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https://github.com/alpha74/java_streams_guide

A shortguide to Java 8 streams on Collections
https://github.com/alpha74/java_streams_guide

java java-8 java-collections java-stream java-stream-api java-stream-collector

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A shortguide to Java 8 streams on Collections

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README 

        
## Java streams() Guide



- A guide to Java 8 streams on Collections



-------



### Overview 



A birds-eye view of all major methods based on `Collections` and `stream()`



![image](https://github.com/user-attachments/assets/fa11c742-8bf6-46dd-adbc-04056b48485d)



- [Youtube source](https://www.youtube.com/watch?v=33JrZGtKOEE&list=PLUDwpEzHYYLvTPVqVIt7tlBohABLo4gyg&index=1&ab_channel=SDET-QA)



- `Collections`

        - It represents a a group of objects as a single entity.



- `Stream`

        - It is used to process data from collections.



- `Non-terminal Operation`

        - It is an operation which adds a listener to the stream which may modify the stream elements.



- `Terminal Operation`

        - It is an operations which returns a result after stream processing.



-------



#### filter()



- Takes a `predicate` as a argument, which returns a `boolean` as output.



- Example of filter() on a List of integers.



```

List numbersList = Arrays.asList(10,15,20,25,30);

```



```

// Create list of even numbers: using collect()

List evenList1 = numbersList.stream()

        .filter(n -> n % 2 == 0)

        .collect(Collectors.toList());



// Same: Create list of even numbers: using toList() shorthard

List evenList2 = numbersList.stream()

        .filter(n -> n % 2 == 0)

        .toList();

```



- Takes a `consumer` expression as input and does not return anything.



```

// Print even numbers

numbersList.stream().filter(n -> n % 2 == 0).forEach(n -> System.out.println(n));



// Same: Print even numbers using shorthand method reference

numbersList.stream().filter(n -> n % 2 == 0).forEach(System.out::println);

```



- `System.out` is a static method.



- Example of filter() on a List of strings.

 

```

List stringList = List.of("OptimusPrime", null, "Megatron", "Bumblebee", "Ratchet", null);



// Filter non-null values which have len > 7

List longStringList = stringList.stream().filter(s -> s != null && s.length() > 7).toList();

```



-------



#### map()



- It accepts a function or consumer as input.



- Example for converting each string in a List to UpperCase



```

List vehicles = List.of("car", "bus", "train", "place", "ship");



// Convert each item to upper case

List vehiclesUpperCase = vehicles.stream()

      .map(name -> name.toUpperCase())

      .collect(Collectors.toList());

```



- Example for calculating len of each string and appending to same item



```

// Find length of each element and append to same item

List vehicleLenList = vehicles.stream()

        .map(name -> name + " len is " + name.length())

        .collect(Collectors.toList());

```



-------



#### flatMap()



- Given a collection of objects, `map()` takes one input and returns one output, whereas `flatMap()` takes one input, but returns a stream of objects.



- Example to combine a list of list of objects into single list using `flatMap()`



```

List list1 = Arrays.asList(1,2);

List list2 = Arrays.asList(3,4);

List list3 = Arrays.asList(5,6);



List> listList = Arrays.asList(list1, list2, list3);



// Combine all lists into single list

List combinedList = listList.stream().flatMap(x -> x.stream()).collect(Collectors.toList());



// Output: [1,2,3,4,5,6]

```



```

// Operating on each item in each list and combining to single list

combinedList = listList.stream().flatMap(x -> x.stream()).map(n -> n +10).collect(Collectors.toList());

```



-------



#### distinct()



- Find distinct elements from a given collection.



```

List vehicleList = Arrays.asList("car", "bike", "car", "bike", "truck", "ship");

List uniqueVehicleList = vehicleList.stream().distinct().collect(Collectors.toList());

```



#### count() & limit()



- `count()` returns count of objects as `long`.



```

List vehicleList = Arrays.asList("car", "bike", "car", "bike", "truck", "ship");

long count = vehicleList.stream().distinct().count();                // Returns 4

```



- `limt()` is used to collect a limited number of objects from a stream.

