Java Stream Stream processing

I. The concept of flow

Structure of 1.

Flow to obtain
Conversion operations: There can be more than one
Terminate operation: There can be only one

Type 2.

Stream () : single pipe

parallelStream()

Multi-pipe, parallel streaming processing, implemented using ForkJoinPool
Enforce order: forEachOrdered()

List<Integer> list = Arrays.asList(1.2.3.4.5.6.7);

// Result: 1234567
list.stream().forEach(System.out::print);

// Result: 5726134
list.parallelStream().forEach(System.out::print);

// Result: 1234567
list.parallelStream().forEachOrdered(System.out::print);
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3. functional interfaces

An interface has one and only one abstract method

Common interface

interface	parameter	The return value	category
Consumer	T	void	Consumer interface
Supplier	None	T	Supply interface
Function	T	R	Functional interface
Predicate	T	boolean	Assertion interface

Lambda expressions

‐> {code statements}
Used to simplify writing functional interfaces
Can delay running, improve performance

Second, stream acquisition

1. Collection subinterface

Call the stream() method directly
List
Set

Vector

List<String> list = new ArrayList<>();
Stream<String> stream1 = list.stream();

Set<String> set = new HashSet<>();
Stream<String> stream2 = set.stream();

Vector<String> vector = new Vector<>();
Stream<String> stream3 = vector.stream();
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2. Map

It is not a Collection subinterface, and its K-V data structure does not conform to the characteristics of confluence elements. Therefore, it needs to obtain the data according to Key, Value, and Entry respectively

Map<String, String> map = new HashMap<>();
// ...
Stream<String> keyStream = map.keySet().stream();
Stream<String> valueStream = map.values().stream();
Stream<Map.Entry<String, String>> entryStream = map.entrySet().stream();
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3. Array

You can’t add a default method to the array, so use the stream.of () method

String[] array = { "Zhang Wuji"."Zhang Cuishan"."Zhang Sanfeng"."Zhang Yi Yuan" };
Stream<String> stream = Stream.of(array);
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Second, conversion operation

1. filter( T -> boolean )

Filter data, reserving elements with true condition

List<Integer> list = Arrays.asList(20.23.25.28.30.33.37.40);
// Filter data sets greater than or equal to 30 from the specified data set
List<Integer> collect = list.stream().filter(x -> x >= 30).collect(Collectors.toList());
// Result: [30, 33, 37, 40]
System.out.println(collect);
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2. map( T -> R )

Convert data and store the converted data in reflux

List<String> list = Arrays.asList("1"."2"."3"."4"."5"."6");

List<Long> collect1 = list.stream().map(x -> Long.parseLong(x)).collect(Collectors.toList());
// Result: [1, 2, 3, 4, 5, 6]
System.out.println(collect1);

// Result: 111111
list.stream().mapToInt(x -> x.length()).forEach(System.out::print);
System.out.println("");

// Result: 111111
list.stream().mapToLong(x -> x.length()).forEach(System.out::print);
System.out.println("");

// Result: 1.01.01.01.01.01.0
list.stream().mapToDouble(x -> x.length()).forEach(System.out::print);
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3. flatMap( T -> Stream )

Map elements in a flow to a flow, and connect each flow to a flow

List<List<String>>  list = new ArrayList<List<String>>(){{
            add(Lists.newArrayList("a"."b"."c"));
            add(Lists.newArrayList("d"."e"."f"));
            add(Lists.newArrayList("j"."k"."y"));
        }};
// Results: [a, B, C], [D, e, f], [J, k, Y]]
System.out.println(list);
List<String> collect = list.stream().flatMap(List::stream).collect(Collectors.toList());
[a, b, C, D, e, f, j, k, y]
System.out.println(collect);
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4. distinct()

The element is de-duplicated, and the bottom layer uses equals() to compare

List<String> list = Arrays.asList("a"."b"."ab"."abc"."a"."ab"."a"."abcd"."bd"."abc");
List<String> collect = list.stream().distinct().collect(Collectors.toList());
// Result: [a, b, ab, ABC, abcd, bd]
System.out.println(collect);
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5. sorted( T -> boolean )

Order elements by implementing the Comparable interface or custom comparator

List<Integer> list = Arrays.asList(5.3.7.1.4.6);
List<Integer> collect = list.stream().sorted((a, b) -> a.compareTo(b)).collect(Collectors.toList());
// Result: [1, 3, 4, 5, 6, 7]
System.out.println(collect);
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6. limit( num )

Limit the number of elements returned

List<String> list = Arrays.asList("a"."b"."ab"."abc"."a"."ab"."a"."abcd"."bd"."abc");
List<String> collect = list.stream().limit(3).collect(Collectors.toList());
// Result: [a, b, ab]
System.out.println(collect);
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7. skip( num )

