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Capture Type information
import java.lang.reflect.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;
/**
* CaptureType is useful to capture the generic type of a parametrized class. For example:
* <pre>
* {@code
*
* CaptureType<String> t1 = new CaptureType<String>() {};
* t1.getRawType().equals(String.class); //true
*
* CaptureType<List<List<String>>> t1 = new CaptureType<List<List<String>>>() {};
* t1.getParamADT().getRawType().equals(List.class);//true
* }
* </pre>
* A more complex example:
* <pre>
* {@code
*
* class X<T> extends CaptureType<T>{}
*
* class Y<A, B> extends X<B> {}
*
* class Z<Q> extends Y<Q, Map<Integer, List<List<List<Integer>>>>> {}
*
* Z<String> z = new Z<>();
* TypeADT param = z.getParamADT();
* equals(param.getRawType(), Map.class);
* List<TypeADT> parameters = param.getParameters();
* equals(parameters.get(0).getRawType(), Integer.class);
* equals(parameters.get(1).getRawType(), List.class);
* equals(parameters.get(1).getParameters().get(0).getRawType(), List.class);
* equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getRawType(), List.class);
* equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getParameters().get(0).getRawType(), Integer.class);
* }
* </pre>
* <p>
* This class is only useful to obtain one parametrized type. But a similar strategy can be used to obtain any number of generic types.
* </p>
* This class captures information using Reflection. It captures using the <a href="http://gafter.blogspot.in/2006/12/super-type-tokens.html">super token</a>.
* So in cases like:
* <pre>
* {@code
*
* class Sample<T> extends CaptureType<T>{
* List<T> t = new ArrayList<T>();
* }
* public void run(){
* Sample<String> sample = new Sample<String>();
* }
* }
* </pre>
* It is impossible to obtain the type information of {@code sample} and also {@code sample.t} as it is erased. In such cases {@code claz.getRawType();} will thrown an exception. {@code CaptureType} class will only work,
* if the type is mentioned while declaring the Class (at any point in hierarchy). For example in:
* <pre>
* {@code
*
* class A<Z> extends CaptureType<Z>{}
* class B<E,T> extends A<T>{}
* class C <X,Y,Z> extends B<X, String>
* }
* </pre>
* It is possible to obtain the type {@code String}, because it is mentioned at class level and can be mapped by following hierarchy. So `Z` in {@code A} is {@code String}
* </p>
* <p>Note: Above, it would had been possible to obtain type information of {@code sample} object had it been an instance variable rather an local variable</p>
*
* @param <T> Any type parameter
* @author Jatin
*/
public abstract class CaptureType<T> {
/**
* {@link java.lang.reflect.Type} object of the corresponding generic type. This method is useful to obtain every kind of information (including annotations) of the generic type.
*
* @return Type object. null if type could not be obtained (This happens in case of generic type whose information cant be obtained using Reflection). Please refer documentation of {@link com.types.CaptureType}
*/
public Type getTypeParam() {
Class<?> bottom = getClass();
Map<TypeVariable<?>, Type> reifyMap = new LinkedHashMap<>();
for (; ; ) {
Type genericSuper = bottom.getGenericSuperclass();
if (!(genericSuper instanceof Class)) {
ParameterizedType generic = (ParameterizedType) genericSuper;
Class<?> actualClaz = (Class<?>) generic.getRawType();
TypeVariable<? extends Class<?>>[] typeParameters = actualClaz.getTypeParameters();
Type[] reified = generic.getActualTypeArguments();
assert (typeParameters.length != 0);
for (int i = 0; i < typeParameters.length; i++) {
reifyMap.put(typeParameters[i], reified[i]);
}
}
if (bottom.getSuperclass().equals(CaptureType.class)) {
bottom = bottom.getSuperclass();
break;
}
bottom = bottom.getSuperclass();
}
TypeVariable<?> var = bottom.getTypeParameters()[0];
while (true) {
Type type = reifyMap.get(var);
if (type instanceof TypeVariable) {
var = (TypeVariable<?>) type;
} else {
return type;
}
}
}
/**
* Returns the raw type of the generic type.
* <p>For example in case of {@code CaptureType<String>}, it would return {@code Class<String>}</p>
* For more comprehensive examples, go through javadocs of {@link com.types.CaptureType}
*
* @return Class object
* @throws java.lang.RuntimeException If the type information cant be obtained. Refer documentation of {@link com.types.CaptureType}
* @see com.types.CaptureType
*/
public Class<T> getRawType() {
Type typeParam = getTypeParam();
if (typeParam != null)
return getClass(typeParam);
else throw new RuntimeException("Could not obtain type information");
}
/**
* Gets the {@link java.lang.Class} object of the argument type.
