问题
The used method reference has return type Integer. But an incompatible String is allowed in the following example.
How to fix the method with declaration to get the method reference type safe without manually casting?
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)
}
}
USE CASE: a type safe but generic Builder.
I tried to implement a generic builder without annotation processing (autovalue) or compiler plugin (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());
}
}
回答1:
In the first example, MyInterface::getLength and "I am NOT an Integer" helped to resolve the generic parameters T and R to MyInterface and Serializable & Comparable<? extends Serializable & Comparable<?>>respectively.
// 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::getLength is not always a Function<MyInterface, Integer> unless you explicitly say so, which would lead to a compile-time error as the second example showed.
// 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");
回答2:
Its the type inference that is playing its role here. Consider the generic R in the method signature:
<R> Builder<T> with(Function<T, R> getter, R returnValue)
In the case as listed:
Builder.of(MyInterface.class).with(MyInterface::getLength, "I am NOT an Integer");
the type of R is successfully inferred as
Serializable, Comparable<? extends Serializable & Comparable<?>>
and a String does imply by this type, hence the compilation succeeds.
To explicitly specify the type of R and find out the incompatibility, one can simply change the line of code as :
Builder.of(MyInterface.class).<Integer>with(MyInterface::getLength, "not valid");
回答3:
It is because your generic type parameter R can be inferred to be Object, i.e. the following compiles:
Builder.of(MyInterface.class).with((Function<MyInterface, Object>) MyInterface::getLength, "I am NOT an Integer");
回答4:
This answer is based on the other answers which explain why it does not work as expected.
SOLUTION
The following code solves the problem by splitting the bifunction "with" into two fluent functions "with" and "returning":
class Builder<T> {
...
class BuilderMethod<R> {
final Function<T, R> getter;
BuilderMethod(Function<T, R> getter) {
this.getter = getter;
}
Builder<T> returning(R returnValue) {
return Builder.this.with(getter, returnValue);
}
}
<R> BuilderMethod<R> with(Function<T, R> getter) {
return new BuilderMethod<>(getter);
}
...
}
MyInterface z = Builder.of(MyInterface.class).with(MyInterface::getLength).returning(1L).with(MyInterface::getNullLength).returning(null).build();
System.out.println("length:" + z.getLength());
// YIPPIE COMPILATION ERRROR:
// The method returning(Long) in the type BuilderExample.Builder<BuilderExample.MyInterface>.BuilderMethod<Long> is not applicable for the arguments (String)
MyInterface zz = Builder.of(MyInterface.class).with(MyInterface::getLength).returning("NOT A NUMBER").build();
System.out.println("length:" + zz.getLength());
(is somewhat unfamiliar)
来源：https://stackoverflow.com/questions/58337639/why-is-lambda-return-type-not-checked-at-compile-time