J8 lambda表达式案例

1.使用() -> {} 替代匿名类

(params) -> expression
(params) -> statement
(params) -> { statements }
没带大括号的时候，默认返回expression或statement的值，即 return expression or statement。反之，如果有大括号的时候，同时方法必须有返回值的时候，就需要带return

new Thread( () -> System.out.println("In Java8!") ).start();

new ArrayList().sort((o1,o2)->o1==null?1:-1);

new ArrayList().sort((o1,o2)->{
     if(o1==null)
         return 1;
     else
         return 0;
 });

2.使用Lambda表达式遍历List集合

new ArrayList().forEach(n -> System.out.println(n));

// Even better use Method reference feature of Java 8
// method reference is denoted by :: (double colon) operator
// looks similar to score resolution operator of C++
new ArrayList().forEach(System.out::println);

//多个入参
new ArrayList().sort(Lambda::testDoubleColon);

3.使用Lambda表达式和函数接口

List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");

//带J的字符串
System.out.println("Languages which starts with J :");
filter(languages, (str)->str.startsWith("J"));

//以a结束
System.out.println("Languages which ends with a ");
filter(languages, (str)->str.endsWith("a"));

//返回true
System.out.println("Print all languages :");
filter(languages, (str)->true);

//返回false
System.out.println("Print no language : ");
filter(languages, (str)->false);

 //字符串长度大于4
System.out.println("Print language whose length greater than 4:");
filter(languages, (str)->str.length() > 4);

filter方法

public static void filter(List names, Predicate condition) {
        for(String name: names)  {
            if(condition.test(name)) {
                System.out.println(name + " ");
            }
        }
    }

Predicate提供and(), or() 和 xor()可以进行逻辑操作

List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");

Predicate startsWithJ = (n) -> n.startsWith("J");
Predicate fourLetterLong = (n) -> n.length() == 4;

//startsWithJ.and(fourLetterLong) 与操作
languages.stream().filter(startsWithJ.and(fourLetterLong))
.forEach((n) -> System.out.print("\nName, which starts with 'J' and four letter long is : " + n));

4.使用Lambda实现Map 和 Reduce

• stream函数中的map，允许你重新构造新的对象集合
  reduce(result,element) 函数是将集合中所有值结合进一个，类似SQL语句中的sum(), avg() 或count()

List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
double total = 0;
for (Integer cost : costBeforeTax) {
      double price = cost + .12*cost;
      total = total + price;
}
System.out.println("Total : " + total);

 // New way:
double bill = costBeforeTax.stream()
             .map((cost) -> cost + .12*cost)
             .reduce((result, cost) -> result + cost)
             .get();
System.out.println("Total : " + bill);

5. Filter 函数

• Filtering是对大型Collection操作的一个通用操作，Stream提供filter()方法，接受一个Predicate对象，意味着你能传送lambda表达式作为一个过滤逻辑进入这个方法filter 过滤

List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");
List filtered = languages.stream()
              .filter(x -> x.length()> 4)
              .collect(Collectors.toList());
System.out.printf("Original List : %s, filtered list : %s %n", languages, filtered);

6. Collect 函数

• 将stream对象反构造成list或者map

List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");
String languagesString = languages.stream()
                    .map(x -> x.toUpperCase())
                    .collect(Collectors.joining(", "));
System.out.println(languagesString);

7. Distinct 函数

• 过滤集合中重复元素

 List numbers = Arrays.asList(9, 10, 3, 4, 7, 3, 4);
 List distinct = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList());
 System.out.printf("Original List : %s,  Square Without duplicates : %s %n", numbers, distinct);

8. IntSummaryStatistics 对象

• IntSummaryStatistics 求平均、最大、最小、总数

List primes = Arrays.asList(2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
IntSummaryStatistics stats = primes.stream()
                .mapToInt((x) -> x)
                .summaryStatistics();
System.out.println("Highest prime number in List : " + stats.getMax());
System.out.println("Lowest prime number in List : " + stats.getMin());
System.out.println("Sum of all prime numbers : " + stats.getSum());
System.out.println("Average of all prime numbers : " + stats.getAverage());