前言
我们前面说了 lambda表达式，这次我们就将下JDK8的另一个新特性，流(Stream)
stream和lambda搭配使用效果更佳，(如果你没有学过lambda表达式，最好先学习下lambda表达式)
看着逼格更高,也更简洁
我们就拿之前的lambda表达式的举例
我们需要找出集合中所有的 男同学 按照年龄从小到大排序 并且打印出来，我们就这样写
studentList.stream()
.filter(student -> "男".equals(student.getSex()))
.sorted((x, y) -> x.getAge()-y.getAge())
.forEach(student -> System.out.println(JSON.toJSONString(student, true)));
项目代码在 github上 。jdk8 stream流的演示，有一个lambda_demo，找到测试的文件夹就行
(如果文章图片失效也可以在上面看,文章更全更新更及时)
image
定义
Stream（流）是一个来自数据源的元素队列并支持聚合操作
- 元素是特定类型的对象，形成一个队列。 Java中的Stream并不会存储元素，而是按需计算。
- 数据源 流的来源。 可以是集合，数组，I/O channel， 产生器generator 等。
- 聚合操作 类似SQL语句一样的操作， 比如filter, map, reduce, find, match, sorted等。
流的处理流程一般是这样
+--------------------+ +------+ +------+ +---+ +-------+
| stream of elements +-----> |filter+-> |sorted+-> |map+-> |collect|
+--------------------+ +------+ +------+ +---+ +-------+
聚合操作按照分法可以分成两种：中间操作，终端操作
- 中间操作
- 无状态操作
- filter、map、peek等
- 有状态操作
- Sorted、distinct、limit等
- 无状态操作
- 终端操作
- 非短路操作
- foreach、count、collect等
- 短路操作
- findFirst、findAny、anyMatch等
- 非短路操作
中间操作
中间操作和很多命令像不像我们 sql 里面的命令，你可以理解为我们的那些限制语句，通过这些手段得到我们想要的一些数据
终端操作
顾名思义，就是指最后的操作。一个流里面进行完终端操作之后就不能再进行其他操作了
无状态操作
就是不需要全部遍历完之后才能得到，比如 我上面的代码，我只看这个元素符不符合，不符合我就不要，不需要遍历完全部元素。与此相对，有状态操作就是需要整个集合遍历完才行，比如我们的 sorted，我不遍历完所有元素，我怎么知道哪一个最大，哪一个最小
短路操作
就是找到一个我们就不往下执行了。与此相反，非短路操作也很好理解
各个方法演示
我的集合中有如下元素
private static List<Student> studentList = new ArrayList<Student>() {
{
add(new Student("张三丰", 20, "男", "体育",
180, 75, "太上老君"));
add(new Student("张无忌", 18, "男", "语文",
178, 73, "文曲星"));
add(new Student("赵敏", 17, "女", "数学",
170, 50, "太白金星"));
add(new Student("金毛狮王", 25, "男", "体育",
176, 80, "太白金星"));
add(new Student("周芷若", 16, "女", "语文",
168, 48, "太上老君"));
add(new Student("张三", 21, "男", "英语",
172, 65, "如来"));
add(new Student("赵勇", 26, "男", "体育",
188, 80, "太上老君"));
}
};
中间操作
无状态操作
filter
filter，就是过滤掉那些不符合你设定的条件的元素
我们看源码
/**
* Returns a stream consisting of the elements of this stream that match
* the given predicate.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
* <a href="package-summary.html#Statelessness">stateless</a>
* predicate to apply to each element to determine if it
* should be included
* @return the new stream
*/
Stream<T> filter(Predicate<? super T> predicate);
// 再看他的参数，记不记得我当初 讲 lambda 时候讲到的 这个
// Predicate 接口 是输入一个类型，返回一个bool值
@FunctionalInterface
public interface Predicate<T> {
/**
* Evaluates this predicate on the given argument.
