学习链接
Hashmap 底层结构/原理 老话题不再阐述 …
并发hashmap …
Java虚拟机 …
Java内存模型 …
Lambda是一个匿名函数,可以理解为一段可以传递的代码(将代码像数据一样传递);可以写出更简洁、更灵活的代码;作为一种更紧凑的代码风格,是Java语言表达能力得到提升。
@Test
public void test01(){
//匿名内部类
Comparator<Integer> comparator = new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return Integer.compare(o1,o2);
}
@Override
public boolean equals(Object obj) {
return false;
}
};
//调用
TreeSet<Integer> set = new TreeSet<>(comparator);
}
@Test
public void test02(){
// Lambda 表达式
Comparator<Integer> comparator = (a, b) -> Integer.compare(a, b);
TreeSet<Integer> set = new TreeSet<>(comparator);
}
演变过程:
- 垃圾代码 --> 策略模式 --> 匿名内部类 --> Lambda表达式
基础语法:
- 操作符:->
- 左侧:参数列表
- 右侧:执行代码块 / Lambda 体
口诀:
语法格式:
例如 Runnable接口:
public class Test02 {
int num = 10; //jdk 1.7以前 必须final修饰
@Test
public void test01(){
//匿名内部类
new Runnable() {
@Override
public void run() {
//在局部类中引用同级局部变量
//只读
System.out.println("Hello World" + num);
}
};
}
@Test
public void test02(){
//语法糖
Runnable runnable = () -> {
System.out.println("Hello Lambda");
};
}
}
@Test
public void test03(){
Consumer<String> consumer = (a) -> System.out.println(a);
consumer.accept("我觉得还行!");
}
@Test
public void test03(){
Consumer<String> consumer = a -> System.out.println(a);
consumer.accept("我觉得还行!");
}
@Test
public void test04(){
Comparator<Integer> comparator = (a, b) -> {
System.out.println("比较接口");
return Integer.compare(a, b);
};
}
@Test
public void test04(){
Comparator<Integer> comparator = (a, b) -> Integer.compare(a, b);
}
函数式接口:
测试:
@FunctionalInterface
public interface MyFun {
Integer count(Integer a, Integer b);
}
@Test
public void test05(){
MyFun myFun1 = (a, b) -> a + b;
MyFun myFun2 = (a, b) -> a - b;
MyFun myFun3 = (a, b) -> a * b;
MyFun myFun4 = (a, b) -> a / b;
}
public Integer operation(Integer a, Integer b, MyFun myFun){
return myFun.count(a, b);
}
@Test
public void test06(){
Integer result = operation(1, 2, (x, y) -> x + y);
System.out.println(result);
}
**案例一:**调用 Collections.sort() 方法,通过定制排序 比较两个 Employee (先按照年龄比,年龄相同按照姓名比),使用 Lambda 表达式作为参数传递
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Employee {
private Integer id;
private String name;
private Integer age;
private Double salary;
}
List<Employee> emps = Arrays.asList(
new Employee(101, "Z3", 19, 9999.99),
new Employee(102, "L4", 20, 7777.77),
new Employee(103, "W5", 35, 6666.66),
new Employee(104, "Tom", 44, 1111.11),
new Employee(105, "Jerry", 60, 4444.44)
);
@Test
public void test01(){
Collections.sort(emps, (e1, e2) -> {
if (e1.getAge() == e2.getAge()){
return e1.getName().compareTo(e2.getName());
} else {
return Integer.compare(e1.getAge(), e2.getAge());
}
});
for (Employee emp : emps) {
System.out.println(emp);
}
}
