Java8实战Lambda和Stram API学习

Posted 2020-10-31 明湖樵夫

public  class Trader{
    
    private String name;
    private String city;

    public Trader(String n, String c){
        this.name = n;
        this.city = c;
    }

    public String getName(){
        return this.name;
    }

    public String getCity(){
        return this.city;
    }

    public void setCity(String newCity){
        this.city = newCity;
    }

    public String toString(){
        return "Trader:"+this.name + " in " + this.city;
    }
}

 

public class Transaction{

    private Trader trader;
    private int year;
    private int value;

    public Transaction(Trader trader, int year, int value)
    {
        this.trader = trader;
        this.year = year;
        this.value = value;
    }

    public Trader getTrader(){
        return this.trader;
    }

    public int getYear(){
        return this.year;
    }

    public int getValue(){
        return this.value;
    }
    
    public String toString(){
        return "{" + this.trader + ", " +
               "year: "+this.year+", " +
               "value:" + this.value +"}";
    }
}

 

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