lambda表达式

lambda表达式 函数式接口

@FunctionalInterface
public interface MathOperation {

    int operation(int a, int b);

    static int sum(int a, int b) {
        return a + b;
    }
}

可以这样写：

MathOperation operation = (a, b) -> a + b;
operation.operation(2, 3);

方法引用::

以上是两数相加，其实MathOperation已经有sum函数，又可以写为：

MathOperation operation = (a, b) -> MathOperation.sum(a, b);

用 :: 可以简化为：

MathOperation operation = MathOperation::sum;

forEach

List letters = Arrays.asList("a", "b", "c");
        for (String letter : letters) {
            System.out.println(letter);
        }

可以这样写：

List letters = Arrays.asList("a", "b", "c");
        letters.forEach(n -> System.out.println(n));
        // 使用方法引用::简化为：
        letters.forEach(System.out::println);

Predicate

List letters = Arrays.asList("a", "ab", "c");
        filter(letters, n -> n.startsWith("a"));
        
public void filter(List letters, Predicate condition) {
        letters.forEach(letter -> {
            if (condition.test(letter)) {
                System.out.println(letter + ".");
            }
        });
    }

可以使用stream API简化，filter参数也是Predicate：

letters.stream().filter(n -> n.startsWith("a")).forEach(n -> System.out.println(n + "."));

Predicate 有 and、or等default方法：

Predicate predicate = n -> n.startsWith("a");
        letters.stream().filter(predicate.and(n -> n.length() == 2))
                .forEach(n -> System.out.println(n + "."));

Map

List letters = Arrays.asList("a", "ab", "c");
        letters.stream().map((String letter) -> letter + ".").forEach(System.out::println);

reduce()

List letters = Arrays.asList("a", "ab", "c");
        String s = letters.stream().map((String letter) -> letter + ".")
                .reduce((append, letter) -> append + letter).get();
        System.out.println(s);

collect()

List letters = Arrays.asList("ab", "ab", "c");
        List list = letters.stream().filter(letter -> letter.contains("a"))
                .map(String::toUpperCase)
                .filter(letter -> letter.contains("A"))
                .distinct()
                .collect(Collectors.toList());
        

summaryStatistics

List letters = Arrays.asList("1", "2", "3");
        IntSummaryStatistics statistics = letters.stream().mapToInt(Integer::parseInt).summaryStatistics();
        int min = statistics.getMin();
        int max = statistics.getMax();
        long sum = statistics.getSum();
        double average = statistics.getAverage();
        long count = statistics.getCount();