Skip the element

List<String> list = Arrays.asList("a"."b"."ab"."abc"."a"."ab"."a"."abcd"."bd"."abc");
List<String> collect = list.stream().skip(5).collect(Collectors.toList());
// Result: [ab, a, abcd, bd, ABC]
System.out.println(collect);
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8. peek( T -> void )

Elements are picked out for manipulation, but the resulting element is not returned to the stream

List<String> list = Arrays.asList("a"."b"."ab"."abc"."a"."ab"."a"."abcd"."bd"."abc");
// Result: abababcaabaabcdbdabc
list.stream().peek(x -> x.toUpperCase()).forEach(System.out::print);

// Result: ABABABCAABAABCDBDABC
list.stream().map(x -> x.toUpperCase()).forEach(System.out::print);
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3. Terminate the operation

1. forEach

ForEach: supports parallel processing

ForEachOrdered: Forces the processing to be orderly and slow

List<String> list = Arrays.asList("a"."b"."ab");

// Result: A, B, ab
list.stream().forEach(x -> System.out.print(x+' '));
System.out.println("");

// Can be simplified
// Result: A, B, ab
list.forEach(x -> System.out.print(x+' '));
System.out.println("");

// Result: A, B, ab
list.stream().forEachOrdered(x -> System.out.print(x+' '));
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2. collect

ToMap: Transforms the data stream into a Map containing key/value elements
ToSet: Converts a data stream to a Set containing elements that cannot be repeated
ToList: Converts the data stream to a List containing ordered elements
Joining: a joining separator between elements that returns a string
GroupingBy: Groups a List into a Map
Couting: Count elements
MaxBy: Get the largest element
MinBy: Gets the smallest element
SummarizingInt: Summarizes elements of type Integer, which returns IntSummaryStatistics. You can then call a specific method to collect statistics
- GetCount: indicates the number of statistics
- GetSum: sum
- GetMin: obtains the minimum value
- GetMax: obtains the maximum value
- GetAverage: Obtain averages
SummarizingLong: Summary Long element, same as summarizingInt
SummarizingDouble: summarizingDouble element, same as summarizingInt
AveragingInt: Retrieves the average value of an Integer element and returns a Double
AveragingLong: Retrieves the average value of the Long element and returns a Double
AveragingDouble: Retrieves the average value of Double elements and returns a Double

Mapping: Gets a mapping that returns a portion of the original element as a new element

List<String> list0 = Arrays.asList("a"."b"."ab");
Map<String, String> collect0 = list0.stream().collect(Collectors.toMap(String::new, Function.identity()));
{ab=ab, a=a, b=b}
System.out.println(collect0);

List<String> list = Arrays.asList("a"."b"."ab"."a"."b"."ab");
List<String> collect1 = list.stream().collect(Collectors.toList());
// Result: [a, b, ab, a, b, ab]
System.out.println(collect1);

// result: [a, ab, b]
Set<String> collect2 = list.stream().collect(Collectors.toSet());
System.out.println(collect2);

String collect3 = list.stream().collect(Collectors.joining(","));
A,b,ab,a,b,ab
System.out.println(collect3);

Map<String, Long> collect4 = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
{ab=2, a=2, b=2}
System.out.println(collect4);

Long collect = list.stream().collect(Collectors.counting());
// Result: 6
System.out.println(collect);

List<Integer> list1 = Arrays.asList(1.3.5.7.9.11);

Integer collect5 = list1.stream().collect(Collectors.maxBy((a, b) -> a.compareTo(b))).orElse(null);
System.out.println(collect5);
// Result: 11
System.out.println(collect5);

String collect6 = list1.stream().collect(Collectors.minBy((a, b) -> a.compareTo(b))).orElse(null);
// Result: 1
System.out.println(collect6);

List<String> list2 = Arrays.asList("2"."3"."5");
IntSummaryStatistics summaryStatistics = list2.stream().collect(Collectors.summarizingInt(x -> Integer.parseInt(x)));
long sum = summaryStatistics.getSum();
// Result: 10
System.out.println(sum);

Double collect7 = list2.stream().collect(Collectors.averagingInt(x -> Integer.parseInt(x)));
// Result: 3.333333333335
System.out.println(collect7);

List<User> userList = new ArrayList<User>() {{
  add(new User("jack".23));
  add(new User("james".30));
  add(new User("curry".28));
}};
List<String> collect8 = userList.stream().collect(Collectors.mapping(User::getName, Collectors.toList()));
//[jack, james, curry]
System.out.println(collect8);
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3. find

FindFirst: Finds the first element and returns type Optional

FindAny: Generally returns the first element, of type Optional, but is not guaranteed to be the first in parallel cases

