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<Staff> 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<Staff>(){
@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<Staff> 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<Staff> 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<Staff> staffs = Arrays.asList(
new Staff(2L, 25, "王五"),
new Staff(1L, 18, "张三"),
new Staff(3L, 36, "李六"));
Comparator<Staff> 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<Staff> 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<Staff> 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<Staff> 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<Staff> 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。