- Takes `maxsize` as input param.



```

long count = vehicleList.stream().distinct().limit(2).count();        // Returns 2

```



-------



#### min() & max()



- Takes a `comparator()` as input, and returns an `Optional<>` object.



```

List numList = Arrays.asList(1,1,2,2,3,4);



Optional minVal = numList.stream().min((val1, val2) -> {return val1.compareTo(val2);});

System.out.println(minVal.get());   // 1



Optional maxVal = numList.stream().max((val1, val2) -> {return val1.compareTo(val2);});

System.out.println(maxVal.get());   // 4

```



-------



#### reduce()



- Takes input as an `identity` and `accumulator` as params.

- Reduces all elements in the stream to a single object.



```

List numList = Arrays.asList(1,1,2,2,3,4);



// Calculate sum

Optional sumVal = numList.stream().reduce((val, combinedVal) -> {

            return val + combinedVal;

        });



List vehicleList = Arrays.asList("car", "bike", "car", "bike", "truck", "ship");



// Append all strings to single string

Optional appended = vehicleList.stream().reduce((val, combined) -> {

    return val+combined;

});

```



-------



#### toArray()



- It converts the stream of objects to array.



```

List numList = Arrays.asList(1,1,2,2,3,4);



// Convert to array

Object arr[] = numList.stream().toArray();

```



-------



#### sorted()



- Used to sort a stream of objects.



```

List numList = Arrays.asList(2,5,1,6,3,7);



// Sort in ascending order

List ascSorted = numList.stream().sorted().collect(Collectors.toList());



// Sort in descending order

List descSorted = numList.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());

```



-------



#### anyMatch() 



- Returns `true` if condition matches any value in the stream.



```

Set fruits = new HashSet<>();



fruits.add("21 mangoes");

fruits.add("31 apples");

fruits.add("412 oranges");

fruits.add("13 guavas");



boolean res1 = fruits.stream().anyMatch(val -> {return val.contains("4");});        // true

```



#### allMatch() 



- Returns `true` if all elements match the condition, else `false`.



```

boolean res2 = fruits.stream().allMatch(val -> {return val.contains("1");});        // true

```



#### noneMatch()



- Returns `true` if none of the elements match the conditions, else `false`.



```

boolean res3 = fruits.stream().noneMatch(val -> {return val.contains("s");});        // false

```



-------



#### findAny()



- Returns `Optional<>`, or `NoSuchElementException` if no element is found.

- It may or may not return the first matched element.



```

List numList = Arrays.asList(2,5,1,6,3,7);



Optional any = numList.stream().findAny();        // 2 (but non-deterministic, helpful in parallelStream())

```



#### findFirst()



- Returns `Optional<>`, or `NoSuchElementException` if no element is found.

- It strictly returns the first matched element.



```

List numList = Arrays.asList(2,5,1,6,3,7);



Optional first = numList.stream().findFirst();        // 2

```



-------



##### Stream.concat()



- Used to concatenate two `Stream` objects into single `Stream`.



```

List list1 = Arrays.asList("dog", "cat", "rat");

List list2 = Arrays.asList("eagle", "sparrow", "hen");



Stream stream1 = list1.stream();

Stream stream2 = list2.stream();



List concatenated = Stream.concat(stream1, stream2).collect(Collectors.toList());

System.out.println(concatenated);

```



-------



#### Parallel Streams



- All collections support `parallelStream()` method that returns a possibly parallel stream.

- When a stream executes in parallel, the Java Runtime divides the stream into multiple substreams.

- The aggregate operators iterates over these sub-streams in parallel and then combine the result. This can improve the performance.



- Eg: `List parallelStream = list1.parallelStream()`



- Using `stream()`, operations are performed sequentially, but using `parallelStream()`, operations are performed parallely.



```

List vehicleList = Arrays.asList("car", "bike", "car", "bike", "truck", "ship");



// Using stream()

List filterSeq = vehicleList.stream().filter(x -> x.contains("a")).collect(Collectors.toList());



// Using parallelStream()

List filterPar = vehicleList.parallelStream().filter(x -> x.contains("a")).collect(Collectors.toList());



// Convert stream to parallel stream

List filterPar2 = vehicleList.stream().parallel().filter(x -> x.contains("a")).collect(Collectors.toList());

```