**案例二:**声明函数式接口,接口中声明抽象方法,String getValue(String str); 声明类 TestLambda,类中编写方法使用接口作为参数,将一个字符串转换成大写,并作为方法的返回值;再将一个字符串的第二个和第四个索引位置进行截取字串
**案例三:**声明一个带两个泛型的函数式接口,泛型类型为
Java内置四大核心函数式接口:
函数式接口 | 参数类型 | 返回类型 | 用途 |
---|---|---|---|
Consumer 消费型接口 |
T | void | 对类型为T的对象应用操作:void accept(T t) |
Supplier 提供型接口 |
无 | T | 返回类型为T的对象:T get() |
Function 函数型接口 |
T | R | 对类型为T的对象应用操作,并返回结果为R类型的对象:R apply(T t) |
Predicate 断言型接口 |
T | boolean | 确定类型为T的对象是否满足某约束,并返回boolean值:boolean test(T t) |
@Test
public void test01(){
//Consumer
Consumer<Integer> consumer = (x) -> System.out.println("消费型接口" + x);
//test
consumer.accept(100);
}
@Test
public void test02(){
List<Integer> list = new ArrayList<>();
List<Integer> integers = Arrays.asList(1,2,3);
list.addAll(integers);
//Supplier
Supplier<Integer> supplier = () -> (int)(Math.random() * 10);
list.add(supplier.get());
System.out.println(supplier);
for (Integer integer : list) {
System.out.println(integer);
}
}
@Test
public void test03(){
//Function
String oldStr = "abc123456xyz";
Function<String, String> function = (s) -> s.substring(1, s.length()-1);
//test
System.out.println(function.apply(oldStr));
}
@Test
public void test04(){
//Predicate
Integer age = 35;
Predicate<Integer> predicate = (i) -> i >= 35;
if (predicate.test(age)){
System.out.println("你该退休了");
} else {
System.out.println("我觉得还OK啦");
}
}
**定义:**若 Lambda 表达式体中的内容已有方法实现,则我们可以使用“方法引用”
语法格式:
对象::实例方法
@Test
public void test01(){
PrintStream ps = System.out;
Consumer<String> con1 = (s) -> ps.println(s);
con1.accept("aaa");
Consumer<String> con2 = ps::println;
con2.accept("bbb");
}
**注意:**Lambda 表达实体中调用方法的参数列表、返回类型必须和函数式接口中抽象方法保持一致
类::静态方法
@Test
public void test02(){
Comparator<Integer> com1 = (x, y) -> Integer.compare(x, y);
System.out.println(com1.compare(1, 2));
Comparator<Integer> com2 = Integer::compare;
System.out.println(com2.compare(2, 1));
}
类::实例方法
@Test
public void test03(){
BiPredicate<String, String> bp1 = (x, y) -> x.equals(y);
System.out.println(bp1.test("a","b"));
BiPredicate<String, String> bp2 = String::equals;
System.out.println(bp2.test("c","c"));
}
**条件:**Lambda 参数列表中的第一个参数是方法的调用者,第二个参数是方法的参数时,才能使用 ClassName :: Method
格式:
@Test
public void test04(){
Supplier<List> sup1 = () -> new ArrayList();
Supplier<List> sup2 = ArrayList::new;
}
**注意:**需要调用的构造器的参数列表要与函数时接口中抽象方法的参数列表保持一致
语法:
什么是 Stream?
创建流:(的几种方法如下)
/**
* 创建流
*/
@Test
public void test01(){
/**
* 集合流
* - Collection.stream() 穿行流
* - Collection.parallelStream() 并行流
*/
List<String> list = new ArrayList<>();
Stream<String> stream1 = list.stream();
//数组流
//Arrays.stream(array)
String[] strings = new String[10];
Stream<String> stream2 = Arrays.stream(strings);
//Stream 静态方法
//Stream.of(...)
