Java8 Stream API 的使用示例及解析

本文章使用jdk8测试 ，并结合使用lambda测试

测试前准备一些测试数据：

class ObjectDemo {
    private Integer id;
    private String name;
    private Integer classNo;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getClassNo() {
        return classNo;
    }

    public void setClassNo(Integer classNo) {
        this.classNo = classNo;
    }

    @Override
    public String toString() {
        return "ObjectDemo{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", classNo=" + classNo +
                '}';
    }
}
 


class ObjectDemoList {

    List<ObjectDemo> getSomeDemo() {
        //手动简历一些测试对象，加入到list中，模拟为从数据库取出来的数据
        List<ObjectDemo> objectDemoList = new ArrayList<ObjectDemo>();
        ObjectDemo objectDemo1 = new ObjectDemo();
        ObjectDemo objectDemo2 = new ObjectDemo();
        ObjectDemo objectDemo3 = new ObjectDemo();
        ObjectDemo objectDemo4 = new ObjectDemo();
        ObjectDemo objectDemo5 = new ObjectDemo();
        ObjectDemo objectDemo6 = new ObjectDemo();
        ObjectDemo objectDemo7 = new ObjectDemo();
        ObjectDemo objectDemo8 = new ObjectDemo();
        ObjectDemo objectDemo9 = new ObjectDemo();
        ObjectDemo objectDemo10 = new ObjectDemo();

        //手动赋值
        objectDemo1.setId(1);
        objectDemo1.setClassNo(1);
        objectDemo1.setName("object01");

        objectDemo2.setId(2);
        objectDemo2.setClassNo(1);
        objectDemo2.setName("object02");

        objectDemo3.setId(3);
        objectDemo3.setClassNo(1);
        objectDemo3.setName("object03");

        objectDemo4.setId(5);
        objectDemo4.setClassNo(2);
        objectDemo4.setName("object05");

        objectDemo5.setId(6);
        objectDemo5.setClassNo(3);
        objectDemo5.setName("object06");

        objectDemo6.setId(7);
        objectDemo6.setClassNo(3);
        objectDemo6.setName("object07");

        objectDemo7.setId(8);
        objectDemo7.setClassNo(3);
        objectDemo7.setName("object08");

        objectDemo8.setId(9);
        objectDemo8.setClassNo(3);
        objectDemo8.setName("object09");

        objectDemo9.setId(10);
        objectDemo9.setClassNo(4);
        objectDemo9.setName("object10");

        objectDemo10.setId(4);
        objectDemo10.setClassNo(4);
        objectDemo10.setName("object04");

        objectDemoList.add(objectDemo1);
        objectDemoList.add(objectDemo2);
        objectDemoList.add(objectDemo3);
        objectDemoList.add(objectDemo4);
        objectDemoList.add(objectDemo5);
        objectDemoList.add(objectDemo6);
        objectDemoList.add(objectDemo7);
        objectDemoList.add(objectDemo8);
        objectDemoList.add(objectDemo9);
        objectDemoList.add(objectDemo10);
        return objectDemoList;
    }
}

View Code

1:collection.stream().forEach() → collection.forEach()

此方法作用为对colleaction流进行遍历--->

 List<ObjectDemo> objectDemoList = new ObjectDemoList().getSomeDemo();

 //foreach test
 objectDemoList.forEach(item -> {
            System.out.println(item.toString());
 });

控制台数据：

2:collection.stream().collect(toList/toSet/toCollection()) → new CollectionType<>(collection)

此方法作用为对colleaction流进行类型转换，当前为list，转化为set示例--->

 List<ObjectDemo> objectDemoList = new ObjectDemoList().getSomeDemo();

 //type change test of jdk8
 Set<ObjectDemo> objectDemoSet = new HashSet<>(objectDemoList);

 objectDemoSet.forEach(item -> {
        System.out.println(item.toString());
 });

 //type change test of jdk7
 Set<ObjectDemo> objectDemoSet3 = objectDemoList.stream().collect(Collectors.toSet());

控制台数据：

3:collection.stream().toArray() → collection.toArray()

