Java 8 Stream API

Содержание

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Outline Stream Building Blocks Java 8 Default Methods Functional Interfaces Lambda Expressions Method References

Outline

Stream Building Blocks
Java 8
Default Methods
Functional Interfaces
Lambda Expressions
Method References

Слайд 3

Outline Characteristics of Streams Creating Streams Common Functional Interfaces Used Anatomy

Outline

Characteristics of Streams
Creating Streams
Common Functional Interfaces Used
Anatomy of the Stream pipeline
Optional

Class
Common Stream API Methods Used
Examples
Parallel Streams
Unbounded (On the Fly) Streams
What Could Streams Do For BMI
References
Questions?
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Java 8 Target Release Date: 03/18/14 Introduces Default Methods Functional Interfaces

Java 8

Target Release Date: 03/18/14
Introduces
Default Methods
Functional Interfaces
Lambda Expressions
Stream API and

overall improvements to Collections to support Streams
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Default Methods In Context of Support For Streams Java 8 needed

Default Methods

In Context of Support For Streams
Java 8 needed to add

functionality to existing Collection interfaces to support Streams (stream(), forEach())
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Default Methods Problem Pre-Java 8 interfaces couldn’t have method bodies. The

Default Methods

Problem
Pre-Java 8 interfaces couldn’t have method bodies.
The only way to

add functionality to Interfaces was to declare additional methods which would be implemented in classes that implement the interface
It is impossible to add methods to an interface without breaking the existing implementation
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Default Methods Solution Default Methods! Java 8 allows default methods to

Default Methods

Solution
Default Methods!
Java 8 allows default methods to be added to

interfaces with their full implementation
Classes which implement the interface don’t have to have implementations of the default method
Allows the addition of functionality to interfaces while preserving backward compatibility
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Default Methods Example public interface A { default void foo(){ System.out.println("Calling

Default Methods

Example
public interface A {
default void foo(){
System.out.println("Calling A.foo()");
}
public class Clazz

implements A {}
Clazz clazz = new Clazz();
clazz.foo(); // Calling A.foo()
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Functional Interfaces Interfaces with only one abstract method. With only one

Functional Interfaces

Interfaces with only one abstract method.
With only one abstract method,

these interfaces can be easily represented with lambda expressions
Example
@FunctionalInterface
public interface SimpleFuncInterface {
public void doWork();
}
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Lambda expressions A more brief and clearly expressive way to implement

Lambda expressions

A more brief and clearly expressive way to implement functional

interfaces
Format: ->
Example (Functional Interface)
public interface Predicate {
boolean test(T input);
}
Example (Static Method)
public static Collection filter(Predicate predicate,
Collection items) {
Collection result = new ArrayList();
for(T item: items) {
if(predicate.test(item)) {
result.add(item);
}
}
}
Example (Call with Lambda Expression)
Collection myInts = asList(0,1,2,3,4,5,6,7,8,9);
Collection onlyOdds = filter(n -> n % 2 != 0, myInts)
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Method References Event more brief and clearly expressive way to implement

Method References

Event more brief and clearly expressive way to implement functional

interfaces
Format: ::
Example (Functional Interface)
public interface IntPredicates {
boolean isOdd(Integer n) { return n % 2 != 0; }
}
Example (Call with Lambda Expression)
List numbers = asList(1,2,3,4,5,6,7,8,9);
List odds = filter(n -> IntPredicates.isOdd(n), numbers);
Example (Call with Method Reference)
List numbers = asList(1,2,3,4,5,6,7,8,9);
List odds = filter(IntPredicates::isOdd, numbers);
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Characteristics of Streams Streams are not related to InputStreams, OutputStreams, etc.