*
* @param t the input argument
* @return {@code true} if the input argument matches the predicate,
* otherwise {@code false}
*/
boolean test(T t);
}
所以我们使用的时候，返回的是一个bool值，如下所示
Equals 方法返回的是一个bool值
studentList.stream()
.filter(student -> "男".equals(student.getSex()))
.forEach(student -> System.out.println(JSON.toJSONString(student, true)));
结果,我们看到它里面已经过滤掉了女同学
{
"age":20,
"height":180,
"name":"张三丰",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":75
}
{
"age":18,
"height":178,
"name":"张无忌",
"sex":"男",
"subject":"语文",
"teacher":"文曲星",
"weight":73
}
{
"age":25,
"height":176,
"name":"金毛狮王",
"sex":"男",
"subject":"体育",
"teacher":"太白金星",
"weight":80
}
{
"age":21,
"height":172,
"name":"张三",
"sex":"男",
"subject":"英语",
"teacher":"如来",
"weight":65
}
{
"age":26,
"height":188,
"name":"赵勇",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":80
}
map
map的作用是，将一个类型的集合转化为另一个类型的集合
我们来看他的源码
/**
* Returns a stream consisting of the results of applying the given
* function to the elements of this stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
* @return the new stream
*/
<R> Stream<R> map(Function<? super T, ? extends R> mapper);
// 他要传入的是一个 Function 接口，作用是输入一个类型，返回另一个类型
@FunctionalInterface
public interface Function<T, R> {
/**
* Applies this function to the given argument.
*
* @param t the function argument
* @return the function result
*/
R apply(T t);
}
我们用它生成一个 学生选课的集合。我们输入的是一个Student类型的集合，返回的是一个 String类型的集合
@Test
public void mapTest(){
studentList.stream()
.map(student -> student.getSubject())
.forEach(student -> System.out.println(JSON.toJSONString(student, true)));
}
结果如下
"体育"
"语文"
"数学"
"体育"
"语文"
"英语"
"体育"
faltMap
将一个类型的集合转换成另一个类型的流(注意和map区分)
/**
* Returns a stream consisting of the results of replacing each element of
* this stream with the contents of a mapped stream produced by applying
* the provided mapping function to each element. Each mapped stream is
* {@link java.util.stream.BaseStream#close() closed} after its contents
* have been placed into this stream. (If a mapped stream is {@code null}
* an empty stream is used, instead.)
*
* @return the new stream
*/
<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
// 他也是 Function接口，但是另一个参数是继承自 Stream类的
/**
* 将一个类型的集合抓换成流.我们把一个字符串流转换成一个字符流
*/
@Test
public void flatMapTest() {
studentList.stream()
.flatMap(student -> Arrays.stream(student.getName().split("")))
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
peek
peek和foreach很相似，区别是 ，一个是中间操作，一个是终端操作。peek用完之后还能被其他操作进行处理。
/**
* Returns a stream consisting of the elements of this stream, additionally
* performing the provided action on each element as elements are consumed
* from the resulting stream.
*
* @return the new stream
*/
Stream<T> peek(Consumer<? super T> action);
// 我们看到他的函数接口是Consumer，他是输入一个参数，但是不会有返回值
@FunctionalInterface
public interface Consumer<T> {
/**
* Performs this operation on the given argument.
