@Data
@AllArgsConstructor
@NoArgsConstructor
public class Human {
private String name;
private int age;
}
下面的操作都基于这个类来进行操作。这里面使用了Lombok类库,它用注解的方式实现了基本的get和set等方法,让代码看起来更加的优雅。
在Java8之前,对集合排序只能创建一个匿名内部类
new Comparator() {
@Override
public int compare(Human h1, Human h2) {
return h1.getName().compareTo(h2.getName());
}
}
下面是简单的对Humans进行排序(按名称正序)
@Test
public void testSortByName_with_plain_java() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
Collections.sort(humans, new Comparator() {
public int compare(Human h1, Human h2) {
return h1.getName().compareTo(h2.getName());
}
});
Assert.assertThat(humans.get(0), equalTo(new Human("li", 25)));
}
使用JAVA8函数式方式的比较器
(Human h1, Human h2) -> h1.getName().compareTo(h2.getName())
下面是使用JAVA8函数式的比较的例子
@Test
public void testSortByName_with_lambda() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
humans.sort((Human h1, Human h2) -> h1.getName().compareTo(h2.getName()));
Assert.assertThat("tomy", equalTo(humans.get(1).getName()));
}
对于上面的表达式还可以进行简化,JAVA编译器可以根据上下文推测出排序的类型:
(h1, h2) -> h1.getName().compareTo(h2.getName())
简化后的比较器是这样的:
@Test
public void testSortByNameSimplify_with_lambda() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
humans.sort((h1, h2) -> h1.getName().compareTo(h2.getName()));
Assert.assertThat("tomy", equalTo(humans.get(1).getName()));
}
JAVA8还可以提供使用Lambda表达式的静态类型引用,我们在Human类增加一个静态比较方法,如下:
public static int compareByNameThenAge(Human h1, Human h2) {
if (h1.getName().equals(h2.getName())) {
return Integer.compare(h1.getAge(), h2.getAge());
}
return h1.getName().compareTo(h2.getName());
}
然后就可以在humans.sort使用这个引用
@Test
public void testSort_with_givenMethodDefinition() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
humans.sort(Human::compareByNameThenAge);
Assert.assertThat("tomy", is(equalTo(humans.get(1).getName())));
}
JAVA8已经提供了很多方便的比较器供我们使用,比如Comparator.comparing方法,所以可以使用Comparator.comparing方法来实现根据Human的name进行比较的操作:
@Test
public void testSort_with_givenInstanceMethod() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
Collections.sort(humans, Comparator.comparing(Human::getName));
Assert.assertThat("tomy", equalTo(humans.get(1).getName()));
}
JDK8中也提供了一个支持倒序排序的方法方便我们更快的进行倒序
@Test
public void testSort_with_comparatorReverse() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
Comparator comparator = (h1, h2) -> h1.getName().compareTo(h2.getName());
humans.sort(comparator.reversed());
Assert.assertThat("tomy", equalTo(humans.get(0).getName()));
}
Lambda提供了更复杂的表达式,还可以先对name排序再根据age进行排序:
@Test
public void testSort_with_multipleComparator() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("li", 25)
);
Comparator comparator = (h1, h2) -> {
if (h1.getName().equals(h2.getName())) {
return Integer.compare(h1.getAge(), h2.getAge());
}
return h1.getName().compareTo(h2.getName());
};
humans.sort(comparator.reversed());
Assert.assertThat("tomy", equalTo(humans.get(0).getName()));
}
Comparator对这种组合的排序有更优雅实现,从JDK8开始,我们可以使用链式操作进行复合操作来构建更复杂的逻辑:
@Test
public void testSort_with_multipleComparator_composition() throws Exception {
ArrayList humans = Lists.newArrayList(
new Human("tomy", 22),
new Human("tomy", 25)
);
humans.sort(Comparator.comparing(Human::getName).thenComparing(Human::getAge));
Assert.assertThat(humans.get(0), equalTo(new Human("tomy", 22)));
}
JDK8真的是一个非常值得我们学习的版本,它提供了Lambda表达式,带来了函数式编程的理念,让JAVA代码更优雅。