此方法作用为对colleaction流进行类型转换为array--->

此处不做控制台演示

//cast as array test
Object[] test = objectDemoList.toArray();
Arrays.stream(test).forEach(item -> {
       System.out.println(item.toString());
});

4:Arrays.asList().stream() → Arrays.stream() or Stream.of()

此方法作用为将array转换为stream流--->

//test of jdk7
Arrays.asList(test).stream();
//test of jdk8
Arrays.stream(test);
Stream.of(test);

Stream.of()底部还是由Arrays.stream()实现的，因此我们正式使用时用Arrays.stream()就行了

 public static<T> Stream<T> of(T... values) {
         return Arrays.stream(values);
 }

5:IntStream.range(0, array.length).mapToObj(idx -> array[idx]) → Arrays.stream(array)

同上，不做多余介绍，但是看看kdk7的写法吧

1 IntStream.range(0, test.length).mapToObj(id -> test[id])
2 .forEach(item -> {
3             System.out.println(item.toString());
4 });

6:IntStream.range(0, list.size()).mapToObj(idx -> list.get(idx)) → list.stream()

list转化为stram流，同上，不做多余介绍

7:Collections.singleton().stream() → Stream.of()

与topic4相同，但介绍一下jdk7的写法

Collections.singleton(objectDemoList).stream();

8:Collections.emptyList().stream() → Stream.empty()

源码部分：

public static<T> Stream<T> empty() {
        return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
}

作用：返回一个空stream

9:stream.filter().findFirst().isPresent() → stream.anyMatch()

判断此stream种是否存在符合条件的elem:

//test 09 of jdk8
System.out.println(objectDemoList.stream().anyMatch(objectDemo -> objectDemo.getId() > 11));
//test 09 of jdk7
System.out.println(objectDemoList.stream().findFirst().isPresent());

stream.anyMatch()源码：

boolean anyMatch(Predicate<? super T> predicate);

Predicate.test（）:

boolean test(T t);

作用：只要符合test，就会返回一个新的predicate；

而我在jdk8中筛选的是 objectDemo.getId() > 11 的objectDemo对象，因此不会有满足条件的objectDemo对象，验证为false;

而条件为 objectDemo.getId() > 3 时 objectDemoLis 中有满足条件 的objectDemo对象，因此不会有满足条件的objectDemo对象，验证为true;

控制台输出：

10:stream.collect(Collectors.counting()) → stream.count()

作用：计算stream内的elem数量（大小）

//test10 od jdk8 and jdk7
System.out.println(objectDemoList.stream().count());
System.out.println(objectDemoList.size());
System.out.println(objectDemoList.stream().collect(Collectors.counting()));

11:stream.collect(Collectors.maxBy()) → stream.max()

作用：返回该stream中的最大elem，前提是根据哪个属性排序，我这里使用id排序

//test11 of jdk8 and jdk7
System.out.println(objectDemoList.stream().max((o1, o2) ->
        new ObjectDemoList().compare(o1, o2)
));

private Integer compare(ObjectDemo objectDemo, ObjectDemo objectDemo2) {
        if (objectDemo.getId() < objectDemo2.getId()) {
            return -1;
        }
        if (objectDemo.getId().equals(objectDemo2.getId())) {
            return 0;
        }
        if (objectDemo.getId() > objectDemo2.getId()) {
            return 1;
        }
        return -1;

}

stream.max()源码：

Optional<T> max(Comparator<? super T> comparator);

Comparator.compare()源码：

* 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);

compare接口方法当o1>o2时，返回负整数，o1.equal(o2)时返回0，o1>02时返回正整数；

因此我编写了一个compare方法来返回具体结果（学而不精，若有好的意见望下方留言，感谢）

控制台输出：

12:stream.collect(Collectors.mapping()) → stream.map().collect()

先讲讲map()方法的作用：个人认为是将stream中的数据做萃取，就比如我们在数据中select * from [table] 时会出现id,name,age字段，但是若我们需要单独取 id 字段时，

就要 select [table].id from [table] ；这里的map是一样的作用，为我们做数据字段筛选提供了可行条件：

13:stream.collect(Collectors.reducing()) → stream.reduce()

唯一：将stream精简到只剩一个

14:stream.collect(Collectors.summingInt()) → stream.mapToInt().sum()

