java8 lambda 排序_java8新特性（一）使用Lambda表达式集合比较排序

概述

最近开始在手头的项目中用到了一些java8的新特性，用了感觉不错，特分享记录下。

Lambda表达式，是java语言层次上的改变，Lambda允许把函数作为一个方法的参数(函数作为参数传递进方法中)，或者把代码看成数据，一种函数式编程的概念。废话不多说，上代码：

在集合比较排序上的应用

我们先创建一个简单的实体类：

public class Staff {

//ID作为唯一标识

private Long id;

private Integer age;

private String name;

public Staff() {

super();

}

public Staff(Long id, Integer age, String name) {

super();

this.id = id;

this.age = age;

this.name = name;

}

...

@Override

public int hashCode() {

final int prime = 31;

int result = 1;

result = prime * result + ((id == null) ? 0 : id.hashCode());

return result;

}

@Override

public boolean equals(Object obj) {

if (this == obj)

return true;

if (obj == null)

return false;

if (getClass() != obj.getClass())

return false;

Staff other = (Staff) obj;

if (id == null) {

if (other.id != null)

return false;

} else if (!id.equals(other.id))

return false;

return true;

}

@Override

public String toString() {

return "Staff [id=" + id + ", age=" + age + ", name=" + name + "]";

}

在java8以前对于对象排序，需要实现匿名比较器来做比较。

List list2 = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(3L, 36, "李六"));

Collections.sort(list2, new Comparator(){

@Override

public int compare(Staff o1, Staff o2) {

return o1.getAge().compareTo(o2.getAge());

}

});

System.out.println(list2.get(0));

Staff [id=1, age=18, name=张三]

使用Lambda表达式实现

@Test

public void testLambdaSorted() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(3L, 36, "李六"));

staffs.sort((Staff o1, Staff o2) -> o1.getAge().compareTo(o2.getAge()));

assertThat(staffs.get(0), equalTo(new Staff(1L, 18, "张三")));

}

我们可以省略Staff类的指定，编译器能帮自动推定。

staffs.sort((o1, o2) -> o1.getAge().compareTo(o2.getAge()));

@Test

public void testLambdaSorted2() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(3L, 36, "李六"));

staffs.sort((o1, o2) -> o1.getAge().compareTo(o2.getAge()));

assertThat(staffs.get(0), equalTo(new Staff(1L, 18, "张三")));

}

要翻转排序？

@Test

public void testLambdaSortReversed() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(3L, 36, "李六"));

Comparator comparator = (o1, o2) -> o1.getAge().compareTo(o2.getAge());

staffs.sort(comparator.reversed());

assertThat(staffs.get(0), equalTo(new Staff(1L, 18, "张三")));

}

可以直接根据类的getter方法来排序

@Test

public void testLambdaSortedByGetter() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(3L, 36, "李六"));

Collections.sort(staffs, Comparator.comparing(Staff::getAge));

assertThat(staffs.get(0), equalTo(new Staff(1L, 18, "张三")));

}

多条件排序？写一个根据名字排序，同名则再比较年龄的例子：

@Test

public void testLambdaSortedByMultipleCondition() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(4L, 17, "张三"),

new Staff(3L, 36, "李六"));

staffs.sort((lo, ro) -> {

if (lo.getName().equals(ro.getName())) {

return lo.getAge() - ro.getAge();

} else {

return lo.getName().compareTo(ro.getName());

}

});

assertThat(staffs.get(0), equalTo(new Staff(4L, 17, "张三")));

}

使用静态方法排序

1、先在Staff新建一个静态方法：

public static int compareByNameThenAge(Staff lo, Staff ro) {

if (lo.name.equals(ro.name)) {

return lo.age - ro.age;

} else {

return lo.name.compareTo(ro.name);

}

}

2、然后就可以用这个方法进行排序了

staffs.sort(Staff :: compareByNameThenAge);

@Test

public void testLambdaSortedByStaticMethod() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(4L, 17, "张三"),

new Staff(3L, 36, "李六"));

staffs.sort(Staff :: compareByNameThenAge);

assertThat(staffs.get(0), equalTo(new Staff(4L, 17, "张三")));

}

不愿写这个多条件比较方法？可以，JAVA8开始Comparator支持链式排序

staffs.sort(Comparator.comparing(Staff::getName).thenComparing(Staff::getAge));

@Test

public void testLambdaSortedByChainGetter() {

List staffs = Arrays.asList(

new Staff(2L, 25, "王五"),

new Staff(1L, 18, "张三"),

new Staff(4L, 17, "张三"),

new Staff(3L, 36, "李六"));

staffs.sort(Comparator.comparing(Staff::getName).thenComparing(Staff::getAge));

assertThat(staffs.get(0), equalTo(new Staff(4L, 17, "张三")));

}

结语：

这篇文章演示了一个列表排序可以使用java 8 Lambda表达式->的语法糖，以及如何实现更加简洁明了的语义排序代码。