List<String> lst1 = Arrays.asList("Jhonny"."David"."Jack"."Duke"."Jill"."Dany"."Julia"."Jenish"."Divya");
List<String> lst2 = Arrays.asList("Jhonny"."David"."Jack"."Duke"."Jill"."Dany"."Julia"."Jenish"."Divya");
 
Optional<String> findFirst = lst1.parallelStream().filter(s -> s.startsWith("D")).findFirst();
Optional<String> fidnAny = lst2.parallelStream().filter(s -> s.startsWith("J")).findAny();
 
System.out.println(findFirst.get()); // Always print David
System.out.println(fidnAny.get()); // Will randomly print Jack/Jill/Julia
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4. match

AllMatch: Returns Boolean if all elements meet the criteria
AnyMatch: Returns Boolean if any element meets the condition

NoneMatch: All elements do not meet the criteria, return Boolean

List<Integer> list = Arrays.asList(2.3.5.7);

boolean allMatch = list.stream().allMatch(x -> x > 1);
// Result: true
System.out.println(allMatch);

boolean allMatch2 = list.stream().allMatch(x -> x > 2);
// Result: false
System.out.println(allMatch2);

boolean anyMatch = list.stream().anyMatch(x -> x > 2);
// Result: true
System.out.println(anyMatch);

boolean noneMatch1 = list.stream().noneMatch(x -> x > 5);
// Result: false
System.out.println(noneMatch1);

boolean noneMatch2 = list.stream().noneMatch(x -> x > 7);
// Result: true
System.out.println(noneMatch2);
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5. count

Count quantities, returning long, similar to the size() method of the collection

List<String> list = Arrays.asList("a"."b"."ab");
long count = list.stream().count();
// Result: 3
System.out.println(count);
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6. The Max and min

Max: Gets the maximum value. Returns the Optional type

Min: Gets the minimum value and returns the Optional type

List<Integer> list = Arrays.asList(2.3.5.7);

Optional<Integer> max = list.stream().max((a, b) -> a.compareTo(b));
// Result: 7
System.out.println(max.get());

Optional<Integer> min = list.stream().min((a, b) -> a.compareTo(b));
// Result: 2
System.out.println(min.get());
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7. reduce

Specification operation, which specifies the entire data stream into a single value
Two parameters: the initial value of the loop calculation, calculation accumulator

Count, Max, and min are implemented using reduce

List<Integer> list = Arrays.asList(2.3.5.7);

Integer sum1 = list.stream().reduce(0, Integer::sum);
// Result: 17
System.out.println(sum1);

Optional<Integer> reduce = list.stream().reduce((a, b) -> a + b);
// Result: 17
System.out.println(reduce.get());

Integer max = list.stream().reduce(0, Integer::max);
// Result: 7
System.out.println(max);

Integer min = list.stream().reduce(0, Integer::min);
// Result: 0
System.out.println(min);


Optional<Integer> reduce1 = list.stream().reduce((a, b) -> a > b ? b : a);
/ / 2
System.out.println(reduce1.get());
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8. toArray

Array operation to convert a stream of data into an array

List<String> list = Arrays.asList("a"."b"."ab");
String[] strings = list.stream().toArray(String[]::new);
// Result: A, B, ab
for (int i = 0; i < strings.length; i++) {
  System.out.print(strings[i]+"");
}
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9. concat

Merge two streams into one

Stream<String> streamA = Stream.of("Zhang Wuji");
Stream<String> streamB = Stream.of("Zhang Cuishan");
Stream<String> result = Stream.concat(streamA, streamB);
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Java Stream Knowledge brain map – Stream processing

Java Stream Stream processing

I. The concept of flow

Structure of 1.

Type 2.

3. functional interfaces

Lambda expressions

Second, stream acquisition

1. Collection subinterface

2. Map

3. Array

Second, conversion operation

1. filter( T -> boolean )

2. map( T -> R )

3. flatMap( T -> Stream )

4. distinct()

5. sorted( T -> boolean )

6. limit( num )

7. skip( num )

8. peek( T -> void )

3. Terminate the operation

1. forEach

2. collect

3. find

4. match

5. count

6. The Max and min

7. reduce

8. toArray

9. concat

Java Stream Knowledge brain map – Stream processing

Java Stream Stream processing

I. The concept of flow

Structure of 1.

Type 2.

3. functional interfaces

Lambda expressions

Second, stream acquisition

1. Collection subinterface

2. Map

3. Array

Second, conversion operation

1. filter( T -> boolean )

2. map( T -> R )

3. flatMap( T -> Stream )

4. distinct()

5. sorted( T -> boolean )

6. limit( num )

7. skip( num )

8. peek( T -> void )

3. Terminate the operation

1. forEach

2. collect

3. find

4. match

5. count

6. The Max and min

7. reduce

8. toArray

9. concat

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