*
* @param t the input argument
*/
void accept(T t);
}
/**
* peek 方法
*/
@Test
public void peekTest() {
studentList.stream()
.peek(student -> System.out.println(student.getName()))
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
张三丰
{
"age":20,
"height":180,
"name":"张三丰",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":75
}
张无忌
{
"age":18,
"height":178,
"name":"张无忌",
"sex":"男",
"subject":"语文",
"teacher":"文曲星",
"weight":73
}
赵敏
{
"age":17,
"height":170,
"name":"赵敏",
"sex":"女",
"subject":"数学",
"teacher":"太白金星",
"weight":50
}
金毛狮王
{
"age":25,
"height":176,
"name":"金毛狮王",
"sex":"男",
"subject":"体育",
"teacher":"太白金星",
"weight":80
}
周芷若
{
"age":16,
"height":168,
"name":"周芷若",
"sex":"女",
"subject":"语文",
"teacher":"太上老君",
"weight":48
}
张三
{
"age":21,
"height":172,
"name":"张三",
"sex":"男",
"subject":"英语",
"teacher":"如来",
"weight":65
}
赵勇
{
"age":26,
"height":188,
"name":"赵勇",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":80
}
我们看到 人名和student类是交替打印的,也就是执行了一次 peek，执行了一次 foreach，那么每次都是这样吗？不是的，我们来看下一节
有状态操作
sorted
sorted是用来排序的
/**
* Returns a stream consisting of the elements of this stream, sorted
* according to the provided {@code Comparator}
* @return the new stream
*/
Stream<T> sorted(Comparator<? super T> comparator);
// 他提供了一个 Comparator接口
@FunctionalInterface
public interface Comparator<T> {
/**
* Compares its two arguments for order. Returns a negative integer,
* zero, or a positive integer as the first argument is less than, equal
* to, or greater than the second.<p>
*/
int compare(T o1, T o2);
/**
* sorted 方法,按照年龄大小排序
*/
@Test
public void sortedTest() {
studentList.stream()
.peek(student -> System.out.println(student.getName()))
.sorted((x,y) -> x.getAge()-y.getAge())
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
张三丰
张无忌
赵敏
金毛狮王
周芷若
张三
赵勇
{
"age":16,
"height":168,
"name":"周芷若",
"sex":"女",
"subject":"语文",
"teacher":"太上老君",
"weight":48
}
{
"age":17,
"height":170,
"name":"赵敏",
"sex":"女",
"subject":"数学",
"teacher":"太白金星",
"weight":50
}
{
"age":18,
"height":178,
"name":"张无忌",
"sex":"男",
"subject":"语文",
"teacher":"文曲星",
"weight":73
}
{
"age":20,
"height":180,
"name":"张三丰",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":75
}
{
"age":21,
"height":172,
"name":"张三",
"sex":"男",
"subject":"英语",
"teacher":"如来",
"weight":65
}
{
"age":25,
"height":176,
"name":"金毛狮王",
"sex":"男",
"subject":"体育",
"teacher":"太白金星",
"weight":80
}
{
"age":26,
"height":188,
"name":"赵勇",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":80
}
我们看到，这次是先打印peek的内容，再打印 foreach的内容，为什么会这样呢？
因为之前我们除只有无状态的方法，他不需要遍历完其他全部的操作，所以他就交替打印了，也符合流的特性。但是我们这一次有了有状态的操作，就只能等到处理完全部元素之后能进行foreach的遍历操作
distinct
去重
/**
* Returns a stream consisting of the distinct elements (according to
* {@link Object#equals(Object)}) of this stream
*
* @return the new stream
*/
Stream<T> distinct();
/**
* distinct 方法,找出所有老师
*/
@Test
public void distinctTest() {
studentList.stream()
.map(student -> student.getTeacher())
.distinct()
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
"太上老君"
"文曲星"
"太白金星"
"如来"
limit
/**
* Returns a stream consisting of the elements of this stream, truncated
* to be no longer than {@code maxSize} in length.
*
* @param maxSize the number of elements the stream should be limited to
* @return the new stream
* @throws IllegalArgumentException if {@code maxSize} is negative
*/
Stream<T> limit(long maxSize);
/**
* limit 方法,只显示前4个
*/
@Test
public void limitTest() {
studentList.stream()
.limit(4)
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
{
"age":20,
"height":180,
"name":"张三丰",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":75
}
{
"age":18,
"height":178,
"name":"张无忌",
"sex":"男",
"subject":"语文",
"teacher":"文曲星",
"weight":73
}
{
"age":17,
"height":170,
"name":"赵敏",
"sex":"女",
"subject":"数学",
"teacher":"太白金星",
"weight":50
}
{
"age":25,
"height":176,
"name":"金毛狮王",
"sex":"男",
"subject":"体育",
"teacher":"太白金星",
"weight":80
}
skip
/**
* Returns a stream consisting of the remaining elements of this stream
* after discarding the first {@code n} elements of the stream.
