Java 函数式接口

函数式接口(Functional Interface)就是一个有且仅有一个抽象方法，但是可以有多个非抽象方法的接口。函数式接口可以被隐式转换为 lambda 表达式。Lambda 表达式和方法引用（实际上也可认为是Lambda表达式）上。

如定义了一个函数式接口，使用Lambda表达式来表示该接口的一个实现(注：JAVA 8 之前一般是用匿名类实现的)如下：

@FunctionalInterface
public interface CustomInterface {
     T  get();
}
class Demo{
    public static void main(String[] args) {
        String[] arr = {"hello","world"};
        //Arrays.asList(arr) 默认返回类型为ArrayList
        CustomInterface> ci = ()-> Arrays.asList(arr);
        List list = ci.get();
    }
}

jdk1.8中新加入很多函数式接口，以供函数式编程使用，下面介绍一下：

• Supplier 结果提供者，具有生成一个结果的能力，举个栗子：

@Slf4j
public class Demostration  {
    public static final DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    public static  void main(String[] args) {
        //栗子1：返回一个当前日期时间
        Supplier localDate = LocalDateTime::now;
        LocalDateTime dateTime = localDate.get();
        log.info("now:{}",dateTime);
        //返回格式化时间
        Supplier dateTimeFormat = ()->dateTime.format(dateTimeFormatter);
        log.info("date:{}",dateTimeFormat.get());
        //返回数组
        String[] arr = new String[]{"hello","world","every","body!"};
        Supplier> s = () -> Arrays.asList(arr);
        List list1 = s.get();
        log.info("list:{}",list1);

        //栗子2：返回一个supplier
        Demostration obj= new Demostration<>();
        List lis2 = obj.supplier().get();
        Demostration obj2= new Demostration<>();
        List lis3 = obj2.supplier().get();

        //栗子3，生成数据
        String desc = product(() -> new CompanySlave("coder"),e-> "\t姓名:"+e.getName()+"\t薪资:"+e.getSalary()+"\t是否加班:"+e.isWorkOvertime());
        log.info("slave:{}",desc);

        

    }
    public static String product(
            Supplier supplier,
            Function function){
        CompanySlave slave = supplier.get();
        if (slave.getName()==null && "".equals(slave.getName())){
            slave.setName("slave");
        }
        slave.setSalary(BigDecimal.ZERO);
        slave.setWorkOvertime(true);
        return function.apply(slave);
    }
    public Supplier> supplier(){
        // lambda
        // return () -> new ArrayList<>();

        // constructor reference
        return ArrayList::new;
    }
}

@Getter
@Setter
class CompanySlave{
    String name;
    BigDecimal salary;
    boolean workOvertime;

    public CompanySlave() {
    }

    public CompanySlave(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "CompanySlave{" +
                "name='" + name + '\'' +
                ", salary=" + salary +
                ", workOvertime=" + workOvertime +
                '}';
    }
}

        //再看看jdk里Logger的使用
        /**
         * Log a message, which is only to be constructed if the logging level
         * is such that the message will actually be logged.
         * 

         * If the logger is currently enabled for the given message
         * level then the message is constructed by invoking the provided
         * supplier function and forwarded to all the registered output
         * Handler objects.
         * 

         * @param   level   One of the message level identifiers, e.g., SEVERE
         * @param   msgSupplier   A function, which when called, produces the
         *                        desired log message
         * @since 1.8
         */
        public void log(Level level, Supplier msgSupplier) {
            if (!isLoggable(level)) {
                return;
            }
            LogRecord lr = new LogRecord(level, msgSupplier.get());
            doLog(lr);
        }

类似还有限定数据类型的函数式接口：

• IntSupplier
• LongSupplier
• DoubleSupplier
• BooleanSupplier

未完待续。。。