Stream<Integer> stream3 = Stream.of(1, 2, 3);
//无限流
//迭代
Stream<Integer> stream4 = Stream.iterate(0, (i) -> ++i+i++);
stream4.forEach(System.out::println);
//生成
Stream.generate(() -> Math.random())
.limit(5)
.forEach(System.out::println);
}
中间操作:
List<Employee> emps = Arrays.asList(
new Employee(101, "Z3", 19, 9999.99),
new Employee(102, "L4", 20, 7777.77),
new Employee(103, "W5", 35, 6666.66),
new Employee(104, "Tom", 44, 1111.11),
new Employee(105, "Jerry", 60, 4444.44)
);
@Test
public void test01(){
emps.stream()
.filter((x) -> x.getAge() > 35)
.limit(3) //短路?达到满足不再内部迭代
.distinct()
.skip(1)
.forEach(System.out::println);
}
map:
@Test
public void test02(){
List<String> list = Arrays.asList("a", "b", "c");
list.stream()
.map((str) -> str.toUpperCase())
.forEach(System.out::println);
}
flatMap:
public Stream<Character> filterCharacter(String str){
List<Character> list = new ArrayList<>();
for (char c : str.toCharArray()) {
list.add(c);
}
return list.stream();
}
@Test
public void test03(){
List<String> list = Arrays.asList("a", "b", "c");
Test02 test02 = new Test02();
list.stream()
.flatMap(test02::filterCharacter)
.forEach(System.out::println);
}
Comparable:自然排序
@Test
public void test04(){
List<Integer> list = Arrays.asList(1,2,3,4,5);
list.stream()
.sorted() //comparaTo()
.forEach(System.out::println);
}
Comparator:定制排序
@Test
public void test05(){
emps.stream()
.sorted((e1, e2) -> { //compara()
if (e1.getAge().equals(e2.getAge())){
return e1.getName().compareTo(e2.getName());
} else {
return e1.getAge().compareTo(e2.getAge());
}
})
.forEach(System.out::println);
}
终止操作:
public enum Status {
FREE, BUSY, VOCATION;
}
@Test
public void test01(){
List<Status> list = Arrays.asList(Status.FREE, Status.BUSY, Status.VOCATION);
boolean flag1 = list.stream()
.allMatch((s) -> s.equals(Status.BUSY));
System.out.println(flag1);
boolean flag2 = list.stream()
.anyMatch((s) -> s.equals(Status.BUSY));
System.out.println(flag2);
boolean flag3 = list.stream()
.noneMatch((s) -> s.equals(Status.BUSY));
System.out.println(flag3);
// 避免空指针异常
Optional<Status> op1 = list.stream()
.findFirst();
// 如果Optional为空 找一个替代的对象
Status s1 = op1.orElse(Status.BUSY);
System.out.println(s1);
Optional<Status> op2 = list.stream()
.findAny();
System.out.println(op2);
long count = list.stream()
.count();
System.out.println(count);
}
reduce:
/**
* Java:
* - reduce:需提供默认值(初始值)
* Kotlin:
* - fold:不需要默认值(初始值)
* - reduce:需提供默认值(初始值)
*/
@Test
public void test01(){
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
Integer integer = list.stream()
.reduce(0, (x, y) -> x + y);
System.out.println(integer);
}
collect:
List<Employee> emps = Arrays.asList(
new Employee(101, "Z3", 19, 9999.99),
new Employee(102, "L4", 20, 7777.77),
new Employee(103, "W5", 35, 6666.66),
new Employee(104, "Tom", 44, 1111.11),
new Employee(105, "Jerry", 60, 4444.44)
);
@Test
public void test02(){
//放入List
List<String> list = emps.stream()
.map(Employee::getName)
.collect(Collectors.toList());
list.forEach(System.out::println);
//放入Set
Set<String> set = emps.stream()
.map(Employee::getName)
.collect(Collectors.toSet());
set.forEach(System.out::println);
//放入LinkedHashSet
LinkedHashSet<String> linkedHashSet = emps.stream()
.map(Employee::getName)
.collect(Collectors.toCollection(LinkedHashSet::new));
linkedHashSet.forEach(System.out::println);
}
@Test
public void test03(){
//总数
Long count = emps.stream()
.collect(Collectors.counting());
System.out.println(count);
//平均值
Double avg = emps.stream()
.collect(Collectors.averagingDouble(Employee::getSalary));
System.out.println(avg);
//总和
Double sum = emps.stream()
.collect(Collectors.summingDouble(Employee::getSalary));
System.out.println(sum);
//最大值
Optional<Employee> max = emps.stream()
.collect(Collectors.maxBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())));
System.out.println(max.get());
//最小值
Optional<Double> min = emps.stream()
.map(Employee::getSalary)
