Java 8的
三大特性：Stream API, Optional API, 跟CompletableFuture稱為Java8三神器。
还会介绍apache commons的validate。
lambda
import java.util.stream.Collectors;
List<String> lines = Arrays.asList("spring", "node", "mkyong");
记得filter后要赋值
List<String> result = lines.stream()
//convert list to stream
.filter(line -> !"mkyong". equals (line))
//filters the line, equals to "mkyong"
.collect(Collectors.toList());
//collect the output and convert streams to a List
result.forEach(System.out::println);
//output : spring node
List<String> result;
result.forEach(x->func(x.getName()));
返回不是特定字符串的结果：
String x = list.stream()
.filter(str->!StringUtils.equals(getSomething(str), "abc"))
.findAny()
.map(v -> getSomething(v))
.orElse(null);
map映射，可以将某一个类型的list转换成另一类型的list
List<Customer> customers = accounts.stream()
.filter(account -> account.getName()!="aaa")
.filter(account -> account.getAccountNumber()=="12")
.map(account ->{
Customer cust = new Customer();
cust.setCustomerId(Integer.parseInt(account.getAccountNumber()));
cust.setAddress(new Address("NEW", account.getName()));
return cust;
})
.collect(Collectors.toList());
字符串是否含有给定数组字符的一个：
Arrays.stream(items).parallel().anyMatch(inputStr::contains);
boolean ans = list.stream().anyMatch(n->n*(n+1))/4==5);
HashMap key排序
Map<String, String> sortedMap =
map.entrySet().stream()
.sorted(Map.Entry.comparingByValue((String s1, String s2)-> s2.length() - s1.length()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
(e1, e2) -> e1, LinkedHashMap::new));
System.out.println("Sorted Map: " + Arrays.toString(sortedMap.entrySet().toArray()));
groupingBy统计数量
List<String> items =
Arrays.asList("apple", "apple", "banana",
"apple", "orange", "banana", "papaya");
Map<String, Long> result =
items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
Optonal
Optional的引入是为了解决null既可作为不存在或空结果的含糊性。有了Optional.isPresent的接口后，就可以让开发者意识到如何只处理空结果的情况，不用在考虑不存在情况。这个例子是找到第一个以L开头的字符串，若找到将其全大写输出。longest是一个Optional类型可以为空结果，若为空结果时，不会打印任何东西。
public void testOptional(){
Stream<String> names = Stream.of("aamurudu", "Okanbi", "Oduduwa");
Optional<String> longest = names
.filter(name -> name.startsWith("L"))
.findFirst();
longest.ifPresent(name -> {
String s = name.toUpperCase();
System.out.println("The longest name is "+ s);
});
}
一般object变Optional:
Optional<Role> roleOpt = Optional.ofNullable(role); role可以为空，至于Optional.of("h")比如不为null。
Future
这个程序分成5个异步任务，等待5个任务（每个任务产生一个随机等待时间）全部完成后再输出完成信息。Future用于异步任务完成后执行主线程，它来保存异步任务结果，Future<Integer>参数类型就是返回类型。线程池设为10，以免线程不收回。tasks.add用了lambda表达式，类似于匿名类，简化程序。
public class AsyncTask {
public void asyncReturns() throws InterruptedException {
long startTime = System.currentTimeMillis();
ExecutorService executor = Executors.newFixedThreadPool(10);
List<Callable<Integer>> tasks = new ArrayList<>();
// Build parallel tasks
for (int i = 0; i < 5; i++) {
tasks.add(() -> getRandomByRange(waitTime));
}
// Execute all calls at the same time
List<Future<Integer>> futures =
executor.invokeAll(tasks);
for (int i = 0; i < futures.size(); i++) {
try {
Future<Integer> future = futures.get(i);
Integer result = future.get();
System.out.println("task " + i + " takes " + result + " millisec");
} catch (ExecutionException e) {
e.printStackTrace();
}
}
executor.shutdown();
long endTime = System.currentTimeMillis();
System.out.println("done in "+(endTime-startTime)+" millisec");
}
public int getRandomByRange(int upper) throws InterruptedException {
int wait = new Random().nextInt(upper) * 1000;
Thread.sleep(wait);
return wait;
}
int waitTime = 5;
}
假设执行没有返回值的异步任务，一定要shutdown，作用是执行提交任务且终止线程池，否则一直等待。Future的意思就是未来会完成的结果。
ExecutorService executor = Executors.newFixedThreadPool(10);
executor.submit(()-> getRandomByRange2());
executor.submit(()-> getRandomByRange2());
executor.shutdown();
Runnable的lambda表达式
原有：
Runnable task1 = new Runnable(){
@Override
public void run(){
System.out.println("Task #1 is running");
}
};
Thread thread1 = new Thread(task1);
thread1.start();
新的：
Runnable task2 = () -> { System.out.println("Task #2 is running"); };
new Thread(task2).start();
可见lambda表达式简化了很多。
lang3
lang3-> notNull: 查某个参数是否null，如果是出现exception：java.lang.NullPointerException: The validated object is null。但参数可以为空。
声明
test(
@Nullable final String abc)
验证
notNull(para, "para must be not null");
isTrue(para>0, "para must be greater than 0")
检查一个Collection类型(ArrayList, HashSet)是否空(null或无元素)
CollectionUtils.
isEmpty(mylist)
StringUtils.isEmpty("")
数组
asList固定大小不支持加删，但支持改。singletonList大小为1，不支持加删改
List<String> aList = Arrays.asList("a", "b");
List<String> bList = Collections.singletonList("a");
首字母大写
StringUtils.capitalized()
不可改变Map,
Google Guava (Google Collections Library)
ImmutableSet<String> COLOR_NAMES =
ImmutableSet.of("Red")
Map<String, String> COLOR_Map =
ImmutableMap.of("1","Red")
类模板
@Override
public boolean equals(Object other)
{
return EqualsBuilder.reflectionEquals(this, other);
}
@Override
public String toString()
{
return ToStringBuilder.reflectionToString(this);
}
findFirst vs findAny
区别只在于多线程情况下，findFirst仍能保证返回data stream的第一个元素。
ref:
lambda
optional
