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[add] add Stream API examples
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‎java8/README.md

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# Java 8 新特性
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《Java 8 In Action》一书
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源码参考了 **java8/Java8InAction**:[https://github.com/java8/Java8InAction](https://github.com/java8/Java8InAction)
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‎java8/src/main/java/com/brianway/learning/java8/streamapi/BuildingStreams.java

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package com.brianway.learning.java8.streamapi;
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import java.nio.charset.Charset;
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import java.nio.file.Files;
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import java.nio.file.Paths;
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import java.util.Arrays;
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import java.util.function.IntSupplier;
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import java.util.stream.IntStream;
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import java.util.stream.Stream;
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public class BuildingStreams {
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public static void main(String... args) throws Exception {
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// Stream.of
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Stream<String> stream = Stream.of("Java 8", "Lambdas", "In", "Action");
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stream.map(String::toUpperCase).forEach(System.out::println);
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// Stream.empty
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Stream<String> emptyStream = Stream.empty();
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// Arrays.stream
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int[] numbers = {2, 3, 5, 7, 11, 13};
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System.out.println(Arrays.stream(numbers).sum());
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// Stream.iterate
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Stream.iterate(0, n -> n + 2)
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.limit(10)
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.forEach(System.out::println);
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// fibonnaci with iterate
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Stream.iterate(new int[] {0, 1}, t -> new int[] {t[1], t[0] + t[1]})
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.limit(10)
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.forEach(t -> System.out.println("(" + t[0] + ", " + t[1] + ")"));
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Stream.iterate(new int[] {0, 1}, t -> new int[] {t[1], t[0] + t[1]})
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.limit(10)
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.map(t -> t[0])
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.forEach(System.out::println);
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// random stream of doubles with Stream.generate
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Stream.generate(Math::random)
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.limit(10)
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.forEach(System.out::println);
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// stream of 1s with Stream.generate
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IntStream.generate(() -> 1)
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.limit(5)
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.forEach(System.out::println);
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IntStream.generate(new IntSupplier() {
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public int getAsInt() {
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return 2;
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}
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}).limit(5)
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.forEach(System.out::println);
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IntSupplier fib = new IntSupplier() {
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private int previous = 0;
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private int current = 1;
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public int getAsInt() {
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int nextValue = this.previous + this.current;
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this.previous = this.current;
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this.current = nextValue;
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return this.previous;
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}
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};
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IntStream.generate(fib).limit(10).forEach(System.out::println);
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String path = BuildingStreams.class.getResource("/").getPath() + "/data-building.txt";
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long uniqueWords = Files.lines(Paths.get(path), Charset.defaultCharset())
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.flatMap(line -> Arrays.stream(line.split(" ")))
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.distinct()
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.count();
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System.out.println("There are " + uniqueWords + " unique words in data.txt");
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}
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/Dish.java

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package com.brianway.learning.java8.streamapi;
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import java.util.Arrays;
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import java.util.List;
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public class Dish {
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private final String name;
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private final boolean vegetarian;
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private final int calories;
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private final Type type;
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public Dish(String name, boolean vegetarian, int calories, Type type) {
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this.name = name;
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this.vegetarian = vegetarian;
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this.calories = calories;
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this.type = type;
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}
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public String getName() {
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return name;
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}
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public boolean isVegetarian() {
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return vegetarian;
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}
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public int getCalories() {
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return calories;
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}
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public Type getType() {
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return type;
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}
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public enum Type {
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MEAT,
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FISH,
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OTHER
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}
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@Override
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public String toString() {
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return name;
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}
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public static final List<Dish> menu =
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Arrays.asList(new Dish("pork", false, 800, Dish.Type.MEAT),
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new Dish("beef", false, 700, Dish.Type.MEAT),
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new Dish("chicken", false, 400, Dish.Type.MEAT),
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new Dish("french fries", true, 530, Dish.Type.OTHER),
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new Dish("rice", true, 350, Dish.Type.OTHER),
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new Dish("season fruit", true, 120, Dish.Type.OTHER),
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new Dish("pizza", true, 550, Dish.Type.OTHER),
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new Dish("prawns", false, 400, Dish.Type.FISH),
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new Dish("salmon", false, 450, Dish.Type.FISH));
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/GroupingTransactions.java

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package com.brianway.learning.java8.streamapi;
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import static java.util.stream.Collectors.groupingBy;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.HashMap;
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import java.util.List;
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import java.util.Map;
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public class GroupingTransactions {
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public static List<Transaction> transactions = Arrays.asList(
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new Transaction(Currency.EUR, 1500.0),
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new Transaction(Currency.USD, 2300.0),
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new Transaction(Currency.GBP, 9900.0),
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new Transaction(Currency.EUR, 1100.0),
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new Transaction(Currency.JPY, 7800.0),
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new Transaction(Currency.CHF, 6700.0),
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new Transaction(Currency.EUR, 5600.0),
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new Transaction(Currency.USD, 4500.0),
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new Transaction(Currency.CHF, 3400.0),
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new Transaction(Currency.GBP, 3200.0),
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new Transaction(Currency.USD, 4600.0),
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new Transaction(Currency.JPY, 5700.0),
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new Transaction(Currency.EUR, 6800.0));
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public static void main(String... args) {
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groupImperatively();
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groupFunctionally();
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}
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private static void groupImperatively() {
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Map<Currency, List<Transaction>> transactionsByCurrencies = new HashMap<>();
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for (Transaction transaction : transactions) {
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Currency currency = transaction.getCurrency();
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List<Transaction> transactionsForCurrency = transactionsByCurrencies.get(currency);
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if (transactionsForCurrency == null) {
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transactionsForCurrency = new ArrayList<>();
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transactionsByCurrencies.put(currency, transactionsForCurrency);
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}
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transactionsForCurrency.add(transaction);
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}
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System.out.println(transactionsByCurrencies);
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}
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private static void groupFunctionally() {
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Map<Currency, List<Transaction>> transactionsByCurrencies = transactions.stream().collect(groupingBy(Transaction::getCurrency));
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System.out.println(transactionsByCurrencies);
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}
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public static class Transaction {
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private final Currency currency;
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private final double value;
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public Transaction(Currency currency, double value) {
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this.currency = currency;
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this.value = value;
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}
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public Currency getCurrency() {
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return currency;
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}
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public double getValue() {
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return value;
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}
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@Override
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public String toString() {
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return currency + " " + value;
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}
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}
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public enum Currency {
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EUR,
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USD,
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JPY,
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GBP,
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CHF
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}
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/PuttingIntoPractice.java

