Stream flow

The previous article talked about a new feature of Java 8: Lambda expression. If you can use it skillfully in business, you can save money. There is a lot of code and it looks a lot neater. Then this article will introduce another new feature: Stream stream, don’t read it wrong! ! ! Not for playing games steam! !

1. What is a Stream:

Stream is a new concept proposed by Java 8. It is not an input and output Stream (it is not the same as the IO stream. Any relationship ha), but a tool that uses functional programming to operate on collection classes. In short, it is an operation of processing collection data in an internal iteration method. Internal iteration can give more control to the collection class. The functions of Stream and Iterator are similar, except that Iterator is an operation that processes collection data in the form of external iteration.

Of course Stream also has its own characteristics:

1. It is not a data structure and does not store data. It just defines a set of operations on the original data set

2. These operations are lazy, that is, whenever an element in the stream is accessed, this series of operations will be performed on this element

3. Because the data is not saved, each Stream Streams can only be used once.

Implementation diagram of Stream stream:

2. Create stream:

If you want to use Stream stream to To operate a collection, you need to convert the array or collection into a Stream first before you can operate

Official document of Stream:

https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/stream/Stream.html

Let’s look at four first Method:

1.filter: Implement conditional filtering through lambda expressions

2.limit: Intercept the stream, intercept a section of the stream

3.skip: Skip Overflow

4.distinct: Remove duplicates

Create Stream:

public class Test {
    public static void main(String[] args) {
        String [] arr = {"東","南","西","北"};
        //將數(shù)組轉(zhuǎn)換成Stream
        Stream<String> stream = Arrays.stream(arr);
        stream = Stream.of(arr);
        stream = Stream.of("東","南","西","北");
        //將list集合轉(zhuǎn)換成stream
        List<String> list = Arrays.asList("aa","cc","bb","aa","dd");
        stream = list.stream();
        //排序、去重、遍歷
        list.stream().sorted().distinct().forEach(System.out::println);
        //用過Stream流操作完集合后還可以再轉(zhuǎn)換成一個新的集合
        List<String> newList = list.stream().sorted().distinct().collect(Collectors.toList());
        System.out.println(newList.toString());
    }
}

Output:

//The result after traversing and deduplicating:
aa
bb
cc
dd
//After using the Stream stream to operate the collection, it can be converted into a new one Collection
[aa, bb, cc, dd]

Operations of four methods: Person class:

Code comparison of this class Many, so don’t write the get/set method in it. Don’t forget it when you use it! !

public class Person {
    private String  name;
    private Integer age;
    private  String country;
    private  char sex;
    @Override
    public String toString() {
        return "信息表：{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", country='" + country + '\'' +
                ", sex=" + sex +
                '}';
    }
//這里節(jié)省的get/set代碼
//重寫toString（） 和 equals 和 hashcode 方法
    @Override
    public boolean equals(Object o){
        if(this == o)
            return true;
        if(o == null || getClass() != o.getClass())
            return false;
        Person person = (Person) o;
        if(country != null){
            if(this.country.equals(person.country)){
                return  true;
            }else{
                return false;
            }
        }
        return false;
    }
    @Override
    public int hashCode(){
        return Objects.hash(country);
    }
}

Test class:

Combined with lambda expression to write

public class Test {
    public static void main(String[] args) {
        List<Person> perosnList = new ArrayList<>();
        perosnList.add(new Person("王一", 30, "中國", &#39;M&#39;));
        perosnList.add(new Person("張三", 19, "美國", &#39;F&#39;));
        perosnList.add(new Person("李四", 29, "日本", &#39;F&#39;));
        perosnList.add(new Person("小美", 74, "英國", &#39;M&#39;));
        perosnList.add(new Person("熊二", 15, "意大利", &#39;F&#39;));
        perosnList.add(new Person("熊大", 66, "韓國", &#39;F&#39;));
        //返回年齡大于20歲的學(xué)生集合
        System.out.println("返回年齡大于20歲的學(xué)生集合");
        perosnList.stream().filter(p -> p.getAge() > 20).forEach(System.out::println);
        //返回年齡大于50歲的學(xué)生集合
        System.out.println("返回年齡大于50歲的集合");
        List<Person> list = perosnList.stream().filter(p -> p.getAge() > 50).collect(Collectors.toList());
        System.out.println(list);
        //返回年齡大于20歲的中國學(xué)生
        System.out.println("返回年齡大于20歲的中國人");
        perosnList.stream().filter(p -> p.getAge() > 20).filter(p -> p.getCountry().equals("韓國")).forEach(System.out::println);
        //年齡大于20  中國  性別M
        System.out.println("返回年齡大于20  中國  性別M");
        perosnList.stream().filter(p -> p.getAge() > 20 && p.getCountry().equals("中國") && p.getSex() == &#39;M&#39;).forEach(System.out::println);
    }
}

