// Example 187 from page 147

//

import java.util.HashMap;

import java.util.Map;

import java.util.Optional;

class Example187 {

public static void main(String[] args) {

Map<String,String> form = new HashMap<String,String>();

form.put("area", "144.0");

doubleExampleA(form);

doubleExampleB(form);

doubleExampleC(form);

}

public static void doubleExampleA(Map<String,String> form) {

String areaString = form.get("area"), toPrint = "No value";

if (areaString != null) {

try {

double areaValue = Double.parseDouble(areaString);

double result = Math.sqrt(areaValue);

if (!Double.isNaN(result))

toPrint = String.valueOf(result);

} catch (NumberFormatException exn) { }

}

System.out.println(toPrint);

}

public static void doubleExampleB(Map<String,String> form) {

Optional<String> areaString = Optional.<String>ofNullable(form.get("area"));

Optional<Double> areaValue = areaString.flatMap(s -> parseDouble(s));

Optional<Double> result = areaValue.flatMap(v -> sqrt(v));

System.out.println(result.map(String::valueOf).orElse("No value"));

}

public static void doubleExampleC(Map<String,String> form) {

String toPrint = Optional.<String>ofNullable(form.get("area"))

.flatMap(s -> parseDouble(s))

.flatMap(v -> sqrt(v))

.map(String::valueOf)

.orElse("No value");

System.out.println(toPrint);

}

public static Optional<Double> parseDouble(String s) {

try {

return Optional.<Double>of(Double.parseDouble(s));

} catch (NumberFormatException exn) {

return Optional.<Double>empty();

}

}

public static Optional<Double> sqrt(double x) {

return x < 0.0 ? Optional.<Double>empty() : Optional.<Double>of(Math.sqrt(x));

}

}