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@ingramchen
Last active August 29, 2015 14:04
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展示 Java 8 與 Java 7 兩者撰寫上差異
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);
}
}
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