Java 8 新特性
Lambda、函数式接口、Stream、Optional 与新的日期 API。
Lambda 表达式
(params) -> expr is shorthand for an anonymous function, usable only with functional interfaces (one abstract method). It makes passing behaviour as easy as passing a value.
import java.util.ArrayList;
import java.util.List;
public class LambdaDemo {
public static void main(String[] args) {
Runnable r = () -> System.out.println("run");
r.run();
List<String> list = new ArrayList<>(List.of("b", "a"));
list.sort((x, y) -> x.compareTo(y));
System.out.println(list);
list.forEach(s -> System.out.print(s + " "));
System.out.println();
}
}run
[a, b]
a b 函数式接口与方法引用
Function<T,R>, Consumer<T>, Supplier<T> and Predicate<T> are the standard functional interfaces. A method reference Type::method is a further shorthand.
import java.util.function.Function;
import java.util.function.Predicate;
public class FunctionalDemo {
public static void main(String[] args) {
Function<String, Integer> len = String::length;
Predicate<String> empty = String::isEmpty;
System.out.println(len.apply("hello"));
System.out.println(empty.test(""));
System.out.println(len.andThen(n -> n * 2).apply("abc"));
}
}5
true
6Stream 流
A Stream is a pipeline: filter, map, sorted, collect, reduce. It is lazy — nothing runs until a terminal operation is invoked.
import java.util.List;
import java.util.stream.Collectors;
public class StreamDemo {
public static void main(String[] args) {
List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);
List<Integer> evens = nums.stream()
.filter(n -> n % 2 == 0)
.map(n -> n * 10)
.collect(Collectors.toList());
System.out.println(evens);
System.out.println(nums.stream().mapToInt(Integer::intValue).sum());
System.out.println(nums.stream().anyMatch(n -> n > 5));
System.out.println(nums.stream().count());
}
}[20, 40, 60]
21
true
6Optional
Optional makes "maybe absent" explicit and forces the caller to handle it: orElse for defaults, map to transform, ifPresent to run conditionally. Do not use it for fields or parameters.
import java.util.Optional;
public class OptionalDemo {
public static void main(String[] args) {
Optional<String> a = Optional.of("x");
Optional<String> b = Optional.empty();
System.out.println(a.isPresent());
System.out.println(b.isPresent());
System.out.println(b.orElse("default"));
System.out.println(a.map(String::length).orElse(0));
}
}true
false
default
1Lambda 表达式
(参数) -> 表达式 是匿名函数的简写，只能用于函数式接口（只有一个抽象方法的接口）。它让「传行为」变得和传值一样简单。
import java.util.ArrayList;
import java.util.List;
public class LambdaDemo {
public static void main(String[] args) {
Runnable r = () -> System.out.println("run");
r.run();
List<String> list = new ArrayList<>(List.of("b", "a"));
list.sort((x, y) -> x.compareTo(y));
System.out.println(list);
list.forEach(s -> System.out.print(s + " "));
System.out.println();
}
}run
[a, b]
a b 函数式接口与方法引用
Function<T,R>、Consumer<T>、Supplier<T>、Predicate<T> 是标准库提供的常用函数式接口。方法引用 类::方法 是 lambda 的进一步简化。
import java.util.function.Function;
import java.util.function.Predicate;
public class FunctionalDemo {
public static void main(String[] args) {
Function<String, Integer> len = String::length;
Predicate<String> empty = String::isEmpty;
System.out.println(len.apply("hello"));
System.out.println(empty.test(""));
System.out.println(len.andThen(n -> n * 2).apply("abc"));
}
}5
true
6Stream 流
Stream 是「流水线」：filter 筛选、map 转换、sorted 排序、collect 收集、reduce 聚合。它是惰性的 —— 没有终止操作就什么也不做。
import java.util.List;
import java.util.stream.Collectors;
public class StreamDemo {
public static void main(String[] args) {
List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);
List<Integer> evens = nums.stream()
.filter(n -> n % 2 == 0)
.map(n -> n * 10)
.collect(Collectors.toList());
System.out.println(evens);
System.out.println(nums.stream().mapToInt(Integer::intValue).sum());
System.out.println(nums.stream().anyMatch(n -> n > 5));
System.out.println(nums.stream().count());
}
}[20, 40, 60]
21
true
6Optional
Optional 显式表示「可能没有值」，逼调用方处理空的情况。orElse 给默认值、map 链式转换、ifPresent 有值才执行。不要用它做字段类型或参数类型。
import java.util.Optional;
public class OptionalDemo {
public static void main(String[] args) {
Optional<String> a = Optional.of("x");
Optional<String> b = Optional.empty();
System.out.println(a.isPresent());
System.out.println(b.isPresent());
System.out.println(b.orElse("default"));
System.out.println(a.map(String::length).orElse(0));
}
}true
false
default
1