java8-科里化与设计模式

科里化

常用于单位转换：
科里化是一种将具备2个参数（比如，x和y）的函数f转化为使用一个参数的函数g，并且这个函数的返回值也是一个函数，它会作为新函数的一个参数。后者的返回值和初始函数的返回值相同，即f(x,y)=(g(x))(y)。

    static double converter(double x, double conversionFactor, double baseline) {
        return x * conversionFactor + baseline;
    }

    static DoubleUnaryOperator curriedConverter(double y, double z) {
        return (double x) -> x * y + z;
    }

    double waterBoilingPointInF = converter(100, 9.0 / 5, 32);
    System.out.println(waterBoilingPointInF);

    DoubleUnaryOperator convertCtoF = curriedConverter(9.0 / 5, 32);//摄氏转为华氏温度
    DoubleUnaryOperator convertUSDtoGBP = curriedConverter(0.6, 0);//美元到人民币
    DoubleUnaryOperator convertKmtoMi = curriedConverter(0.6214, 0);//公里到英里

    System.out.println(convertCtoF.applyAsDouble(100.0));
    System.out.println(convertUSDtoGBP.applyAsDouble(100.0));
    System.out.println(convertUSDtoGBP.applyAsDouble(200.0));

调试

java8中涉及到lambda表达式的栈跟踪不那么友好，较难理解。
调试时可以使用日志调试。

       Stream.of(2, 3, 4, 5)
                .peek(x -> System.out.println("taking from stream: " + x))
                .map(x -> x + 17)
                .peek(x -> System.out.println("after map: " + x))
                .filter(x -> x % 2 == 0)
                .peek(x -> System.out.println("after filter: " + x))
                .limit(3)
                .peek(x -> System.out.println("after limit: " + x))
                .collect(toList());

设计模式

策略模式：

public class StrategyMain {

    public static void main(String[] args) {
        // old school
        Validator v1 = new Validator(new IsNumeric());
        System.out.println(v1.validate("aaaa"));
        Validator v2 = new Validator(new IsAllLowerCase ());
        System.out.println(v2.validate("bbbb"));


        // with lambdas
        Validator v3 = new Validator((String s) -> s.matches("\\d+"));
        System.out.println(v3.validate("aaaa"));
        Validator v4 = new Validator((String s) -> s.matches("[a-z]+"));
        System.out.println(v4.validate("bbbb"));
    }

    interface ValidationStrategy {
        public boolean execute(String s);
    }

    static private class IsAllLowerCase implements ValidationStrategy {
        public boolean execute(String s){
            return s.matches("[a-z]+");
        }
    }
    static private class IsNumeric implements ValidationStrategy {
        public boolean execute(String s){
            return s.matches("\\d+");
        }
    }

    static private class Validator{
        private final ValidationStrategy strategy;
        public Validator(ValidationStrategy v){
            this.strategy = v;
        }
        public boolean validate(String s){
            return strategy.execute(s); }
    }
}

参考

Java 8新特性终极指南 - ImportNew