Java 8 Lambda Expressions, Functional Interfaces, and Stream API Tutorial

Java 8 brings lambda expressions, enabling functional programming by passing behavior as immutable values to functions. A functional interface is an interface with a single abstract method, optionally annotated with @FunctionalInterface, and can contain default and static methods.

Example code shows how to use built‑in functional interfaces such as Predicate , Consumer , Function , Supplier , UnaryOperator , and BinaryOperator , as well as a custom interface Worker with a test method.

public class Test {
    public static void main(String[] args) {
        Predicate
predicate = x -> x > 185;
        Student student = new Student("9龙", 23, 175);
        System.out.println("9龙的身高高于185吗？：" + predicate.test(student.getStature()));

        Consumer
consumer = System.out::println;
        consumer.accept("命运由我不由天");

        Function
function = Student::getName;
        String name = function.apply(student);
        System.out.println(name);

        Supplier
supplier = () -> Integer.valueOf(BigDecimal.TEN.toString());
        System.out.println(supplier.get());

        UnaryOperator
unaryOperator = uglily -> !uglily;
        Boolean apply2 = unaryOperator.apply(true);
        System.out.println(apply2);

        BinaryOperator
operator = (x, y) -> x * y;
        Integer integer = operator.apply(2, 3);
        System.out.println(integer);

        test(() -> "我是一个演示的函数式接口");
    }

    /**
     * 演示自定义函数式接口使用
     */
    public static void test(Worker worker) {
        String work = worker.work();
        System.out.println(work);
    }

    public interface Worker {
        String work();
    }
}

The article then moves to the Stream API, explaining lazy (intermediate) and eager (terminal) evaluation. It demonstrates common stream operations:

collect(Collectors.toList()) – converts a stream to a list.

filter – uses a Predicate to keep elements, e.g., students with height < 180.

map – transforms each element, e.g., extracting student names.

flatMap – flattens multiple streams into one.

max / min – finds the maximum or minimum element using a Comparator , returning an Optional .

count – counts elements after a filter.

reduce – aggregates values, illustrated by summing integers.

// collect example
List
studentList = Stream.of(
        new Student("路飞", 22, 175),
        new Student("红发", 40, 180),
        new Student("白胡子", 50, 185)
).collect(Collectors.toList());
System.out.println(studentList);

// filter example
List
list = students.stream()
        .filter(stu -> stu.getStature() < 180)
        .collect(Collectors.toList());
System.out.println(list);

// map example
List
names = students.stream()
        .map(Student::getName)
        .collect(Collectors.toList());
System.out.println(names);

// flatMap example
List
studentList = Stream.of(students,
        asList(new Student("艾斯", 25, 183), new Student("雷利", 48, 176)))
        .flatMap(students1 -> students1.stream())
        .collect(Collectors.toList());
System.out.println(studentList);

// max/min example
Optional
max = students.stream()
        .max(Comparator.comparing(stu -> stu.getAge()));
Optional
min = students.stream()
        .min(Comparator.comparing(stu -> stu.getAge()));
if (max.isPresent()) System.out.println(max.get());
if (min.isPresent()) System.out.println(min.get());

// count example
long count = students.stream().filter(s -> s.getAge() < 45).count();
System.out.println("年龄小于45岁的人数是：" + count);

// reduce example
Integer reduce = Stream.of(1, 2, 3, 4).reduce(0, (acc, x) -> acc + x);
System.out.println(reduce);

Advanced collectors are covered, including:

partitioningBy – splits a stream into two groups based on a predicate.

groupingBy – groups elements by a key function, similar to SQL's GROUP BY.

joining – concatenates strings from a stream with optional delimiters, prefix, and suffix.

// partitioningBy example
Map
> listMap = students.stream()
        .collect(Collectors.partitioningBy(student ->
                student.getSpecialities().contains(SpecialityEnum.SING)));

// groupingBy example
Map
> listMap = students.stream()
        .collect(Collectors.groupingBy(student ->
                student.getSpecialities().get(0)));

// joining example
String names = students.stream()
        .map(Student::getName)
        .collect(Collectors.joining(",", "[", "]"));
System.out.println(names);

The article concludes that using Java 8's functional features makes code more expressive and concise, encouraging readers to refactor existing codebases with lambda expressions and stream operations.