Như tất cả chúng ta đều biết rằng nó bị xóa. Nhưng nó có thể được biết trong một số trường hợp trong đó loại được đề cập rõ ràng trong hệ thống phân cấp lớp:
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;
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();
}
}
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;
}
}
Và bây giờ bạn có thể làm những việc như:
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);
}
}
Thêm thông tin ở đây . Nhưng một lần nữa, gần như không thể truy xuất:
class SomeClass<T> extends CaptureType<T>{}
SomeClass<String> claz = new SomeClass<>();
nơi nó bị xóa.
List<Integer> list1 = new ArrayList<Integer>(); List<String> list2 = (List<String>)list1; list2.add("foo"); // perfectly legalBạn không thể làm điều đó trong Java, bạn gặp lỗi biên dịch kiểu không khớp!