第五章(4)流的規約操作reduce
阿新 • • 發佈:2018-11-02
我們之前見識了流的終端操作返回了boolen,List,Optional等。那麼這一章節我們介紹諸如求和,找出最大值最小值的玩法。
1.元素求和(或積等其他運算)
這個,用之前的方法,我相信大家都會得:
List<Integer> nums = Arrays.asList(1,2,3,4,5,6,7);
int sum = 0;
for(Integer integer:nums) {
sum+=integer;
}
System.out.println(sum);//28
下面是流操作的求和運算:
List<Integer> nums = Arrays.asList(1,2,3,4,5,6,7); int sum = nums.stream().reduce(0, (a,b)->a+b); System.out.println(sum);//28
解釋一下上面程式碼的意思:
reduce接受兩個引數:
•一個初始值,這裡是0;
•一個BinaryOperator<T>來將兩個元素結合起來產生一個新值,這裡我們用的是lambda(a, b) -> a + b。
你也可以使用方法引用的方式上程式碼變得更簡潔:
int sum = nums.stream().reduce(0, Integer::sum);
此外reduce還有還有一個過載的方法,它沒有初始值,使用Optional接收結果:
List<Integer> nums = Arrays.asList(1,2,3,4,5,6,7); Optional<Integer> optional = nums.stream().reduce(Integer::sum); System.out.println(optional.get());//28
2.最大值和最小值
List<Integer> nums = Arrays.asList(1,2,3,4,5,6,7);
Optional<Integer> optional = nums.stream().reduce(Integer::max);
System.out.println(optional.get());//7
Optional<Integer> optional2 = nums.stream().reduce(Integer::min);
System.out.println(optional2.get());//1
練習題:使用reduce來統計一下選單中有多少菜:
當然,你可以使用Stream中的count方法來統計:
long count = menu.stream().count();
如果使用規約reduce,需要藉助於map:
int count = menu.stream().map(d ->1).reduce(0, Integer::sum);
System.out.println(count);
把每個菜的物件對映為數值1,讓他們相加。
習題:
交易員類:
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;
}
}
交易類:
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 +"}";
}
}
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));
八道題:
1.找出2011年的交易,降序排列
2.交易員在哪些城市不同的城市工作
3.找來自劍橋的交易員,按名字排序
4.返回所有交易員的姓名字串,按字母順序排序
5.有沒有在米蘭工作的?
6.列印在劍橋工作的所有交易員的交易金額
7.所有交易額中最高交易額是多少
8.找出交易額最小的交易
答案:
System.out.println("找出2011年的交易,降序排列");
List<Transaction> transactions2011 = transactions.parallelStream()
.filter(t->t.getYear()==2011)
.sorted(Comparator.comparing(Transaction::getValue).reversed())
.collect(Collectors.toList());
transactions2011.parallelStream().forEach(t->System.out.println(t.getYear()+":"+t.getValue()));
System.out.println("----------------------------------------------------");
System.out.println("交易員在哪些城市不同的城市工作");
List<String> citys = transactions.parallelStream()
.map(Transaction::getTrader)
.map(Trader::getCity)
.distinct()
.collect(Collectors.toList());
citys.parallelStream().forEach(System.out::println);
System.out.println("----------------------------------------------------");
System.out.println("找來自劍橋的交易員,按名字排序");
List<Trader> traders = transactions.parallelStream()
.map(Transaction::getTrader)
.filter(t ->"Cambridge".equals(t.getCity()))
.distinct()
.sorted(Comparator.comparing(Trader::getName))
.collect(Collectors.toList());
traders.parallelStream().forEach(trader->System.out.println(trader.getName()+":"+trader.getCity()));
System.out.println("-----------------------------------------------------");
System.out.println("返回所有交易員的姓名字串,按字母順序排序");
String names = transactions.parallelStream()
.map(transaction -> transaction.getTrader().getName())
.distinct()
.sorted()
.reduce("", (n1,n2)->n1+n2);
System.out.println(names);
System.out.println("------------------------------------------------------");
System.out.println("有沒有在米蘭工作的?");
boolean b = transactions.parallelStream()
.anyMatch(t -> t.getTrader().getCity().equals("Milan"));
System.out.println(b);
System.out.println("-------------------------------------------------------");
System.out.println("列印在劍橋工作的所有交易員的交易金額");
List<Integer> values = transactions.parallelStream()
.filter(transaction -> "Cambridge".equals(transaction.getTrader().getCity()))
.map(Transaction::getValue)
.collect(Collectors.toList());
values.parallelStream().forEach(System.out::println);
System.out.println("-------------------------------------------------------");
System.out.println("所有交易額中最高交易額是多少");
transactions.parallelStream().
map(Transaction::getValue)
.reduce(Integer::max)
.ifPresent(System.out::println);
System.out.println("---------------------------------------------------------");
System.out.println("找出交易額最小的交易");
Optional<Transaction> smallestTransaction = transactions.stream().reduce((t1, t2) -> t1.getValue() < t2.getValue() ? t1 : t2);
smallestTransaction.ifPresent(t -> System.out.println(t.getValue()));
//或者
Optional<Transaction> smallestTransaction2 = transactions.stream() .min(Comparator.comparing(Transaction::getValue));
smallestTransaction2.ifPresent(t -> System.out.println(t.getValue()));