.collect(Collectors.minBy(Double::compare));
System.out.println(min.get());
}
@Test
public void test04(){
//分组
Map<Integer, List<Employee>> map = emps.stream()
.collect(Collectors.groupingBy(Employee::getId));
System.out.println(map);
//多级分组
Map<Integer, Map<String, List<Employee>>> mapMap = emps.stream()
.collect(Collectors.groupingBy(Employee::getId, Collectors.groupingBy((e) -> {
if (e.getAge() > 35) {
return "开除";
} else {
return "继续加班";
}
})));
System.out.println(mapMap);
//分区
Map<Boolean, List<Employee>> listMap = emps.stream()
.collect(Collectors.partitioningBy((e) -> e.getSalary() > 4321));
System.out.println(listMap);
}
@Test
public void test05(){
//总结
DoubleSummaryStatistics dss = emps.stream()
.collect(Collectors.summarizingDouble(Employee::getSalary));
System.out.println(dss.getMax());
System.out.println(dss.getMin());
System.out.println(dss.getSum());
System.out.println(dss.getCount());
System.out.println(dss.getAverage());
//连接
String str = emps.stream()
.map(Employee::getName)
.collect(Collectors.joining("-")); //可传入分隔符
System.out.println(str);
}
**案例一:**给定一个数字列表,如何返回一个由每个数的平方构成的列表呢?(如:给定【1,2,3,4,5】,返回【1,4,9,16,25】)
@Test
public void test01(){
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
list.stream()
.map((x) -> x * x)
.forEach(System.out::println);
}
**案例二:**怎样使用 map 和 reduce 数一数流中有多少个 Employee 呢?
List<Employee> emps = Arrays.asList(
new Employee(101, "Z3", 19, 9999.99),
new Employee(102, "L4", 20, 7777.77),
new Employee(103, "W5", 35, 6666.66),
new Employee(104, "Tom", 44, 1111.11),
new Employee(105, "Jerry", 60, 4444.44)
);
@Test
public void test02(){
Optional<Integer> result = emps.stream()
.map((e) -> 1)
.reduce(Integer::sum);
System.out.println(result.get());
Fork / Join 框架:
Fork / Join 框架与传统线程池的区别:
public class ForkJoinCalculate extends RecursiveTask<Long> {
private static final long serialVersionUID = 1234567890L;
private long start;
private long end;
private static final long THRESHPLD = 10000;
public ForkJoinCalculate(long start, long end) {
this.start = start;
this.end = end;
}
@Override
protected Long compute() {
long length = end - start;
if (length <= THRESHPLD) {
long sum = 0;
for (long i = start; i <= end; i++) {
sum += i;
}
} else {
long middle = (start + end) / 2;
ForkJoinCalculate left = new ForkJoinCalculate(start, end);
left.fork(); //拆分子任务 压入线程队列
ForkJoinCalculate right = new ForkJoinCalculate(middle + 1, end);
right.fork();
return left.join() + right.join();
}
return null;
}
}
public class TestForkJoin {
/**
* ForkJoin 框架
*/
@Test
public void test01(){
Instant start = Instant.now();
ForkJoinPool pool = new ForkJoinPool();
ForkJoinCalculate task = new ForkJoinCalculate(0, 100000000L);
Long sum = pool.invoke(task);
System.out.println(sum);
Instant end = Instant.now();
System.out.println(Duration.between(start, end).getNano());
}
/**
* 普通 for循环
*/
@Test
public void test02(){
Instant start = Instant.now();
Long sum = 0L;
for (long i = 0; i < 100000000L; i++) {
sum += i;
}
Instant end = Instant.now();
System.out.println(Duration.between(start, end).getNano());
}
}
Java 8 并行流 / 串行流:
@Test
public void test03(){
//串行流(单线程):切换为并行流 parallel()
//并行流:切换为串行流 sequential()
LongStream.rangeClosed(0, 100000000L)
.parallel() //底层:ForkJoin
.reduce(0, Long::sum);
}
**定义:**Optional 类 (java.util.Optional) 是一个容器类,代表一个值存在或不存在,原来用 null 表示一个值不存在,现在用 Optional 可以更好的表达这个概念;并且可以避免空指针异常
常用方法:
Optional.of(T t):
@Test
public void test01(){
Optional<Employee> op = Optional.of(new Employee());
Employee employee = op.get();
}
Optional.empty(T t):
@Test
public void test02(){
Optional<Employee> op = Optional.empty();
Employee employee = op.get();
}
Optional.ofNullable(T t):
@Test
public void test03(){
Optional<Employee> op = Optional.ofNullable(new Employee());
Employee employee = op.get();
}
isPresent():
@Test
public void test03(){
Optional<Employee> op = Optional.ofNullable(new Employee());
if (op.isPresent()) {
Employee employee = op.get();
