jukzi:
La referencia método usado tiene tipo de retorno Integer. Sin embargo, una incompatibles Stringestá permitido en el siguiente ejemplo.
Cómo fijar el método de withdeclaración para obtener el tipo de seguridad Método de referencia sin poner manualmente?
import java.util.function.Function;
public class MinimalExample {
static public class Builder<T> {
final Class<T> clazz;
Builder(Class<T> clazz) {
this.clazz = clazz;
}
static <T> Builder<T> of(Class<T> clazz) {
return new Builder<T>(clazz);
}
<R> Builder<T> with(Function<T, R> getter, R returnValue) {
return null; //TODO
}
}
static public interface MyInterface {
Integer getLength();
}
public static void main(String[] args) {
// missing compiletimecheck is inaceptable:
Builder.of(MyInterface.class).with(MyInterface::getLength, "I am NOT an Integer");
// compile time error OK:
Builder.of(MyInterface.class).with((Function<MyInterface, Integer> )MyInterface::getLength, "I am NOT an Integer");
// The method with(Function<MinimalExample.MyInterface,R>, R) in the type MinimalExample.Builder<MinimalExample.MyInterface> is not applicable for the arguments (Function<MinimalExample.MyInterface,Integer>, String)
}
}
Caso de uso: una caja fuerte, pero el tipo genérico del constructor.
Traté de poner en práctica un constructor genérico sin procesamiento de anotación (autovalor) o plug-in compilador (Lombok)
import java.lang.reflect.Array;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;
public class BuilderExample {
static public class Builder<T> implements InvocationHandler {
final Class<T> clazz;
HashMap<Method, Object> methodReturnValues = new HashMap<>();
Builder(Class<T> clazz) {
this.clazz = clazz;
}
static <T> Builder<T> of(Class<T> clazz) {
return new Builder<T>(clazz);
}
Builder<T> withMethod(Method method, Object returnValue) {
Class<?> returnType = method.getReturnType();
if (returnType.isPrimitive()) {
if (returnValue == null) {
throw new IllegalArgumentException("Primitive value cannot be null:" + method);
} else {
try {
boolean isConvertable = getDefaultValue(returnType).getClass().isAssignableFrom(returnValue.getClass());
if (!isConvertable) {
throw new ClassCastException(returnValue.getClass() + " cannot be cast to " + returnType + " for " + method);
}
} catch (IllegalArgumentException | SecurityException e) {
throw new RuntimeException(e);
}
}
} else if (returnValue != null && !returnType.isAssignableFrom(returnValue.getClass())) {
throw new ClassCastException(returnValue.getClass() + " cannot be cast to " + returnType + " for " + method);
}
Object previuos = methodReturnValues.put(method, returnValue);
if (previuos != null) {
throw new IllegalArgumentException("Value alread set for " + method);
}
return this;
}
static HashMap<Class, Object> defaultValues = new HashMap<>();
private static <T> T getDefaultValue(Class<T> clazz) {
if (clazz == null || !clazz.isPrimitive()) {
return null;
}
@SuppressWarnings("unchecked")
T cachedDefaultValue = (T) defaultValues.get(clazz);
if (cachedDefaultValue != null) {
return cachedDefaultValue;
}
@SuppressWarnings("unchecked")
T defaultValue = (T) Array.get(Array.newInstance(clazz, 1), 0);
defaultValues.put(clazz, defaultValue);
return defaultValue;
}
public synchronized static <T> Method getMethod(Class<T> clazz, java.util.function.Function<T, ?> resolve) {
AtomicReference<Method> methodReference = new AtomicReference<>();
@SuppressWarnings("unchecked")
T proxy = (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[] { clazz }, new InvocationHandler() {
@Override
public Object invoke(Object p, Method method, Object[] args) {
Method oldMethod = methodReference.getAndSet(method);
if (oldMethod != null) {
throw new IllegalArgumentException("Method was already called " + oldMethod);
}
Class<?> returnType = method.getReturnType();
return getDefaultValue(returnType);
}
});
resolve.apply(proxy);
Method method = methodReference.get();
if (method == null) {
throw new RuntimeException(new NoSuchMethodException());
}
return method;
}
// R will accep common type Object :-( // see https://stackoverflow.com/questions/58337639
<R, V extends R> Builder<T> with(Function<T, R> getter, V returnValue) {
Method method = getMethod(clazz, getter);
return withMethod(method, returnValue);
}
//typesafe :-) but i dont want to avoid implementing all types
Builder<T> withValue(Function<T, Long> getter, long returnValue) {
return with(getter, returnValue);
}
Builder<T> withValue(Function<T, String> getter, String returnValue) {
return with(getter, returnValue);
}
T build() {
@SuppressWarnings("unchecked")
T proxy = (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[] { clazz }, this);
return proxy;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) {
Object returnValue = methodReturnValues.get(method);
if (returnValue == null) {
Class<?> returnType = method.getReturnType();
return getDefaultValue(returnType);
}
return returnValue;
}
}
static public interface MyInterface {
String getName();
long getLength();
Long getNullLength();
Long getFullLength();
Number getNumber();
}
public static void main(String[] args) {
MyInterface x = Builder.of(MyInterface.class).with(MyInterface::getName, "1").with(MyInterface::getLength, 1L).with(MyInterface::getNullLength, null).with(MyInterface::getFullLength, new Long(2)).with(MyInterface::getNumber, 3L).build();
System.out.println("name:" + x.getName());
System.out.println("length:" + x.getLength());
System.out.println("nullLength:" + x.getNullLength());
System.out.println("fullLength:" + x.getFullLength());
System.out.println("number:" + x.getNumber());
// java.lang.ClassCastException: class java.lang.String cannot be cast to long:
// RuntimeException only :-(
MyInterface y = Builder.of(MyInterface.class).with(MyInterface::getLength, "NOT A NUMBER").build();
// java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
// RuntimeException only :-(
System.out.println("length:" + y.getLength());
}
}
Andrew Tobilko:
En el primer ejemplo, MyInterface::getLengthy "I am NOT an Integer"ayudó a resolver los parámetros genéricos Ty Rde MyInterfacey Serializable & Comparable<? extends Serializable & Comparable<?>>, respectivamente.
// it compiles since String is a Serializable
Function<MyInterface, Serializable> function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");
MyInterface::getLengthNo es siempre una Function<MyInterface, Integer>menos que usted diga explícitamente, lo que llevaría a un error en tiempo de compilación como el segundo ejemplo mostró.
// it doesn't compile since String isn't an Integer
Function<MyInterface, Integer> function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");