* The TypeSafe classes, and their descendants, need a mechanism to find out what type has been used as a parameter
* for the concrete matcher. Unfortunately, this type is lost during type erasure so we need to use reflection
* to get it back, by picking out the type of a known parameter to a known method.
* The catch is that, with bridging methods, this type is only visible in the class that actually implements
* the expected method, so the ReflectiveTypeFinder needs to be applied to that class or a subtype.
* For example, the abstract <code>TypeSafeDiagnosingMatcher&lt;T&gt;</code> defines an abstract method
* <pre>protected abstract boolean matchesSafely(T item, Description mismatchDescription);</pre>
* By default it uses <code>new ReflectiveTypeFinder("matchesSafely", 2, 0); </code> to find the
* parameterised type. If we create a <code>TypeSafeDiagnosingMatcher&lt;String&gt;</code>, the type
* finder will return <code>String.class</code>.
* A <code>FeatureMatcher</code> is an abstract subclass of <code>TypeSafeDiagnosingMatcher</code>.
* Although it has a templated implementation of <code>matchesSafely(&lt;T&gt;, Description);</code>, the
* actual run-time signature of this is <code>matchesSafely(Object, Description);</code>. Instead,
* we must find the type by reflecting on the concrete implementation of
* <pre>protected abstract U featureValueOf(T actual);</pre>
* a method which is declared in <code>FeatureMatcher</code>.
* In short, use this to extract a type from a method in the leaf class of a templated class hierarchy.

* @author Steve Freeman
* @author Nat Pryce
package org.hamcrest.internal;

import java.lang.reflect.Method;

public class ReflectiveTypeFinder {
private final String methodName;
private final int expectedNumberOfParameters;
private final int typedParameter;

public ReflectiveTypeFinder(String methodName, int expectedNumberOfParameters, int typedParameter) {
this.methodName = methodName;
this.expectedNumberOfParameters = expectedNumberOfParameters;
this.typedParameter = typedParameter;

public Class<?> findExpectedType(Class<?> fromClass) {
for (Class<?> c = fromClass; c != Object.class; c = c.getSuperclass()) {
for (Method method : c.getDeclaredMethods()) {
if (canObtainExpectedTypeFrom(method)) {
return expectedTypeFrom(method);
throw new Error("Cannot determine correct type for " + methodName + "() method.");

* @param method The method to examine.
* @return true if this method references the relevant type

private boolean canObtainExpectedTypeFrom(Method method) {
return method.getName().equals(methodName)
&& method.getParameterTypes().length == expectedNumberOfParameters
&& !method.isSynthetic();

* @param method The method from which to extract
* @return The type we're looking for

private Class<?> expectedTypeFrom(Method method) {
return method.getParameterTypes()[typedParameter];