}
}
不再一一例举…
public interface MyFun {
default String getName(){
return "libo";
}
default Integer getAge(){
return 22;
}
}
类优先原则:
public interface MyFun {
static void getAddr(){
System.out.println("addr");
}
static String Hello(){
return "Hello World";
}
}
传统的日期格式化:
@Test
public void test01(){
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Callable<Date> task = () -> sdf.parse("20200517");
ExecutorService pool = Executors.newFixedThreadPool(10);
ArrayList<Future<Date>> result = new ArrayList<>();
for (int i = 0; i < 10; i++) {
result.add(pool.submit(task));
}
for (Future<Date> future : result) {
try {
System.out.println(future.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
pool.shutdown();
}
加锁:
public class DateFormatThreadLocal {
private static final ThreadLocal<DateFormat> df = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
public static Date convert(String source) throws ParseException{
return df.get().parse(source);
}
}
@Test
public void test02(){
Callable<Date> task = () -> DateFormatThreadLocal.convert("20200517");
ExecutorService pool = Executors.newFixedThreadPool(10);
ArrayList<Future<Date>> result = new ArrayList<>();
for (int i = 0; i < 10; i++) {
result.add(pool.submit(task));
}
for (Future<Date> future : result) {
try {
System.out.println(future.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
pool.shutdown();
}
DateTimeFormatter:
@Test
public void test03(){
DateTimeFormatter dtf = DateTimeFormatter.ISO_LOCAL_DATE;
Callable<LocalDate> task = () -> LocalDate.parse("20200517",dtf);
ExecutorService pool = Executors.newFixedThreadPool(10);
ArrayList<Future<LocalDate>> result = new ArrayList<>();
for (int i = 0; i < 10; i++) {
result.add(pool.submit(task));
}
for (Future<LocalDate> future : result) {
try {
System.out.println(future.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
pool.shutdown();
}
ISO 标准:
常用方法:
方法名 | 返回值类型 | 解释 |
---|---|---|
now( ) | static LocalDateTime | 从默认时区的系统时钟获取当前日期 |
of(int year, int month, int dayOfMonth, int hour, int minute, int second) | static LocalDateTime | 从年,月,日,小时,分钟和秒获得 LocalDateTime的实例,将纳秒设置为零 |
plus(long amountToAdd, TemporalUnit unit) | LocalDateTime | 返回此日期时间的副本,并添加指定的数量 |
get(TemporalField field) | int | 从此日期时间获取指定字段的值为 int |
@Test:
@Test
public void test01(){
//获取当前时间日期 now
LocalDateTime ldt1 = LocalDateTime.now();
System.out.println(ldt1);
//指定时间日期 of
LocalDateTime ldt2 = LocalDateTime.of(2020, 05, 17, 16, 24, 33);
System.out.println(ldt2);
//加 plus
LocalDateTime ldt3 = ldt2.plusYears(2);
System.out.println(ldt3);
//减 minus
LocalDateTime ldt4 = ldt2.minusMonths(3);
System.out.println(ldt4);
//获取指定的你年月日时分秒... get
System.out.println(ldt2.getDayOfYear());
System.out.println(ldt2.getHour());
System.out.println(ldt2.getSecond());
}
LocalDate / LocalTime 不再一一例举…
Instant:以 Unix 元年 1970-01-01 00:00:00 到某个时间之间的毫秒值
@Test:
@Test
public void test02(){
// 默认获取 UTC 时区 (UTC:世界协调时间)
Instant ins1 = Instant.now();
System.out.println(ins1);
//带偏移量的时间日期 (如:UTC + 8)
OffsetDateTime odt1 = ins1.atOffset(ZoneOffset.ofHours(8));
System.out.println(odt1);
//转换成对应的毫秒值
long milli1 = ins1.toEpochMilli();
System.out.println(milli1);
//构建时间戳
Instant ins2 = Instant.ofEpochSecond(60);
System.out.println(ins2);
}
@Test:
@Test
public void test03(){
//计算两个时间之间的间隔 between
Instant ins1 = Instant.now();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
Instant ins2 = Instant.now();
Duration dura1 = Duration.between(ins1, ins2);
System.out.println(dura1.getSeconds());
System.out.println(dura1.toMillis());
}
@Test
public void test04(){
LocalDate ld1 = LocalDate.of(2016, 9, 1);
LocalDate ld2 = LocalDate.now();
Period period = Period.between(ld1, ld2); // ISO 标准
System.out.println(period.getYears());
System.out.println(period.toTotalMonths());
}
操纵日期:
@Test:
@Test
public void test01(){
//TemporalAdjusters:时间校正器
LocalDateTime ldt1 = LocalDateTime.now();
System.out.println(ldt1);
//指定日期时间中的 年 月 日 ...
