JDK8新特性：使用stream、Comparator和Method Reference实现集合的优雅排序

大家对java接口Comparator和Comparable都不陌生，JDK8里面Comparable还和以前一样，没有什么改动；但是Comparator在之前基础上增加了很多static和default方法。本文主要结合JDK的stream编程，学习下Comparator。阅读本文需要一些前置知识，可以参考如下文章。

JDK8新特性：接口的静态方法和默认方法
http://blog.csdn.net/aitangyong/article/details/54134385

JDK8新特性：函数式接口@FunctionalInterface的使用说明

http://blog.csdn.net/aitangyong/article/details/54137067

JDK8新特性：lambda入门
http://blog.csdn.net/aitangyong/article/details/54317539

JDK8新特性：使用Method References实现方法复用，简化lambda表达式
http://blog.csdn.net/aitangyong/article/details/54586197

可以使用Stream.sort对集合进行排序，sort有2个重载方法，区别如下。

// Student实现Comparable接口,默认按照id升序排列
public class Student implements Comparable{

	private int id;

	private int age;

	private String name;

	private Address address;

	public Student(int id, int age, String name, Address address) {
		this.id = id;
		this.age = age;
		this.name = name;
		this.address = address;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Address getAddress() {
		return address;
	}

	public void setAddress(Address address) {
		this.address = address;
	}

	@Override
	public String toString() {
		return "Student [id=" + id + ", age=" + age + ", name=" + name + ", address=" + address + "]";
	}

	@Override
	public int compareTo(Student o) {
		return this.id - o.id;
	}

}

stream().sorted()/Comparator.naturalOrder()/Comparator.reverseOrder()，要求元素必须实现Comparable接口。

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class TestComparator {

	public static void main(String[] args) {
		List students = buildStudents();
		
		// 按照默认顺序排序
		List ascList1 = students.stream().sorted().collect(Collectors.toList());
		System.out.println(ascList1);
		
		// 按照自然序排序(其实就是默认顺序)
		List ascList2 = students.stream().sorted(Comparator.naturalOrder()).collect(Collectors.toList());
		System.out.println(ascList2);
		
		// 按照默认顺序的相反顺序排序
		List descList = students.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
		System.out.println(descList);

	}

	private static List buildStudents() {
		List students = new ArrayList<>();
		students.add(new Student(10, 20, "aty", new Address("d")));
		students.add(new Student(1, 22, "qun", new Address("c")));
		students.add(new Student(1, 26, "Zen", new Address("b")));
		students.add(new Student(5, 23, "aty", new Address("a")));
		return students;
	}

}

如果Student没有实现Comparable接口，效果如下：

接下来测试，都不要求Student实现Comparable接口，这里直接给出Student和Address实体类。

public class Student {

	private int id;

	private int age;

	private String name;

	private Address address;

	public Student(int id, int age, String name, Address address) {
		this.id = id;
		this.age = age;
		this.name = name;
		this.address = address;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Address getAddress() {
		return address;
	}

	public void setAddress(Address address) {
		this.address = address;
	}

	@Override
	public String toString() {
		return "Student [id=" + id + ", age=" + age + ", name=" + name + ", address=" + address + "]";
	}

}

public class Address {
	private String address;

	public Address(String address) {
		super();
		this.address = address;
	}

	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}

	@Override
	public String toString() {
		return "Address [address=" + address + "]";
	}
	

}

Comparator.comparing(Function keyExtractor)生成1个Comparator对象，要求keyExtractor.apply()返回值一定要实现Comparable接口。比如下面代码extractIdWay1和extractIdWay2都是等价的，从Student对象中提取id属性，而id是int类型(Integer实现了Comparable) 。

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class TestComparator {

	public static void main(String[] args) {
		List students = buildStudents();

		// 使用lambda表达式创建Function对象
		Function extractIdWay1 = (student) -> student.getId();
		
		// 使用方法引用简化lambda
		Function extractIdWay2 = Student::getId;
		
		// Comparator.comparing(Function keyExtractor)
		Comparator byId = Comparator.comparing(extractIdWay2);
		
		// 升序
		List ascList = students.stream().sorted(byId).collect(Collectors.toList());
		System.out.println(ascList);
		
		// 降序
		List descList = students.stream().sorted(byId.reversed()).collect(Collectors.toList());
		System.out.println(descList);

	}

	private static List buildStudents() {
		List students = new ArrayList<>();
		students.add(new Student(10, 20, "aty", new Address("d")));
		students.add(new Student(1, 22, "qun", new Address("c")));
		students.add(new Student(1, 26, "Zen", new Address("b")));
		students.add(new Student(5, 23, "aty", new Address("a")));
		return students;
	}

