K, V>>[] buckets;

  public SomeMap(final List<KeyValPair<K, V>>[] buckets) {
    this.buckets = buckets;
  }

  public boolean containsKey(final K k) {
    return buckets[hash(k)].stream().map(KeyValPair::getKey).anyMatch(k::equals);
  }

  /**
   * Dummy implemetation for testing. It always returns 0.
   *
   * @param k
   * @return 0
   */
  private int hash(final K k) {
    return 0;
  }
}

class SomeMapWithNullableBucket<K, V> {

  private List<KeyValPair<K, V>>[] buckets;

  public SomeMapWithNullableBucket() {
    this.buckets = new List[1];
  }

  public boolean containsKey(final K k) {
    return Optional.ofNullable(buckets[hash(k)])
        .map(Collection::stream)
        .filter(s -> s.map(KeyValPair::getKey).anyMatch(k::equals))
        .isPresent();
// OR
//    return Optional.ofNullable(buckets[hash(k)])
//        .map(Collection::stream)
//        .map(s -> s.map(KeyValPair::getKey))
//        .map(s -> s.filter(k::equals))
//        .flatMap(Stream::findAny)
//        .isPresent();


    // This one causes NullPointerException
    // return buckets[hash(k)].stream().map(KeyValPair::getKey).anyMatch(k::equals);
  }

  /**
   * Dummy implemetation for testing. It always returns 0.
   *
   * @param k
   * @return 0
   */
  private int hash(final K k) {
    return 0;
  }
}