JAVA 8 实战记录

常用API

实际案例

//Trader类
public class Trader{
	private final String name;
	private final 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 String toString(){
	return "Trader:"+this.name + " in " + this.city;
	}  
}

//Transaction类
public class Transaction{
private final Trader trader;
private final int year;
private final 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 +"}";
	}
}

//赋值
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 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)
);

找出2011年的所有交易并按交易额排序（从低到高）

List tr2011 =transactions.stream().filter(transaction -> transaction.getYear() == 2011).sorted(comparing(Transaction::getValue)).collect(toList());

交易员都在哪些不同的城市工作过

List cities =transactions.stream().map(transaction -> transaction.getTrader().getCity()).distinct().collect(toList());

可以去掉 distinct() ，改用 toSet() ，这样就会把流转换为集合)

Set cities =transactions.stream().map(transaction -> transaction.getTrader().getCity()).collect(toSet());

查找所有来自于剑桥的交易员，并按姓名排序

List traders =transactions.stream().map(Transaction::getTrader).filter(trader -> trader.getCity().equals("Cambridge")).distinct().sorted(comparing(Trader::getName)).collect(toList());

返回所有交易员的姓名字符串，按字母顺序排序

String traderStr =transactions.stream().map(transaction -> transaction.getTrader().getName()).distinct().sorted().reduce("", (n1, n2) -> n1 + n2);

此解决方案效率不高（所有字符串都被反复连接，每次迭代的时候都要建立一个新的 String 对象),使用 joining （其内部会用到 StringBuilder ）

String traderStr =transactions.stream().map(transaction -> transaction.getTrader().getName()).distinct().sorted().collect(joining());

有没有交易员是在米兰工作的

boolean milanBased =transactions.stream().anyMatch(transaction -> transaction.getTrader().getCity().equals("Milan"));

打印生活在剑桥的交易员的所有交易额

transactions.stream().filter(t -> "Cambridge".equals(t.getTrader().getCity())).map(Transaction::getValue).forEach(System.out::println);

所有交易中，最高的交易额是多少

Optional highestValue =transactions.stream().map(Transaction::getValue).reduce(Integer::max);

找到交易额最小的交易

Optional smallestTransaction =transactions.stream().reduce((t1, t2) ->t1.getValue() < t2.getValue() ? t1 : t2);

流支持 min 和 max 方法,可以接受一个 Comparator 作为参数，指定计算最小或最大值时要比较哪个键值：

Optional smallestTransaction =transactions.stream().min(comparing(Transaction::getValue));