Characteristics of Streams

Streams are not related to InputStreams, OutputStreams, etc.
Streams

are NOT data structures but are wrappers around Collection that carry values from a source through a pipeline of operations.
Streams are more powerful, faster and more memory efficient than Lists
Streams are designed for lambdas
Streams can easily be output as arrays or lists
Streams employ lazy evaluation
Streams are parallelizable
Streams can be “on-the-fly”
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Creating Streams From individual values Stream.of(val1, val2, …) From array Stream.of(someArray)

Creating Streams

From individual values
Stream.of(val1, val2, …)
From array
Stream.of(someArray)
Arrays.stream(someArray)
From List

(and other Collections)
someList.stream()
someOtherCollection.stream()
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Common Functional Interfaces Used Predicate Represents a predicate (boolean-valued function) of

Common Functional Interfaces Used

Predicate
Represents a predicate (boolean-valued function) of one argument
Functional

method is boolean Test(T t)
Evaluates this Predicate on the given input argument (T t)
Returns true if the input argument matches the predicate, otherwise false
Supplier
Represents a supplier of results
Functional method is T get()
Returns a result of type T
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Common Functional Interfaces Used Function Represents a function that accepts one

Common Functional Interfaces Used

Function
Represents a function that accepts one argument and

produces a result
Functional method is R apply(T t)
Applies this function to the given argument (T t)
Returns the function result
Consumer
Represents an operation that accepts a single input and returns no result
Functional method is void accept(T t)
Performs this operation on the given argument (T t)
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Common Functional Interfaces Used UnaryOperator Represents an operation on a single

Common Functional Interfaces Used

UnaryOperator
Represents an operation on a single operands that

produces a result of the same type as its operand
Functional method is R Function.apply(T t)
Applies this function to the given argument (T t)
Returns the function result
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Common Functional Interfaces Used BiFunction Represents an operation that accepts two

Common Functional Interfaces Used

BiFunction
Represents an operation that accepts two arguments and

produces a result
Functional method is R apply(T t, U u)
Applies this function to the given arguments (T t, U u)
Returns the function result
BinaryOperator
Extends BiFunction
Represents an operation upon two operands of the same type, producing a result of the same type as the operands
Functional method is R BiFunction.apply(T t, U u)
Applies this function to the given arguments (T t, U u) where R,T and U are of the same type
Returns the function result
Comparator
Compares its two arguments for order.
Functional method is int compareTo(T o1, T o2)
Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
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Anatomy of the Stream Pipeline A Stream is processed through a

Anatomy of the Stream Pipeline

A Stream is processed through a pipeline

of operations
A Stream starts with a source data structure
Intermediate methods are performed on the Stream elements. These methods produce Streams and are not processed until the terminal method is called.
The Stream is considered consumed when a terminal operation is invoked. No other operation can be performed on the Stream elements afterwards
A Stream pipeline contains some short-circuit methods (which could be intermediate or terminal methods) that cause the earlier intermediate methods to be processed only until the short-circuit method can be evaluated.
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Anatomy of the Stream Pipeline Intermediate Methods map, filter, distinct, sorted,

Anatomy of the Stream Pipeline

Intermediate Methods
map, filter, distinct, sorted, peek, limit,


parallel
Terminal Methods
forEach, toArray, reduce, collect, min,
max, count, anyMatch, allMatch, noneMatch, findFirst, findAny, iterator
Short-circuit Methods
anyMatch, allMatch, noneMatch, findFirst, findAny,limit
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Optional Class A container which may or may not contain a

Optional Class

A container which may or may not contain a non-null

value
Common methods
isPresent() – returns true if value is present
Get() – returns value if present
orElse(T other) – returns value if present, or other
ifPresent(Consumer) – runs the lambda if value is present
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Common Stream API Methods Used Void forEach(Consumer) Easy way to loop

Common Stream API Methods Used

Void forEach(Consumer)
Easy way to loop over Stream

elements
You supply a lambda for forEach and that lambda is called on each element of the Stream
Related peek method does the exact same thing, but returns the original Stream
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Common Stream API Methods Used Void forEach(Consumer) Example Employees.forEach(e -> e.setSalary(e.getSalary()

Common Stream API Methods Used

Void forEach(Consumer)
Example
Employees.forEach(e -> e.setSalary(e.getSalary() * 11/10))
Give all

employees a 10% raise
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Common Stream API Methods Used Void forEach(Consumer) Vs. For Loops List

Common Stream API Methods Used

Void forEach(Consumer)
Vs. For Loops
List employees = getEmployees();
for(Employee

e: employees) {
e.setSalary(e.getSalary() * 11/10);
}
Advantages of forEach
Designed for lambdas to be marginally more succinct
Lambdas are reusable
Can be made parallel with minimal effort
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Common Stream API Methods Used Stream map(Function) Produces a new Stream