Look at the result:

Return the collection of students older than 20 years old
Information table: {name='Wang Yi', age=30, country='China', sex=M}
Information table: {name='李思', age=29, country='Japan', sex=F}
Information table: {name='Xiaomei', age=74, country='UK', sex=M}
Information table: {name='Xiong Da', age=66, country='Korea', sex=F}
Return the collection of people older than 50 years old
[Information table: {name='小Beauty', age=74, country='UK', sex=M}, information table: {name='Xiong Da', age=66, country='South Korea', sex=F}]
Return age greater than 20-year-old Chinese
Information table: {name='Xiong Da', age=66, country='South Korea', sex=F}
Return age greater than 20 China GenderM
Information table: { name='Wang Yi', age=30, country='China', sex=M}

Summary:

Using Stream can be easily operated Arrays or collections can be combined with Lambda expressions to make an expression neat and clear. In fact, since it is a new feature exited by Java, it must be useful.

3. Stream map mapping stream

public class Test {
    public static void main(String[] args) {
        //map的作用是迭代取到每個list元素，再通過map里面的函數(shù)進(jìn)行相應(yīng)的操作
        List<String> list1 = Arrays.asList("a","bb","ccc","dddd");
        //通過map取到每個集合元素的長度并返回
        Stream<Integer> stream = list1.stream().map(p->p.length());
        stream.forEach(System.out::println);
        System.out.println("----------------");
        List<String> userList = new ArrayList<>();
        userList.add("周杰倫.tom");
        userList.add("尼古拉斯.趙四");
        userList.add("牛頓.巴基斯");
        userList.add("趙少華.思密達(dá)");
        List<String> uList = userList.stream().map(p->p.substring(p.indexOf(".")+1,
                p.length())).collect(Collectors.toList());
        System.out.println(uList.toString());
    }
}

Output:

1
2
3
4
----------------
[tom, Zhao Si, Bakis, Smecta]

4. Stream search and There is also a

method in the matching Stream:

Returns whether any element in this stream matches the provided word

Demo:

public class Test {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("周杰倫","王力宏","孫燕姿","林俊杰");
        boolean flag1 = list.stream().anyMatch(ele->ele.contains("燕"));
        System.out.println("有沒有名字包含燕的同學(xué)："+flag1);
        //判斷開頭:
        boolean flag2 = list.stream().anyMatch(ele->ele.startsWith("王"));
        System.out.println("有沒有名字開頭是王的同學(xué)："+flag2);
        //判斷結(jié)尾:
        boolean flag3 = list.stream().anyMatch(ele->ele.endsWith("杰"));
        System.out.println("有沒有名字結(jié)尾是杰的同學(xué)："+flag3);
        // anyMatch是匹配所有的，要滿足條件
        boolean flag4 = list.stream().anyMatch(ele->ele.length()>2);
        System.out.println("所有同學(xué)的名字都是兩個字以上的嗎"+flag4);
        boolean flag5 = list.stream().anyMatch(ele->ele.startsWith("王"));
        System.out.println("所有同學(xué)的名字都有王嗎？"+flag5);
        //noneMatch
        boolean flag6 = list.stream().noneMatch(ele->ele.contains("燕"));
        System.out.println("集合中都沒有包含&#39;燕&#39;這個字嗎"+flag5);
    }
}

Output:

Are there any students whose names contain Yan: true
Are there any students whose names start with Wang: true
Are there any students whose names end with Jie: true
Do all students’ names have more than two characters? true
All students’ names have Wang ? true
The collection does not contain the word 'Yan' true

Using the method in anyMatch() can easily match the information of this stream.

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