作用：为每个elem或者elem中的某个属性求总和：

类似的方法还有 stream.mapToDouble().sum() 、 stream.mapToLong().sum()

//test13 of jdk8 and jdk7
System.out.println(objectDemoList.stream().map(ObjectDemo::getId).collect(Collectors.toList()));
System.out.println("sum(id)=" + objectDemoList.stream().collect(Collectors.summingInt(ObjectDemo::getId)));//jdk7
System.out.println(objectDemoList.stream().map(ObjectDemo::getClassNo).collect(Collectors.toList()));
System.out.println("sum(classNo)=" + objectDemoList.stream().mapToInt(ObjectDemo::getClassNo).sum());//jdk8

控制台输出：

15:stream.distinct

作用：去重（与sql中的distinct作用一致）

 //去重前
 System.out.println(objectDemoList.stream().map(ObjectDemo::getClassNo).collect(Collectors.toList()));
 //去重后
 System.out.println(objectDemoList.stream().map(ObjectDemo::getClassNo).distinct().collect(Collectors.toList()));

控制台输出：

16:!stream.anyMatch() → stream.noneMatch()

stream.anyMatch()：stream中只要含有满足条件的elem就返回true，否则false

stream.noneMatch():stream中只要没有满足条件的elem就返回true，否则false

System.out.println(objectDemoList.stream().anyMatch(objectDemo -> objectDemo.getId() == 11));
System.out.println(objectDemoList.stream().noneMatch(objectDemo -> objectDemo.getClassNo() == 5));

控制台输出：

17:!stream.anyMatch(x -> !(...)) → stream.allMatch()

作用：全部符合条件才为true,否则为false

18:stream.sorted

作用：将elem根据某一属性进行排序（因为无法直接比较两个对象的大小）

源码中这么写的：传入的对象要有可比较性！！！

* Returns a stream consisting of the elements of this stream, sorted
* according to natural order.  If the elements of this stream are not
* {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
* when the terminal operation is executed.

Stream<T> sorted();

19：stream.filter()

作用：过滤掉你不需要的对象

20：stream.concat()

作用：对两个stream进行拼接并转化为一个stream

List<ObjectDemo> objectDemoList2 = new ArrayList<>();
        ObjectDemo objectDemo13 = new ObjectDemo();
        objectDemo13.setId(5);
        objectDemo13.setName("object13");
        objectDemo13.setClassNo(5);
        objectDemoList2.add(objectDemo13);

Stream streamT = Stream.concat(objectDemoList.stream(), objectDemoList2.stream());
System.out.println(streamT.collect(Collectors.toList()));

21：Collector.grouping()

与数据库中的group by 一样的作用

posted @ 2018-11-15 17:31 竹根七阅读(808) 评论(0) 收藏举报

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竹根七

学而思之，学以致用

Java8 Stream API 的使用示例及解析

1:collection.stream().forEach() → collection.forEach()

2:collection.stream().collect(toList/toSet/toCollection()) → new CollectionType<>(collection)

3:collection.stream().toArray() → collection.toArray()

4:Arrays.asList().stream() → Arrays.stream() or Stream.of()

5:IntStream.range(0, array.length).mapToObj(idx -> array[idx]) → Arrays.stream(array)

6:IntStream.range(0, list.size()).mapToObj(idx -> list.get(idx)) → list.stream()

7:Collections.singleton().stream() → Stream.of()

8:Collections.emptyList().stream() → Stream.empty()

9:stream.filter().findFirst().isPresent() → stream.anyMatch()

10:stream.collect(Collectors.counting()) → stream.count()

11:stream.collect(Collectors.maxBy()) → stream.max()

12:stream.collect(Collectors.mapping()) → stream.map().collect()

13:stream.collect(Collectors.reducing()) → stream.reduce()

14:stream.collect(Collectors.summingInt()) → stream.mapToInt().sum()

15:stream.distinct

16:!stream.anyMatch() → stream.noneMatch()

17:!stream.anyMatch(x -> !(...)) → stream.allMatch()

18:stream.sorted

19：stream.filter()

20：stream.concat()

21：Collector.grouping()

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