Common Stream API Methods Used

Stream map(Function)
Produces a new Stream that is

the result of applying a Function to each element of original Stream
Example
Ids.map(EmployeeUtils::findEmployeeById)
Create a new Stream of Employee ids
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Common Stream API Methods Used Stream filter(Predicate) Produces a new Stream

Common Stream API Methods Used

Stream filter(Predicate)
Produces a new Stream that contains

only the elements of the original Stream that pass a given test
Example
employees.filter(e -> e.getSalary() > 100000)
Produce a Stream of Employees with a high salary
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Common Stream API Methods Used Optional findFirst() Returns an Optional for

Common Stream API Methods Used

Optional findFirst()
Returns an Optional for the first

entry in the Stream
Example
employees.filter(…).findFirst().orElse(Consultant)
Get the first Employee entry that passes the filter
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Common Stream API Methods Used Object[] toArray(Supplier) Reads the Stream of

Common Stream API Methods Used

Object[] toArray(Supplier)
Reads the Stream of elements into

a an array
Example
Employee[] empArray = employees.toArray(Employee[]::new);
Create an array of Employees out of the Stream of Employees
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Common Stream API Methods Used List collect(Collectors.toList()) Reads the Stream of

Common Stream API Methods Used

List collect(Collectors.toList())
Reads the Stream of elements into

a List or any other collection
Example
List empList =
employees.collect(Collectors.toList());
Create a List of Employees out of the Stream of Employees
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Common Stream API Methods Used List collect(Collectors.toList()) partitioningBy You provide a

Common Stream API Methods Used

List collect(Collectors.toList())
partitioningBy
You provide a Predicate. It builds

a Map where true maps to a List of entries that passed the Predicate, and false maps to a List that failed the Predicate.
Example
Map> richTable =
googlers().collect
(partitioningBy(e -> e.getSalary() > 1000000));
groupingBy
You provide a Function. It builds a Map where each output value of the Function maps to a List of entries that gave that value.
Example
Map> deptTable =
employeeStream().collect(groupingBy(Employee::getDepartment));
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Common Stream API Methods Used T reduce(T identity, BinaryOperator) You start

Common Stream API Methods Used

T reduce(T identity, BinaryOperator)
You start with a

seed (identity) value, then combine this value with the first Entry in the Stream, combine the second entry of the Stream, etc.
Example
Nums.stream().reduce(1, (n1,n2) -> n1*n2)
Calculate the product of numbers
IntStream (Stream on primative int] has build-in sum()
Built-in Min, Max methods
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Common Stream API Methods Used Stream limit(long maxSize) Limit(n) returns a

Common Stream API Methods Used

Stream limit(long maxSize)
Limit(n) returns a stream of

the first n elements
Example
someLongStream.limit(10)
First 10 elements
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Common Stream API Methods Used Stream skip(long n) skip(n) returns a

Common Stream API Methods Used

Stream skip(long n)
skip(n) returns a stream starting

with element n
Example
twentyElementStream.skip(5)
Last 15 elements
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Common Stream API Methods Used Stream sorted(Comparator) Returns a stream consisting

Common Stream API Methods Used

Stream sorted(Comparator)
Returns a stream consisting of the

elements of this stream, sorted according to the provided Comparator
Example
empStream.map(…).filter(…).limit(…)
.sorted((e1, e2) -> e1.getSalary() - e2.getSalary())
Employees sorted by salary
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Common Stream API Methods Used Optional min(Comparator) Returns the minimum element

Common Stream API Methods Used

Optional min(Comparator)
Returns the minimum element in this

Stream according to the Comparator
Example
Employee alphabeticallyFirst =
ids.stream().map(EmployeeSamples::findGoogler)
.min((e1, e2) ->
e1.getLastName()
.compareTo(e2.getLastName()))
.get();
Get Googler with earliest lastName
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Common Stream API Methods Used Optional max(Comparator) Returns the minimum element

Common Stream API Methods Used

Optional max(Comparator)
Returns the minimum element in this

Stream according to the Comparator
Example
Employee richest =
ids.stream().map(EmployeeSamples::findGoogler)
.max((e1, e2) -> e1.getSalary() -
e2.getSalary())
.get();
Get Richest Employee
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Common Stream API Methods Used Stream distinct() Returns a stream consisting

