展示 Java 8 與 Java 7 兩者撰寫上差異

gistfile1.java

package com.liquable.nemo.model;

import static java.util.Arrays.asList;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.summingLong;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.function.Function;

import com.google.common.collect.ImmutableSet;

/**
 * 主要有兩個方法 <code>java7_imperative()</code> 和 <code>java8_functional()</code>，分別展示 
 * imperative 與 functional 風格的寫法。
 * 
 * 程式的目的在求誰是最多人共有的朋友。
 * 
 * @author ingram
 *
 */
public class UserTest {

  /**
   * Java 7 external iteration, 共 21 行。
   */
  public void java7_imperative(List<User> users) {
    final Map<Friend, Long> friendCounts = new HashMap<>();
    for (final User user : users) {
      for (final Friend friend : user.getFriends()) {
        if (!friendCounts.containsKey(friend)) {
          friendCounts.put(friend, 1L);
        } else {
          friendCounts.put(friend, friendCounts.get(friend) + 1);
        }
      }
    }
    Friend topFriend = null;
    long max = -1;
    for (final Entry<Friend, Long> entry : friendCounts.entrySet()) {
      if (entry.getValue() > max) {
        topFriend = entry.getKey();
        max = entry.getValue();
      }
    }
    if (topFriend != null) {
      userDao.saveMostCommonFriend(topFriend);
    }
  }

  /**
   * Java 8 internal iteration, 共 6 行。
   */
  public void java8_functional(List<User> users) {
    users.stream().flatMap(user -> user.getFriends().stream())
         .collect(groupingBy(Function.identity(), summingLong(friend -> 1)))
         .entrySet().stream() // Entry<Friend, Long> friendCount
         .max(Entry.comparingByValue())
         .map(Entry::getKey)
         .ifPresent(userDao::saveMostCommonFriend);
  }

  public static void main(String[] args) {
    final Friend[] f = { new Friend("a"), new Friend("b"), new Friend("c"), new Friend("d") };
    final User[] users = new User[6];
    users[0] = new User(f[0], f[1]);
    users[1] = new User(f[1], f[2]);
    users[2] = new User(f[3], f[2]);
    users[3] = new User(f[0], f[1], f[2], f[3]);
    users[4] = new User(f[1]);
    users[5] = new User();

    new UserTest().java7_imperative(asList(users));
    new UserTest().java8_functional(asList(users));
  }

  UserDao userDao = new UserDao();

}

class Friend {

  String name;

  Friend(String name) {
    this.name = name;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null) return false;
    if (getClass() != obj.getClass()) return false;
    final Friend other = (Friend) obj;
    if (name == null) {
      if (other.name != null) return false;
    } else if (!name.equals(other.name)) return false;
    return true;
  }

  @Override
  public int hashCode() {
    return ((name == null) ? 0 : name.hashCode());
  }

  @Override
  public String toString() {
    return name;
  }
}

class User {

  Set<Friend> friends;

  User(Friend... friends) {
    this.friends = ImmutableSet.copyOf(friends);
  }

  Set<Friend> getFriends() {
    return friends;
  }
}

class UserDao {
  // ... do database save...
  public void saveMostCommonFriend(Friend friend) {
    System.out.println("saved: " + friend);
  }
}