* If this stream contains fewer than {@code n} elements then an
* empty stream will be returned.
* @throws IllegalArgumentException if {@code n} is negative
*/
Stream<T> skip(long n);
/**
* skip 方法,跳过前4个
*/
@Test
public void skipTest() {
studentList.stream()
.skip(4)
.forEach(stu -> System.out.println(JSON.toJSONString(stu,true)));
}
{
"age":16,
"height":168,
"name":"周芷若",
"sex":"女",
"subject":"语文",
"teacher":"太上老君",
"weight":48
}
{
"age":21,
"height":172,
"name":"张三",
"sex":"男",
"subject":"英语",
"teacher":"如来",
"weight":65
}
{
"age":26,
"height":188,
"name":"赵勇",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":80
}
skip+limit可以完成分页功能
/**
* limit+skip 方法,完成分页功能
*/
@Test
public void skipAndLimitTest() {
studentList.stream()
.skip(1 * 4)
.limit(4)
.forEach(stu -> System.out.println(JSON.toJSONString(stu, true)));
}
终端操作
短路
anyMatch
/**
* Returns whether any elements of this stream match the provided
* predicate. May not evaluate the predicate on all elements if not
* necessary for determining the result. If the stream is empty then
* {@code false} is returned and the predicate is not evaluated.
* @return {@code true} if any elements of the stream match the provided
* predicate, otherwise {@code false}
*/
boolean anyMatch(Predicate<? super T> predicate);
/**
* anyMatch方法,判断是否有一个满足条件
*/
@Test
public void anyMatchTest() {
final boolean b = studentList.stream()
.peek(student -> System.out.println(student))
.allMatch(student -> student.getAge() > 100);
System.out.println(b);
}
为啥说是短路操作。我们测试一下就知道
我们看到，它只打印了一个元素。因为第一个就不满足，他就不会再往下执行了，直接返回false
Student(name=张三丰, age=20, sex=男, subject=体育, height=180, weight=75, teacher=太上老君)
false
allMatch
/**
* Returns whether all elements of this stream match the provided predicate.
* May not evaluate the predicate on all elements if not necessary for
* determining the result. If the stream is empty then {@code true} is
* returned and the predicate is not evaluated
* @return {@code true} if either all elements of the stream match the
* provided predicate or the stream is empty, otherwise {@code false}
*/
// 我们看到他的接口是一个 Predicate ，这个我们之前介绍过，返回的是一个bool值
boolean allMatch(Predicate<? super T> predicate);
/**
* allMatch方法,判断是否全部满足输入的条件
*/
@Test
public void allMatchTest() {
final boolean b = studentList.stream()
.allMatch(student -> student.getAge() < 100);
System.out.println(b);
}
findFirst
/**
* Returns an {@link Optional} describing the first element of this stream,
* or an empty {@code Optional} if the stream is empty. If the stream has
* no encounter order, then any element may be returned.
* @return an {@code Optional} describing the first element of this stream,
* or an empty {@code Optional} if the stream is empty
* @throws NullPointerException if the element selected is null
*/
// 他返回的是一个 Optional对象
Optional<T> findFirst();
/**
* findFirst方法
*/
@Test
public void findFirstTest() {
final Optional<Student> first = studentList.stream()
.peek(student -> System.out.println(student))
.findFirst();
System.out.println(first.get());
}
findAny
/**
* Returns an {@link Optional} describing some element of the stream, or an
* empty {@code Optional} if the stream is empty.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* <p>The behavior of this operation is explicitly nondeterministic; it is
* free to select any element in the stream. This is to allow for maximal
* performance in parallel operations; the cost is that multiple invocations
* on the same source may not return the same result. (If a stable result
* is desired, use {@link #findFirst()} instead.)