Common Stream API Methods Used

Stream distinct()
Returns a stream consisting of the

distinct elements of this stream
Example
List ids2 =
Arrays.asList(9, 10, 9, 10, 9, 10);
List emps4 =
ids2.stream().map(EmployeeSamples::findGoogler)
.distinct()
.collect(toList());
Get a list of distinct Employees
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Common Stream API Methods Used Boolean anyMatch(Predicate), allMatch(Predicate), noneMatch(Predicate) Returns true

Common Stream API Methods Used

Boolean anyMatch(Predicate), allMatch(Predicate), noneMatch(Predicate)
Returns true if Stream

passes, false otherwise
Lazy Evaluation
anyMatch processes elements in the Stream one element at a time until it finds a match according to the Predicate and returns true if it found a match
allMatch processes elements in the Stream one element at a time until it fails a match according to the Predicate and returns false if an element failed the Predicate
noneMatch processes elements in the Stream one element at a time until it finds a match according to the Predicate and returns false if an element matches the Predicate
Example
employeeStream.anyMatch(e -> e.getSalary() > 500000)
Is there a rich Employee among all Employees?
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Common Stream API Methods Used long count() Returns the count of

Common Stream API Methods Used

long count()
Returns the count of elements in

the Stream
Example
employeeStream.filter(somePredicate).count()
How many Employees match the criteria?
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Parallel Streams Helper Methods For Timing private static void timingTest(Stream testStream)

Parallel Streams

Helper Methods For Timing
private static void timingTest(Stream testStream) {
long startTime

= System.nanoTime();
testStream.forEach(e -> doSlowOp());
long endTime = System.nanoTime();
System.out.printf(" %.3f seconds.%n",
deltaSeconds(startTime, endTime));
}
private static double deltaSeconds(long startTime, long endTime) {
return((endTime - startTime) / 1000000000);
}
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Parallel Streams Helper Method For Simulating Long Operation void doSlowOp() {

Parallel Streams

Helper Method For Simulating Long Operation
void doSlowOp() {
try {
TimeUnit.SECONDS.sleep(1);
} catch

(InterruptedException ie) {
// Nothing to do here.
}
}
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Parallel Streams Main Code System.out.print("Serial version [11 entries]:"); timingTest(googlers()); int numProcessorsOrCores

Parallel Streams

Main Code
System.out.print("Serial version [11 entries]:");
timingTest(googlers());
int numProcessorsOrCores =
Runtime.getRuntime().availableProcessors();
System.out.printf("Parallel version on %s-core

machine:",
numProcessorsOrCores);
timingTest(googlers().parallel() );
Слайд 42

Parallel Streams Results Serial version [11 entries]: 11.000 seconds. Parallel version on 4-core machine: 3.000 seconds.

Parallel Streams

Results
Serial version [11 entries]: 11.000 seconds.
Parallel version on 4-core machine:

3.000 seconds.
Слайд 43

(On The Fly) Streams Stream generate(Supplier) The method lets you specify

(On The Fly) Streams

Stream generate(Supplier)
The method lets you specify a Supplier
This

Supplier is invoked each time the system needs a Stream element
Example
List emps =
Stream.generate(() -> randomEmployee())
.limit(n)
.collect(toList());
Stream iterate(T seed, UnaryOperator f)
The method lets you specify a seed and a UnaryOperator.
The seed becomes the first element of the Stream, f(seed) becomes the second element of the Stream, f(second) becomes the third element, etc.
Example
List powersOfTwo =
Stream.iterate(1, n -> n * 2)
.limit(n)
.collect(toList());
The values are not calculated until they are needed
To avoid unterminated processing, you must eventually use a size-limiting method
This is less of an actual Unbounded Stream and more of an “On The Fly” Stream
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References Stream API http://download.java.net/jdk8/docs/api/java/util/stream/Stream.html Java 8 Explained: Applying Lambdas to Java

References

Stream API
http://download.java.net/jdk8/docs/api/java/util/stream/Stream.html
Java 8 Explained: Applying Lambdas to Java Collections
http://zeroturnaround.com/rebellabs/java-8-explained-applying-lambdas-to-java-collections/
Java 8 first

steps with Lambdas and Streams
https://blog.codecentric.de/en/2013/10/java-8-first-steps-lambdas-streams/
Java 8Tutorial: Lambda Expressions, Streams, and More
http://www.coreservlets.com/java-8-tutorial/
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Java Collections Framework
Следующая -
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