Java VM Type Erasure

Container.java

package te;

import java.util.ArrayList;
import java.util.List;

public class Container<T> {
    public List<T> elems = new ArrayList<T>();

    public Container(T arg) {
        this.elems.add(arg);
        this.elems.add(arg);
    }

    @Override
    public String toString() {
        return "Container{" + "elems=" + elems + '}';
    }
}

Document.java

package te;

import java.util.Arrays;
import java.util.List;

public class Document {
    public List<String> items = Arrays.asList(new String[] { "apple", "bus" });
    public Container<String> strings = new Container<String>("many");
    public Container<int[]> ints = new Container<int[]>(new int[] { 42 });
//    Container<Integer[]> ints = new Container<>(new Integer[] { 42 });
//    Container<List<Integer>> ints = new Container<>(Arrays.asList(new Integer[] { 42 }));

//    @Override
//    public String toString() {
//        return "Document{" + "items=" + items + '}';
//    }

    @Override
    public String toString() {
        return "Document{" + "items=" + items + ",\n strings=" + strings + ",\n ints=" + ints + '}';
    }
}

Main.java

package te;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;

/* 0. (slide) title
 * switch to netbeans
 * document
 * document + container
 * simplify setup -> just container
 * string container
 * int container
 *   print i[0]
 *   print returned structure nested types
 *   supply type via obj.getClass()
 * int container + TypeToken
 *   supply type via TypeToken
 * TypeToken -> ParameterizedType
 * flashback to 1.4...
 * list in 1.4 - list14()
 * list in 1.5 - list15()
 * return parameterized list + 1.7 diamond - list17returnParameterized()
 * javap:
 *   cd ~/Work/TypeErasure
 *   javap -classpath build/classes te.Main
 *   javap -classpath build/classes te.Document
 * 1. (slide) function & field signatures & template class _code_ does not contain parameterized type information
 * but, inspect container element(s) -> special case, knowledge about container structure
 * scala console
 *   cd ~/Work/TypeErasure
 *   javap -classpath build/classes te.Model
 *   scala -cp build/classes:lib/Google_Gson/gson-2.1.jar
 *   val m = new te.Model
 *   val fs = m.getClass.getDeclaredFields
 *   import collection.JavaConversions._
 *   fs.foreach(f => println(f.getGenericType))
 * why it doesn't work for Container?
 *   val d = new te.Document
 *   d.getClass.getDeclaredFields.foreach(f => println(f.getGenericType))
 *   d.ints.getClass.getDeclaredFields.foreach(f => println(f.getGenericType))
 * plain container
 *   val c = new te.Container[String]("try me")
 *   c.getClass.getDeclaredFields.foreach(f => println(f.getGenericType))
 * 2. (slide) field reflection metadata contains information about parameterizing type
 *   Field.getGenericType()
 * 3. (slide) (anonymous) derived (template specialized) classes for which bytecode is generated, encode parameterizing type information
 *   Class.getGenericSuperclass()
 * summary:
 *   {}
 *   cd ~/Work/TypeErasure
 *   javap -classpath build/classes te.Main\$1
 *   javap -classpath build/classes te.Main\$2
 *   container()
 *   containerArray()
 * workarounds besides reflection tricks:
 * sample data
 *   List x = new List( { "string" } )
 * annotation
 *   switch to intellij, open model.Person
 *   Option[Int] (Scala Int => Java int) => Option[Object]
 *   @OptionType(classOf[Int])
 * scalap
 *   cd ~/Work/BIS/bis/backend/services
 *   javap -classpath target/scala-2.9.1/classes model.Person
 *   scalap -classpath target/scala-2.9.1/classes model.Person
 * TypeToken -> GenericArrayType
 * switch to 1.6, fix <> diamond errors
 * containerArray()
 */

public class Main {
    private static Gson gson = new Gson();

    private static int i = 0;
    private static void p(Object o) { System.out.println(++i + ". " + o); }
    private static void h(String h) { System.out.println("\n--- " + h.toUpperCase()); }

