Java8 Lamdba表达式 002

本篇将讲述lamdba表达式的排序，本例包含一个Player对象的集合[稍后定义]，通过每个player的分数高低对列表的player进行排序，类定义001如下 

public class SortingPlayer {
	
	public static void main(String[] args) {
		List<Player> playerList = new ArrayList<>();
		playerList.add(new Player("Black", "White", 9));
		playerList.add(new Player("John", "Hello", 2));
		playerList.add(new Player("Machicel", "Jackson", 7));
		playerList.add(new Player("Ani", "Hessius", 4));
		playerList.add(new Player("Mark", "Towns", 3));
		playerList.add(new Player("Huge", "Nana", 6));
	}
}

class Player{
	private String firstName;
	private String lastName;
	private int goals;
	
	public Player(String firstName, String lastName, int goals) {
		this.firstName = firstName;
		this.lastName = lastName;
		this.goals = goals;
	}
	
	public String getFirstName() {
		return firstName;
	}
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}
	public String getLastName() {
		return lastName;
	}
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
	public int getGoals() {
		return goals;
	}
	public void setGoals(int goals) {
		this.goals = goals;
	}
}

002下面的代码在添加在001main方法list后

Comparator<Player> byGoals = Comparator.comparing(Player::getGoals);
System.out.println("== Sort by Number of Goals ==");
playerList.stream().sorted(byGoals)
		.map(p -> p.getFirstName() + " " + p.getLastName() + " - "
				+ p.getGoals())
		.forEach(element -> System.out.println(element));

使用Player对象的getter方法[根据你想排序的那个字段，本例使用goals]创建一个Comparator - Player::getGoals 。然后使用混合的lamdba表达式和streams，forEach()，展示排序后的集合。

java8中集合排序又新加了可以极大提高开发效率的3个新特性。分别是lamdba表达式，方法引用以及stream。对于方法引用和stream这里只做简单介绍，Stream可以在集合数据[collections]中使用,它允许集合中的元素进行函数式操作。Stream不存储数据，它能让获得的集合具有更多功能。

002中，Comparator根据计算的goals生成，Player::getGoals。然后根据byGoals生成stream。stream提供sorted()功能，它接收一个Comparator，Comparator在传递给sorted()时已经完成初始化，然后调用map()功能，map使用一个lamdba表达式拼一个firstname,lastname,&goals的字符串。最后由于List<Player>是可迭代的，它包含forEach()方法，forEach()方法允许集合中的每个元素应用表达式或状态组。本例中，每个元素都在命令行打印，由于map()功能已经在stream中应用,因此最后结果是打印按照goals排好序的每个元素的firstname,lastname,&goals拼的字符串。如下所示：

== Sort by Number of Goals ==
John Hello - 2
Mark Towns - 3
Ani Hessius - 4
Huge Nana - 6
Machicel Jackson - 7
Black White - 9

除了使用002所示的方法进行排序，我们还可以使用Collections集合提供的sort()方法：Collections.sort()，见003

System.out.println("== utilize the Collections.sort()method ==");
Collections.sort(playerList, (p1,p2) 
		-> p1.getLastName().compareTo(p2.getLastName()));
playerList.stream().forEach((p) -> {
	System.out.println(p.getLastName());
});

== utilize the Collections.sort()method ==
Hello
Hessius
Jackson
Nana
Towns
White

ps:本文示例引自 Josh Juneau所著 Java 8 Recipes, 2nd Edition

mission completed！

O(∩_∩)O~