簡介

說實話，不太理解反射機制。
不過好像應該是對於特定對於泛型的介面。

code

package com.company;

import java.lang.reflect.*;
import java.util.*;

public class GenericReflectionTest {
    public static void main(String[] args){
        String name;
        if(args.length > 0) name = args[0];
        else{
            try(Scanner in = new Scanner(System.in)){
                System.out.println("Enter class name (e.g. java.util.Collections)");
                name = in.next();
            }
        }
        try{
            // print generic info for class and public methods
            Class<?> cl = Class.forName(name);
            printClass(cl);
        }catch(ClassNotFoundException e){
            e.printStackTrace();
        }
    }
    public static void printClass(Class<?> cl){
        System.out.print(cl);
        printTypes(cl.getTypeParameters(), "<", ",",">", true);
        Type sc = cl.getGenericSuperclass();
        if(sc != null){
            System.out.print(" extends ");
            printType(sc, false);
        }
        printTypes(cl.getGenericInterfaces(), " implements ", ",", "", false);
        System.out.println();
    }

    public static void printMethod(Method m){
        String name = m.getName();
        System.out.print(Modifier.toString(m.getModifiers()));
        System.out.print(" ");
        printTypes(m.getTypeParameters(), "<", ",", ">", true);

        printType(m.getGenericReturnType(), false);
        System.out.print(" ");
        System.out.print(name);
        System.out.print("(");
        printTypes(m.getGenericParameterTypes(), "", ", ", "", false);
        System.out.println(")");
    }

    public static void printTypes(Type[] types, String pre, String sep, String suf, boolean isDefinition){
        if(pre.equals(" extends ") && Arrays.equals(types, new Type[] {Object.class})) return;
        if(types.length > 0) System.out.print(pre);
        for(int i=0; i<types.length; i++){
            if(i > 0) System.out.print(sep);
            printType(types[i], isDefinition);
        }
        if(types.length > 0) System.out.print(suf);
    }

    public static void printType(Type type, boolean isDefinition){
        if(type instanceof Class) {
            Class<?> t = (Class<?>) type;
            System.out.print(t.getName());
        }else if(type instanceof TypeVariable){
            TypeVariable<?> t = (TypeVariable<?>) type;
            System.out.print(t.getName());
            if(isDefinition){
                printTypes(t.getBounds(), " extends ", " & ", "", false);
            }
        }else if(type instanceof WildcardType){
            WildcardType t = (WildcardType) type;
            System.out.print("?");
            printTypes(t.getUpperBounds(), " extends ", " & ", "", false);
            printTypes(t.getLowerBounds(), " super ", " & ", "", false);
        }else if(type instanceof ParameterizedType){
            ParameterizedType t = (ParameterizedType) type;
            Type owner = t.getOwnerType();
            if(owner != null){
                printType(owner, false);
                System.out.print(".");
            }
            printType(t.getRawType(), false);
            printTypes(t.getActualTypeArguments(), "<", ", ", ">", false);
        }else if(type instanceof GenericArrayType){
            GenericArrayType t = (GenericArrayType) type;
            System.out.print("");
            printType(t.getGenericComponentType(), isDefinition);
            System.out.print("[]");
        }
    }

}
 

result

java.util.Collections
class java.util.Collections extends java.lang.Object