Stream learning record

One 、 summary

stream Flow is Java8 , which deals with key abstractions of collections , He can specify what you want to do with the collection , You can perform very complex lookups 、 Filtering and mapping operations . Use StreamAPI Operates on the collection data , It's kind of like using SQL Execute database query , You can also use StreamAPI To execute operations in parallel .

in short ：StreamAPI Provides an efficient and easy-to-use way to process data .

characteristic :

• It's not a data structure , It doesn't save data
• The original data source will not be modified , It saves the data after the operation to another object
• Lazy evaluation , The stream is in the reconstruction process , It's just a record of the operation , Not immediately executed , The timing calculation will not be performed until the termination operation is executed

Two 、 Operation classification

• No state ： The processing of an element is not affected by the previous element
• A stateful ： The operation cannot continue until all elements have been obtained
• Non short circuit operation ： All elements must be processed to get the final result
• Short circuit operation ： When you meet some elements that meet the conditions, you can get the final result , Such as A||B, as long as A by true, There is no need to judge B Result

3、 ... and 、 The specific use

1. Common methods for creating streams
   1.1 Use Collection Under the stream() and parallelStream() Method

  List<AnalysisOrderCaseGroupDO> list = page.getList();
  List<Long> caseGroupIds = list.stream().map(AnalysisOrderCaseGroupDO::getId)
            .collect(Collectors.toList());

 dataList.parallelStream().forEach(d -> geometryBaseService.doSimplify(d, zoom));

1.2 Use Arrays Medium stream() Method , Convert array to stream

Integer[] nums = new Integer[10];
Stream<Integer> stream = Array.stream(nums);

Here is an example used in enumerations ：

public enum ModeEnum{
    
	// File operation mode 
	READ_ONLY(1, " read-only "）,
	EDITABLE(2, " Editable "),
	;

	Integer code;
	String desc;
	
	public static String descOf(Integer code){
    
	 	if(code == null ){
    
	 		return "";
	 	}
	 	return Arrays.Stream(ModeEnum.values())
	 		.filter(o -> Objects.equals(o.getCode(), code))
	 		.findAny().map(ModeEnum::getDesc).orElse("");
	}
}

1.3 Use Stream Static methods in ：of()、iterator() 、generate()

A small case ：

public class StreamOfTest {
    
    public static void main(String[] args) {
    
        String  idStr = "12, 22, 33,44";
        List<Long> ids = Stream.of(idStr.split(","))
                .map(String::trim)
                .map(Long::valueOf)
                .collect(Collectors.toList());
        System.out.println(ids);

		Stream<Integer> stream2 = Stream.iterate(0, (x) -> x + 2).limit(6);
        stream2.forEach(System.out::println);

        Stream<Double> stream3 = Stream.generate(Math::random).limit(2);
        stream3.forEach(System.out::println);
    }
}

1.4 Use BufferReader.lines() Method , Turn each line into a stream