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/54317539JDK8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<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 + "]";	}	@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<Student> students = buildStudents();				// 按照默认顺序排序		List<Student> 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<Student> 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<Student> descList = students.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());		System.out.println(descList);	}	private static List<Student> buildStudents() {		List<Student> 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<Student> students = buildStudents();		// 使用lambda表达式创建Function对象		Function<Student, Integer> extractIdWay1 = (student) -> student.getId();				// 使用方法引用简化lambda		Function<Student, Integer> extractIdWay2 = Student::getId;				// Comparator.comparing(Function keyExtractor)		Comparator<Student> byId = Comparator.comparing(extractIdWay2);				// 升序		List<Student> ascList = students.stream().sorted(byId).collect(Collectors.toList());		System.out.println(ascList);				// 降序		List<Student> descList = students.stream().sorted(byId.reversed()).collect(Collectors.toList());		System.out.println(descList);	}	private static List<Student> buildStudents() {		List<Student> 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<Student> students = buildStudents();		Comparator<Address> cmpAddr = Comparator.comparing(Address::getAddress);		Comparator<Student> byAddress = Comparator.comparing(Student::getAddress, cmpAddr);		List<Student> sortedAddressList = students.stream().sorted(byAddress).collect(Collectors.toList());		System.out.println(sortedAddressList);	}	private static List<Student> buildStudents() {		List<Student> 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<Student> byName = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER);List<Student> 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<Student> students = buildStudents();	Comparator<Student> byAge1 = Comparator.comparingInt(Student::getAge);	Comparator<Student> byAge2 = Comparator.comparing(Student::getAge);	List<Student> sortedAgeList1 = students.stream().sorted(byAge1).collect(Collectors.toList());	List<Student> sortedAgeList2 = students.stream().sorted(byAge2).collect(Collectors.toList());	System.out.println(sortedAgeList1);	System.out.println(sortedAgeList2);}private static List<Student> buildStudents() {	List<Student> 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<Student> students = buildStudents();		Comparator<Student> nullNotAllowed = Comparator.comparing(Student::getId);		Comparator<Student> allowNullComparator = Comparator.nullsFirst(nullNotAllowed);				// 正常排序		List<Student> result1 = students.stream().sorted(allowNullComparator).collect(Collectors.toList());		System.out.println(result1);				// 抛异常		List<Student> result2 = students.stream().sorted(nullNotAllowed).collect(Collectors.toList());		System.out.println(result2);	}	private static List<Student> buildStudents() {		List<Student> 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<Student> students = buildStudents();		// id升序	Comparator<Student> byIdASC = Comparator.comparing(Student::getId);		// named不分区大小写降序	Comparator<Student> byNameDESC = Comparator.comparing(Student::getName, String.CASE_INSENSITIVE_ORDER)			.reversed();	// 联合排序	Comparator<Student> finalComparator = byIdASC.thenComparing(byNameDESC);		List<Student> result = students.stream().sorted(finalComparator).collect(Collectors.toList());	System.out.println(result);}private static List<Student> buildStudents() {	List<Student> 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;}