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package com.brianway.learning.java8.streamapi;
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import static java.util.Comparator.comparing;
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import static java.util.stream.Collectors.toList;
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import java.util.Arrays;
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import java.util.List;
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public class PuttingIntoPractice {
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public static void main(String... args) {
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Trader raoul = new Trader("Raoul", "Cambridge");
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Trader mario = new Trader("Mario", "Milan");
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Trader alan = new Trader("Alan", "Cambridge");
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Trader brian = new Trader("Brian", "Cambridge");
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List<Transaction> transactions = Arrays.asList(
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new Transaction(brian, 2011, 300),
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new Transaction(raoul, 2012, 1000),
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new Transaction(raoul, 2011, 400),
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new Transaction(mario, 2012, 710),
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new Transaction(mario, 2012, 700),
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new Transaction(alan, 2012, 950)
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);
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// Query 1: Find all transactions from year 2011 and sort them by value (small to high).
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List<Transaction> tr2011 = transactions.stream()
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.filter(transaction -> transaction.getYear() == 2011)
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.sorted(comparing(Transaction::getValue))
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.collect(toList());
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System.out.println(tr2011);
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// Query 2: What are all the unique cities where the traders work?
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List<String> cities =
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transactions.stream()
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.map(transaction -> transaction.getTrader().getCity())
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.distinct()
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.collect(toList());
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System.out.println(cities);
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// Query 3: Find all traders from Cambridge and sort them by name.
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List<Trader> traders =
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transactions.stream()
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.map(Transaction::getTrader)
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.filter(trader -> trader.getCity().equals("Cambridge"))
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.distinct()
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.sorted(comparing(Trader::getName))
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.collect(toList());
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System.out.println(traders);
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// Query 4: Return a string of all traders’ names sorted alphabetically.
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String traderStr =
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transactions.stream()
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.map(transaction -> transaction.getTrader().getName())
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.distinct()
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.sorted()
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.reduce("", (n1, n2) -> n1 + n2);
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System.out.println(traderStr);
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// Query 5: Are there any trader based in Milan?
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boolean milanBased =
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transactions.stream()
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.anyMatch(transaction -> transaction.getTrader()
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.getCity()
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.equals("Milan")
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);
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System.out.println(milanBased);
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// Query 6: Update all transactions so that the traders from Milan are set to Cambridge.
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transactions.stream()
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.map(Transaction::getTrader)
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.filter(trader -> trader.getCity().equals("Milan"))
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.forEach(trader -> trader.setCity("Cambridge"));
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System.out.println(transactions);
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// Query 7: What's the highest value in all the transactions?
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int highestValue =
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transactions.stream()
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.map(Transaction::getValue)
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.reduce(0, Integer::max);
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System.out.println(highestValue);
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}
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/StreamBasic.java

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package com.brianway.learning.java8.streamapi;
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import static java.util.Comparator.comparing;
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import static java.util.stream.Collectors.toList;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.Comparator;
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import java.util.List;
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public class StreamBasic {
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public static void main(String... args) {
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// Java 7
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getLowCaloricDishesNamesInJava7(Dish.menu).forEach(System.out::println);
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System.out.println("---");
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// Java 8
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getLowCaloricDishesNamesInJava8(Dish.menu).forEach(System.out::println);
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}
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public static List<String> getLowCaloricDishesNamesInJava7(List<Dish> dishes) {
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List<Dish> lowCaloricDishes = new ArrayList<>();
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for (Dish d : dishes) {
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if (d.getCalories() < 400) {
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lowCaloricDishes.add(d);
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}
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}
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List<String> lowCaloricDishesName = new ArrayList<>();
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Collections.sort(lowCaloricDishes, new Comparator<Dish>() {
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public int compare(Dish d1, Dish d2) {
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return Integer.compare(d1.getCalories(), d2.getCalories());
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}
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});
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for (Dish d : lowCaloricDishes) {
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lowCaloricDishesName.add(d.getName());
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}
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return lowCaloricDishesName;
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}
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public static List<String> getLowCaloricDishesNamesInJava8(List<Dish> dishes) {
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return dishes.stream()
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.filter(d -> d.getCalories() < 400)
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.sorted(comparing(Dish::getCalories))
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.map(Dish::getName)
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.collect(toList());
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}
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}

‎java8/src/main/java/com/brianway/learning/java8/streamapi/Trader.java

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package com.brianway.learning.java8.streamapi;
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public class Trader {
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private String name;
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private String city;
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public Trader(String n, String c) {
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this.name = n;
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this.city = c;
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}
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public String getName() {
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return this.name;
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}
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public String getCity() {
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return this.city;
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}
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public void setCity(String newCity) {
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this.city = newCity;
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}
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public String toString() {
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return "Trader:" + this.name + " in " + this.city;
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}
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}

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