* <p>If the type is an {@link java.lang.reflect.ParameterizedType}, then it returns its {@link java.lang.reflect.ParameterizedType#getRawType()}</p>
*
* @param type The type
* @param <A> type of class object expected
* @return The Class<A> object of the type
* @throws java.lang.RuntimeException If the type is a {@link java.lang.reflect.TypeVariable}. In such cases, it is impossible to obtain the Class object
*/
public static <A> Class<A> getClass(Type type) {
if (type instanceof GenericArrayType) {
Type componentType = ((GenericArrayType) type).getGenericComponentType();
Class<?> componentClass = getClass(componentType);
if (componentClass != null) {
return (Class<A>) Array.newInstance(componentClass, 0).getClass();
} else throw new UnsupportedOperationException("Unknown class: " + type.getClass());
} else if (type instanceof Class) {
Class claz = (Class) type;
return claz;
} else if (type instanceof ParameterizedType) {
return getClass(((ParameterizedType) type).getRawType());
} else if (type instanceof TypeVariable) {
throw new RuntimeException("The type signature is erased. The type class cant be known by using reflection");
} else throw new UnsupportedOperationException("Unknown class: " + type.getClass());
}
/**
* This method is the preferred method of usage in case of complex generic types.
* <p>It returns {@link com.types.TypeADT} object which contains nested information of the type parameters</p>
*
* @return TypeADT object
* @throws java.lang.RuntimeException If the type information cant be obtained. Refer documentation of {@link com.types.CaptureType}
*/
public TypeADT getParamADT() {
return recursiveADT(getTypeParam());
}
private TypeADT recursiveADT(Type type) {
if (type instanceof Class) {
return new TypeADT((Class<?>) type, null);
} else if (type instanceof ParameterizedType) {
ArrayList<TypeADT> generic = new ArrayList<>();
ParameterizedType type1 = (ParameterizedType) type;
return new TypeADT((Class<?>) type1.getRawType(),
Arrays.stream(type1.getActualTypeArguments()).map(x -> recursiveADT(x)).collect(Collectors.toList()));
} else throw new UnsupportedOperationException();
}
}
import java.util.List;
/**
*
* <p>A simple class which stores the following :
* <li>The Raw type of Class</li>
* <li>The List of parametrized types</li>
* </p>
*<p>For example in case of an object declared as:
* <pre>
* {@code
* HashMap<String, List<Integer>> map = new HashMap<>();
* }
* </pre>
* </p>
* The information about above object can be stored in this class, where {@link TypeADT#getRawType()} would return {@code Class<HashMap>} and {@link TypeADT#getParameters()} would return a list of
* {@link com.types.TypeADT}. This list would be of size-2 and calling {@code getRawType} on them would return:
* <p><li>{@code Class<String>}</li>
* <li>{@code Class<List>}. Calling {@code getParameters().get(0).getRawType} on this second object would return {@code Class<Integer>}</li>
* </p>
*
* @author Jatin
*
*/
public class TypeADT {
private final Class<?> reify;
private final List<TypeADT> parametrized;
TypeADT(Class<?> reify, List<TypeADT> parametrized) {
this.reify = reify;
this.parametrized = parametrized;
}
public Class<?> getRawType() {
return reify;
}
public List<TypeADT> getParameters() {
return parametrized;
}
}
static void test1() {
CaptureType<String> t1 = new CaptureType<String>() {
};
equals(t1.getRawType(), String.class);
}
static void test2() {
CaptureType<List<String>> t1 = new CaptureType<List<String>>() {
};
equals(t1.getRawType(), List.class);
equals(t1.getParamADT().getParameters().get(0).getRawType(), String.class);
}
private static void test3() {
CaptureType<List<List<String>>> t1 = new CaptureType<List<List<String>>>() {
};
equals(t1.getParamADT().getRawType(), List.class);
equals(t1.getParamADT().getParameters().get(0).getRawType(), List.class);
}
static class Test4 extends CaptureType<List<String>> {
}
static void test4() {
Test4 test4 = new Test4();
equals(test4.getParamADT().getRawType(), List.class);
}
static class PreTest5<S> extends CaptureType<Integer> {
}
static class Test5 extends PreTest5<Integer> {
}
static void test5() {
Test5 test5 = new Test5();
equals(test5.getTypeParam(), Integer.class);
}
static class PreTest6<S> extends CaptureType<S> {
}
static class Test6 extends PreTest6<Integer> {
}
static void test6() {
Test6 test6 = new Test6();
equals(test6.getTypeParam(), Integer.class);
}
class X<T> extends CaptureType<T> {
}
class Y<A, B> extends X<B> {
}
class Z<Q> extends Y<Q, Map<Integer, List<List<List<Integer>>>>> {
}
void test7(){
Z<String> z = new Z<>();
TypeADT param = z.getParamADT();
equals(param.getRawType(), Map.class);
List<TypeADT> parameters = param.getParameters();
equals(parameters.get(0).getRawType(), Integer.class);
equals(parameters.get(1).getRawType(), List.class);
equals(parameters.get(1).getParameters().get(0).getRawType(), List.class);
equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getRawType(), List.class);
equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getParameters().get(0).getRawType(), Integer.class);
}
static void test8() throws IllegalAccessException, InstantiationException {
CaptureType<int[]> type = new CaptureType<int[]>() {
};
equals(type.getRawType(), int[].class);
}
static void test9(){
CaptureType<String[]> type = new CaptureType<String[]>() {
};
equals(type.getRawType(), String[].class);
}
static class SomeClass<T> extends CaptureType<T>{}
static void test10(){
SomeClass<String> claz = new SomeClass<>();
try{
claz.getRawType();
throw new RuntimeException("Shouldnt come here");
}catch (RuntimeException ex){
}
}
static void equals(Object a, Object b) {
if (!a.equals(b)) {
throw new RuntimeException("Test failed. " + a + " != " + b);
}
}
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