Learn-Java-By-Example/src/main/java/com/lianglliu/java8/collectors/usage/GroupingTransactions.java at master · LiangLliu/Learn-Java-By-Example · GitHub

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package com.lianglliu.java8.collectors.usage;

import com.lianglliu.java8.collectors.model.Currency;
import com.lianglliu.java8.collectors.model.Transaction;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupingTransactions {

    public static List<Transaction> transactions = Arrays.asList(
            new Transaction(Currency.EUR, 1500.0),
            new Transaction(Currency.USD, 2300.0),
            new Transaction(Currency.GBP, 9900.0),
            new Transaction(Currency.EUR, 1100.0),
            new Transaction(Currency.JPY, 7800.0),
            new Transaction(Currency.CHF, 6700.0),
            new Transaction(Currency.EUR, 5600.0),
            new Transaction(Currency.USD, 4500.0),
            new Transaction(Currency.CHF, 3400.0),
            new Transaction(Currency.GBP, 3200.0),
            new Transaction(Currency.USD, 4600.0),
            new Transaction(Currency.JPY, 5700.0),
            new Transaction(Currency.EUR, 6800.0)
    );

    public static void main(String[] args) {
        groupImperatively();
        groupFunctionally();
    }

    /**
     * <pre> {@code
     *  var transactionsForCurrency =
     *      transactionsByCurrencies.get(currency);
     *  if(transactionsForCurrency ==null)
     *  {
     *      transactionsForCurrency = new ArrayList<>();
     *      transactionsByCurrencies.put(currency, transactionsForCurrency);
     *  }
     *  transactionsForCurrency.add(transaction);
     *
     *  等价于 ==>>
     *
     *  var transactionsForCurrency =
     *      transactionsByCurrencies.computeIfAbsent(currency, k -> new ArrayList<>());
     *  transactionsForCurrency.add(transaction);
     * }</pre>
     */
    private static void groupImperatively() {
        Map<Currency, List<Transaction>> transactionsByCurrencies = new HashMap<>();
        for (Transaction transaction : transactions) {
            Currency currency = transaction.getCurrency();

            var transactionsForCurrency =
                    transactionsByCurrencies.computeIfAbsent(currency, k -> new ArrayList<>());
            transactionsForCurrency.add(transaction);
        }
        System.out.println(transactionsByCurrencies);
    }

    private static void groupFunctionally() {
        Map<Currency, List<Transaction>> transactionsByCurrencies = transactions.stream()
                .collect(Collectors.groupingBy(Transaction::getCurrency));
        System.out.println(transactionsByCurrencies);
    }
}