    public static void main(String[] args) {
        //gsonDocument();
        //gsonContainer();
        //gsonContainerInt();
        //p(list14());
        //p(list15());
        //p(list17returnParameterized());
        //container();
        containerArray();
    }

    public static void gsonDocument() {
        h("document");
        Document doc = new Document();
        p(doc);
        String docJson = gson.toJson(doc);
        p(docJson);
        Document docFromJson = gson.fromJson(docJson, Document.class);
        p(docFromJson);
    }

    public static void gsonContainer() {
        h("string container");
        Container<String> stringContainer = new Container<String>("alone");
        p(stringContainer);
        String stringContainerJson = gson.toJson(stringContainer);
        p(stringContainerJson);
        Container<String> stringContainerFromJson = gson.fromJson(stringContainerJson, Container.class);
        p(stringContainerFromJson);
    }

    public static void gsonContainerInt() {
        h("int array container");
        Container<int[]> intArrayContainer = new Container<int[]>(new int[] { 42 });
        p(intArrayContainer);
//        Container<List<Integer>> intArrayContainer = new Container<List<Integer>>(Arrays.asList(new Integer[] { 42 }));
        String intArrayContainerJson = gson.toJson(intArrayContainer);
        p(intArrayContainerJson);

        h("int array container from json");
//        Container<int[]> intArrayContainerFromJson = gson.fromJson(intArrayContainerJson,
            // Container.class);
//        Container<int[]> intArrayContainerFromJson = gson.fromJson(intArrayContainerJson,
//                intArrayContainer.getClass());
        Container<int[]> intArrayContainerFromJson = gson.fromJson(intArrayContainerJson,
                new TypeToken<Container<int[]>>(){}.getType());
//        p(intArrayContainerFromJson);
        int[] i = intArrayContainerFromJson.elems.get(0);
        p(i[0]);
        p(intArrayContainerFromJson.elems);
        p(intArrayContainerFromJson.elems.get(0));
        Object o = intArrayContainerFromJson.elems.get(0);
        p(o.getClass().getName());
        Object n;
        if (o instanceof List)
            n = ((List) o).get(0);
        else
            n = ((int[]) o)[0];
        p(n.getClass().getName());
    }

    public static List list14() {
        List l = new ArrayList();
        l.add("j");
        l.add(new Integer(10));
        Integer i = (Integer) l.get(1);
        String s = (String) l.get(0);
        return l;
    }

    public static List<String> list15() {
        List<String> l = new ArrayList<String>();
        l.add("j");
//        l.add(new Integer(10));
        String s = l.get(0);
        return l;
    }

    public static List<String> list17returnParameterized() {
        List<String> l = new ArrayList<String>();
        l.add("j");
        String s = l.get(0);
        return l;
    }

    public static Container container() {
        Container<String> c = new Container<String>("j") {};
        Type t = c.getClass().getGenericSuperclass();
        if (t instanceof ParameterizedType) {
            ParameterizedType p = (ParameterizedType) t;
            Type r = p.getActualTypeArguments()[0];
            p(r);
            p(r.getClass().getName());
        }
        return c;
    }

    public static Container containerArray() {
        Container<int[]> c = new Container<int[]>(new int[] { 42 }) {};
        Type t = c.getClass().getGenericSuperclass();
        if (t instanceof ParameterizedType) {
            ParameterizedType p = (ParameterizedType) t;
            Type r = p.getActualTypeArguments()[0];
            p(r);
            p(r.getClass().getName());
            if (r instanceof GenericArrayType) {
                GenericArrayType g = (GenericArrayType) r;
                p(g.getGenericComponentType());
                p(g.getGenericComponentType().getClass().getName());
            }
        }
        return c;
    }
}

Model.java

package te;

import java.util.List;

public class Model {
    List<String> lstr;
    List<Integer> lint;
    List<int[]> larr;
}