*
* @return an {@code Optional} describing some element of this stream, or an
* empty {@code Optional} if the stream is empty
* @throws NullPointerException if the element selected is null
* @see #findFirst()
*/
// 注意 他的解释中说了，findAny可以发挥并行操作的性能，但是如果你在并行的时候想要一个稳定的结果，要用 findFirst。
Optional<T> findAny();
/**
* 因为我们使用的是串行的操作，所以并不影响结果，和findFirst 的结果一样
*/
@Test
public void findAnyTest() {
final Optional<Student> first = studentList.stream()
.peek(student -> System.out.println(student))
.findAny();
System.out.println(first.get());
}
max
/**
* Returns the maximum element of this stream according to the provided
* {@code Comparator}. This is a special case of a
* <a href="package-summary.html#Reduction">reduction</a>.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
* <a href="package-summary.html#Statelessness">stateless</a>
* {@code Comparator} to compare elements of this stream
* @return an {@code Optional} describing the maximum element of this stream,
* or an empty {@code Optional} if the stream is empty
* @throws NullPointerException if the maximum element is null
*/
// 返回的是一个 Optional类
Optional<T> max(Comparator<? super T> comparator);
/**
* max 方法测试，输出最大的年龄。如果从这里点进入就是使用的 intStream接口,和之前的还不一样
*/
@Test
public void maxTest() {
final OptionalInt max = studentList.stream()
.mapToInt(stu -> stu.getAge())
.max();
System.out.println(max.getAsInt());
}
终端非短路
collect
@Test
public void collectTest(){
final List<Student> list = studentList.stream()
.filter(student -> "女".equals(student.getSex()))
.collect(Collectors.toList());
System.out.println(JSON.toJSONString(list,true));
}
[
{
"age":17,
"height":170,
"name":"赵敏",
"sex":"女",
"subject":"数学",
"teacher":"太白金星",
"weight":50
},
{
"age":16,
"height":168,
"name":"周芷若",
"sex":"女",
"subject":"语文",
"teacher":"太上老君",
"weight":48
}
]
groupBy 进行分类
/**
* Returns a {@code Collector} implementing a "group by" operation on
* input elements of type {@code T}, grouping elements according to a
* classification function, and returning the results in a {@code Map}.
*
* <p>The classification function maps elements to some key type {@code K}.
* The collector produces a {@code Map<K, List<T>>} whose keys are the
* values resulting from applying the classification function to the input
* elements, and whose corresponding values are {@code List}s containing the
* input elements which map to the associated key under the classification
* function.
*/
// 可以看到，他的参数也是一个 Function接口，
public static <T, K> Collector<T, ?, Map<K, List<T>>>
groupingBy(Function<? super T, ? extends K> classifier) {
return groupingBy(classifier, toList());
}
@Test
public void collectTest(){
// final List<Student> list = studentList.stream()
// .filter(student -> "女".equals(student.getSex()))
// .collect(Collectors.toList());
// 按照性别分类
final Map<String, List<Student>> list = studentList.stream()
.collect(Collectors.groupingBy(s -> s.getSex()));
System.out.println(JSON.toJSONString(list,true));
}
{
"女":[
{
"age":17,
"height":170,
"name":"赵敏",
"sex":"女",
"subject":"数学",
"teacher":"太白金星",
"weight":50
},
{
"age":16,
"height":168,
"name":"周芷若",
"sex":"女",
"subject":"语文",
"teacher":"太上老君",
"weight":48
}
],
"男":[
{
"age":20,
"height":180,
"name":"张三丰",
"sex":"男",
"subject":"体育",
"teacher":"太上老君",
"weight":75
},
{
"age":18,
"height":178,
"name":"张无忌",
"sex":"男",
"subject":"语文",
"teacher":"文曲星",
"weight":73
},
{
"age":25,
"height":176,
"name":"金毛狮王",
"sex":"男",
"subject":"体育",
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