}

由于Student.getAddress()返回的对象没有实现Comparable接口，所以不能通过Comparator.comparing()创建一个Comparator对象。

如果我们想安装Address(没有实现Comparable接口)排序怎么办呢？使用另一种形式的comparing方法：

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class TestComparator {

	public static void main(String[] args) {
		List students = buildStudents();

		Comparator
 cmpAddr = Comparator.comparing(Address::getAddress);
		Comparator byAddress = Comparator.comparing(Student::getAddress, cmpAddr);
		List sortedAddressList = students.stream().sorted(byAddress).collect(Collectors.toList());
		System.out.println(sortedAddressList);
	}

	private static List buildStudents() {
		List students = new ArrayList<>();
		students.add(new Student(10, 20, "aty", new Address("d")));
		students.add(new Student(1, 22, "qun", new Address("c")));
		students.add(new Student(1, 26, "Zen", new Address("b")));
		students.add(new Student(5, 23, "aty", new Address("a")));
		return students;
	}

}

这种形式的comparing()接收2个参数，第一个参数提取要排序的key，第二个参数指定排序的Comparator。自己指定比较器，可以灵活定制比较逻辑。比如，我们想实现字符串不区分大小写比较。

//getName()返回String本身已经实现了Comparable,但是我们可以自己传递一个不区分大小写的比较器
Comparator byName = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER);
List sortedNameList = students.stream().sorted(byName).collect(Collectors.toList());
System.out.println(sortedNameList);

comparingDouble()、comparingLong()、comparingInt()不过是comparing()更具体的版本，使用方式相同。

public static void main(String[] args) {
	List students = buildStudents();

	Comparator byAge1 = Comparator.comparingInt(Student::getAge);
	Comparator byAge2 = Comparator.comparing(Student::getAge);
	List sortedAgeList1 = students.stream().sorted(byAge1).collect(Collectors.toList());
	List sortedAgeList2 = students.stream().sorted(byAge2).collect(Collectors.toList());
	System.out.println(sortedAgeList1);
	System.out.println(sortedAgeList2);
}

private static List buildStudents() {
	List students = new ArrayList<>();
	students.add(new Student(10, 20, "aty", new Address("d")));
	students.add(new Student(1, 22, "qun", new Address("c")));
	students.add(new Student(1, 26, "Zen", new Address("b")));
	students.add(new Student(5, 23, "aty", new Address("a")));
	return students;
}

Comparator.nullsFirst()和Comparator.nullsLast()，前面我们创建的Student列表中没有null，如果有null的话，上面的代码都会抛异常。而这2个方法就是用来处理null的，一个认为null比所有非null都小，一个认为比所有都大。

public class TestComparator {

	public static void main(String[] args) {
		List students = buildStudents();
		Comparator nullNotAllowed = Comparator.comparing(Student::getId);
		Comparator allowNullComparator = Comparator.nullsFirst(nullNotAllowed);
		
		// 正常排序
		List result1 = students.stream().sorted(allowNullComparator).collect(Collectors.toList());
		System.out.println(result1);
		
		// 抛异常
		List result2 = students.stream().sorted(nullNotAllowed).collect(Collectors.toList());
		System.out.println(result2);

	}

	private static List buildStudents() {
		List students = new ArrayList<>();
		students.add(new Student(10, 20, "aty", new Address("d")));
		students.add(new Student(1, 22, "qun", new Address("c")));
		students.add(new Student(1, 26, "Zen", new Address("b")));
		students.add(new Student(5, 23, "aty", new Address("a")));
		students.add(null);
		return students;
	}

}

至此Comparator的static方法已经介绍完毕，接下来我们看下它的default方法。

reversed()前面已经介绍了，返回一个新的比较器(排序顺序相反)

thenComparing()系列方法与comparing()使用方法类似

如果我们先按照id排序，id相等的话再按照name排序，那么可以这样写。

public static void main(String[] args) {
	List students = buildStudents();
	
	// id升序
	Comparator byIdASC = Comparator.comparing(Student::getId);
	
	// named不分区大小写降序
	Comparator byNameDESC = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER)
			.reversed();

	// 联合排序
	Comparator finalComparator = byIdASC.thenComparing(byNameDESC);
	
	List result = students.stream().sorted(finalComparator).collect(Collectors.toList());
	System.out.println(result);
}

private static List buildStudents() {
	List students = new ArrayList<>();
	students.add(new Student(10, 20, "aty", new Address("d")));
	students.add(new Student(1, 22, "qun", new Address("c")));
	students.add(new Student(1, 26, "Zen", new Address("b")));
	students.add(new Student(5, 23, "aty", new Address("a")));
	return students;
}