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This article mainly introduces the example analysis of Stream API operation in Java8, which has a certain reference value, and interested friends can refer to it. I hope you will gain a lot after reading this article.

1. What is StreamAPI?

There are two of the most important changes in Java8. The first is the Lambda expression; the other is m Stream API (java.util.stream.*).

Stream is a key abstract concept of dealing with collections in Java8. It can specify the operations you want to do on the collection, and can perform very complex operations such as finding, filtering, and mapping data.

Using Stream API to manipulate collection data is similar to a database query performed using SQL. You can also use Stream API to perform operations in parallel. In short, Stream API provides an efficient and easy-to-use way to process data.

What on earth is Stream?

Is a data channel for manipulating the sequence of elements generated by data sources (collections, arrays, etc.). "Collection is about data, flow is about computation!"

Note:

Stream itself does not store elements.

Stream does not change the source object. Instead, they return a new Stream that holds the results.

The Stream operation is deferred. This means that they will wait until the results are needed.

Three steps of 2.Stream API operation

Create Stream: a data source (e.g. collection, array) to get a stream.

Intermediate operation: an intermediate chain of operations that processes data from a data source.

Termination operation ((terminal operation)): a termination operation that executes an intermediate operation chain and produces a result.

2.1Create Stream

In Java8, the Collection interface is extended to provide two ways to get the stream:

Default Stream stream (): returns a sequential stream

Default Stream parallelStream (): returns a parallel flow

The static method stream () of Arrays in Java8 can get the array flow:

Static Stream stream (T [] array): returns a stream overloaded form that can handle arrays of corresponding basic types:

Public static IntStream stream (int [] array)

Public static LongStream stream (long [] array)

Public static DoubleStream stream (double [] array)

You can use the static method Stream.of () to create a stream by displaying values. It can receive any number of parameters.

Public static Stream of (T... Values): returns a stream

You can use the static methods Stream.iterate () and Stream.generate () to create an infinite stream.

Iterative public static Stream iterate (final T seed, final UnaryOperator f)

Generate public static Stream generate (Supplier s)

/ / create Stream @ Test public void test1 () {/ / 1.Collection provides two methods: stream () and parallelStream () List list = new ArrayList (); Stream stream1 = list.stream (); / / 2. Get an array stream String [] strings = new String [10]; Stream stream2 = Arrays.stream (strings); / / 3 through stream () in Arrays. Through the static method of () Stream stream3 = Stream.of ("abc", "666", "?") in the Stream class; / / 4. Create infinite flow / / iterative Stream stream4 = Stream.iterate (0, (x)-> x + 2); stream4.limit (5) .forEach (System.out::println); / / generate Stream.generate (()-> Math.random ()) .limit (5) .forEach (System.out::println);}

2.2 Intermediate operation

Multiple intermediate operations can be connected to form a pipeline, and the intermediate operation will not be performed unless the termination operation is triggered on the pipeline!

When terminating the operation, it is all dealt with at one time, which is called "lazy evaluation". Commonly used intermediate operations can be divided into three categories: ① filtering and slicing; ② mapping; and ③ sorting.

2.2.1 screening and slicing of intermediate operations

Here are some code examples to apply. First, we prepare an Employee class, as well as a List collection.

Package com.szh.java8; import lombok.AllArgsConstructor;import lombok.Data;import lombok.NoArgsConstructor; / * * * / @ Data@NoArgsConstructor@AllArgsConstructorpublic class Employee {private Integer id; private String name; private Integer age; private Double salary; public Employee (Integer id) {this.id = id;} public Employee (Integer id,String name) {this.id = id; this.name = name }} List employees = Arrays.asList (new Employee (1001, "Zhang San", 26pr 666.66), new Employee (1002, "Li Si", 50 pr 1111.11), new Employee (1003, "Wang Wu", 189999.99), new Employee (1004, "Zhao Liu", 3588888.88), new Employee (1005, "Tian Qi", 4 4m 3333.33), new Employee (1005, "Tian Qi", 4 4m 3333.33) New Employee (1005, "Tian Qi", 44pm 3333.33)

Screen information for employees under the age of 35. Here, the result is printed by println each time, and then compared. If it is satisfied, it is output.

/ / Internal iteration, completing @ Test public void test2 () {/ / all intermediate operations within Stream API will not do any processing Stream stream = employees.stream () .filter ((e)-> {System.out.println ("test intermediate operations") Return e.getAge () {System.out.println ("short circuit...."); return e.getSalary () > 5000;}) .limit (2) .forEach (System.out::println);}

Filter information about employees whose salary is greater than 5000, but throw away the first two.

@ Test public void test4 () {employees.stream () .filter ((e)-> e.getSalary () > 5000) .skip (2) .forEach (System.out::println);}

De-reprocess all employee information in the collection.

@ Test public void test5 () {employees.stream () .forEach (System.out::println);}

2.2.2 Mapping of intermediate operations

This is still done with the help of the Employee class and List collection in the above example.

First, all strings in the custom List collection are capitalized, and the employee collection is mapped to the name field.

@ Test public void test1 () {List list = Arrays.asList ("aaa", "bbb", "ccc", "ddd", "eee"); list.stream () .map ((str)-> str.toUpperCase ()) .forEach (System.out::println) System.out.println ("- -"); employees.stream () .map (Employee::getName) .forEach (System.out::println);}

Traverses output for each character in each string in the List collection. (① uses map; ② and flatMap)

The main difference between the two approaches is that after getting the Stream stream, the generics in the stream are different.

If we use map, it will return the elements in the strList collection in the form of "aaa", "bbb" and "ccc". At this time, the generics of the Stream stream is still a Stream stream (the data we want in this Stream stream), that is, after we traverse the stream stream for the first time, we get one Character character array after another. Then you need to iterate through the character array twice (that is, the two forEach in the top half of the code).

If we use flatMap, it returns the elements in the strList collection in the form of "a',"b", "c", "d", so the generics of the Stream stream are just an array of Character characters, so you can retrieve the data with a single traversal (that is, the forEach in the bottom half of the code).

@ Test public void test2 () {List strList = Arrays.asList ("aaa", "bbb", "ccc", "ddd", "eee"); Stream stream1 = strList.stream () .map (MyTest6::filterCharacter); stream1.forEach ((sm)-> {sm.forEach (System.out::println);}) System.out.println ("- -"); Stream stream2 = strList.stream () .flatMap (MyTest6::filterCharacter); stream2.forEach (System.out::println);} private static Stream filterCharacter (String str) {List list = new ArrayList () For (Character character: str.toCharArray ()) {list.add (character);} return list.stream ();}

2.2.3 sorting of intermediate operations

Natural sorting of strings in the List collection. Customizes the employee information in the employee collection (first by age, then by name).

@ Test public void test3 () {List strList = Arrays.asList ("ccc", "aaa", "bbb", "eee", "ddd"); strList.stream () .sorted () .forEach (System.out::println) System.out.println ("-") Employees.stream () .sorted ((E1 equals e2)-> {if (e1.getAge (). Equals (e2.getAge () {return e1.getName () .sorted (e2.getName ());} else {return e1.getAge () .compareTo (e2.getAge () }) .forEach (System.out::println);}

Thank you for reading this article carefully. I hope the article "sample Analysis of Stream API Operation in Java8" shared by the editor will be helpful to you. At the same time, I also hope you will support us and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

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