LocalDateTime ldt2 = ldt1.withDayOfMonth(10);
System.out.println(ldt2);
//指定时间校正器
LocalDateTime ldt3 = ldt1.with(TemporalAdjusters.next(DayOfWeek.SUNDAY));
System.out.println(ldt3);
//自定义时间校正器
LocalDateTime ldt5 = ldt1.with((ta) -> {
LocalDateTime ldt4 = (LocalDateTime) ta;
DayOfWeek dow1 = ldt4.getDayOfWeek();
if (dow1.equals(DayOfWeek.FRIDAY)) {
return ldt4.plusDays(3);
} else if (dow1.equals(DayOfWeek.SATURDAY)) {
return ldt4.plusDays(2);
} else {
return ldt4.plusDays(1);
}
});
System.out.println(ldt5);
}
@Test
public void test01(){
//默认格式化
DateTimeFormatter dtf1 = DateTimeFormatter.ISO_DATE_TIME;
LocalDateTime ldt1 = LocalDateTime.now();
String str1 = ldt1.format(dtf1);
System.out.println(str1);
//自定义格式化 ofPattern
DateTimeFormatter dtf2 = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime ldt2 = LocalDateTime.now();
String str2 = ldt2.format(dtf2);
System.out.println(str2);
//解析
LocalDateTime newDate = ldt1.parse(str1, dtf1);
System.out.println(newDate);
}
@Test:
@Test
public void test02(){
//查看支持的时区
Set<String> set = ZoneId.getAvailableZoneIds();
set.forEach(System.out::println);
//指定时区
LocalDateTime ldt1 = LocalDateTime.now(ZoneId.of("Europe/Tallinn"));
System.out.println(ldt1);
//在已构建好的日期时间上指定时区
LocalDateTime ldt2 = LocalDateTime.now(ZoneId.of("Europe/Tallinn"));
ZonedDateTime zdt1 = ldt2.atZone(ZoneId.of("Europe/Tallinn"));
System.out.println(zdt1);
}
一些转换:
@Test
public void test03(){
// Date 转 LocalDateTime
Date date = new Date();
Instant instant = date.toInstant();
ZoneId zoneId = ZoneId.systemDefault();
LocalDateTime localDateTime = instant.atZone(zoneId).toLocalDateTime();
// LocalDateTime 转 Date
LocalDateTime localDateTime = LocalDateTime.now();
ZoneId zoneId = ZoneId.systemDefault();
ZonedDateTime zdt = localDateTime.atZone(zoneId);
Date date = Date.from(zdt.toInstant());
// 原则:利用 时间戳Instant
}
定义注解:
@Repeatable(MyAnnotations.class) //指定容器类
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
String value() default "Java 8";
}
定义容器:
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotations {
MyAnnotation[] value();
}
@Test:
public class Test01 {
//重复注解
@Test
@MyAnnotation("Hello")
@MyAnnotation("World")
public void test01() throws NoSuchMethodException {
Class<Test01> clazz = Test01.class;
Method test01 = clazz.getMethod("test01");
MyAnnotation[] mas = test01.getAnnotationsByType(MyAnnotation.class);
for (MyAnnotation ma : mas) {
System.out.println(ma.value());
}
}
}
Java 8 新增注解:新增ElementType.TYPE_USE 和ElementType.TYPE_PARAMETER(在Target上)