@Data @AllArgsConstructor @NoArgsConstructor public class User { private String name; private int age; }
下面的操做都基于这个类来进行操做。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。java
在Java8以前,对集合排序只能建立一个匿名内部类ide
new Comparator<User>() { @Override public int compare(User h1, User h2) { return h1.getName().compareTo(h2.getName()); } }
下面是简单的对users进行排序(按名称正序)函数
@Test public void testSortByName_with_plain_java() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); Collections.sort(users, new Comparator<User>() { public int compare(User h1, User h2) { return h1.getName().compareTo(h2.getName()); } }); Assert.assertThat(users.get(0), equalTo(new User("leyuan", 25))); }
使用JAVA8函数式方式的比较器code
(User h1, User h2) -> h1.getName().compareTo(h2.getName())
下面是使用JAVA8函数式的比较的例子排序
@Test public void testSortByName_with_lambda() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); users.sort((User h1, User h2) -> h1.getName().compareTo(h2.getName())); Assert.assertThat("shequ", equalTo(users.get(1).getName())); }
对于上面的表达式还能够进行简化,JAVA编译器能够根据上下文推测出排序的类型:ip
(h1, h2) -> h1.getName().compareTo(h2.getName())
简化后的比较器是这样的:get
@Test public void testSortByNameSimplify_with_lambda() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); users.sort((h1, h2) -> h1.getName().compareTo(h2.getName())); Assert.assertThat("shequ", equalTo(users.get(1).getName())); }
JAVA8还能够提供使用Lambda表达式的静态类型引用,咱们在User类增长一个静态比较方法,以下:编译器
public static int compareByNameThenAge(User h1, User h2) { if (h1.getName().equals(h2.getName())) { return Integer.compare(h1.getAge(), h2.getAge()); } return h1.getName().compareTo(h2.getName()); }
而后就能够在humans.sort使用这个引用it
@Test public void testSort_with_givenMethodDefinition() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); users.sort(User::compareByNameThenAge); Assert.assertThat("shequ", is(equalTo(users.get(1).getName()))); }
JAVA8已经提供了不少方便的比较器供咱们使用,好比Comparator.comparing方法,因此能够使用Comparator.comparing方法来实现根据User的name进行比较的操做:io
@Test public void testSort_with_givenInstanceMethod() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); Collections.sort(users, Comparator.comparing(User::getName)); Assert.assertThat("shequ", equalTo(users.get(1).getName())); }
JDK8中也提供了一个支持倒序排序的方法方便咱们更快的进行倒序
@Test public void testSort_with_comparatorReverse() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); Comparator<User> comparator = (h1, h2) -> h1.getName().compareTo(h2.getName()); users.sort(comparator.reversed()); Assert.assertThat("shequ", equalTo(users.get(0).getName())); }
Lambda提供了更复杂的表达式,还能够先对name排序再根据age进行排序:
@Test public void testSort_with_multipleComparator() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("shequ", 22), new User("leyuan", 25) ); Comparator<User> comparator = (h1, h2) -> { if (h1.getName().equals(h2.getName())) { return Integer.compare(h1.getAge(), h2.getAge()); } return h1.getName().compareTo(h2.getName()); }; users.sort(comparator.reversed()); Assert.assertThat("shequ", equalTo(users.get(0).getName())); }
Comparator对这种组合的排序有更优雅实现,从JDK8开始,咱们能够使用链式操做进行复合操做来构建更复杂的逻辑:
@Test public void testSort_with_multipleComparator_composition() throws Exception { ArrayList<User> users = Lists.newArrayList( new User("leyuan", 22), new User("leyuan", 25) ); users.sort(Comparator.comparing(User::getName).thenComparing(User::getAge)); Assert.assertThat(users.get(0), equalTo(new User("leyuan", 22))); }