Java Generics: Useing multiple generic types in one collection type

See also questions close to this topic

• Printing elements in a array list

  I am trying to get an output similar to this -

  John Smith has been added to the student list

  Tom Will has been added to the student list

  your name has been added to the student list

  --Begin--

  Name:John Smith Email:js@qmul.ac.uk Year:2008

  Name:Tom Will Email:tw@qmul.ac.uk Year:2007

  Name:your name Email:your email Year:your year

  --End--

  Tom Will has been removed from the student list

  --Begin--

  Name:John Smith Email:js@qmul.ac.uk Year:2008

  Name:your name Email:your email Year: your year

  --End--

  But it is printing it muddled up. Where am I going wrong in my code?

  A class that represents a student:

  public class Student { 
  
  private String name;
  private String email;
  private int year; //year of registration to the course
  
  /**
   *  Constructor
   *
   *@param  name, email and year of registration
   */
  public Student(String name, String email, int year){
      this.name = name;
      this.email = email;
      this.year = year;
  }
  
  /**
   *  get the name
   *
   *@return the name
   */
  public String getName(){
      return name;
  }
  
  
  /**
   *  A toString() method to give a String representation of a Student
   *
   *@return    The String representation of a Student
   */
  public String toString(){
      return "Name:" + name +" Email:" + email + " Year:" + year;
  }
  

  }

  public class StudentList {
  
  private ArrayList<Student> list; //instance variable
  
  /**
   *  Constructor
   */
  public StudentList(){
      list = new ArrayList<Student>();
  }
  
  /**
   *  a method to print off all ArrayList elements
   */
  
  public void printList(){
      System.out.println("--Begin--");
      for (int i = 0; i < list.size(); i++) {
          System.out.println(list.get(i));
          }       
      System.out.println("--End--");
  
  }
  
  /**
   *  A method to add a student to the list
   *
   *@param    The student
   */
  public void addToList(Student s){
      list.add(s);
  
      System.out.println("--Begin--");
      for (int i = 0; i < list.size(); i++) {
          System.out.println(list.get(i) +"has been addded to the list");
          }
      System.out.println("--End--");
  
  }
  
  /**
   *  A method to remove a student from the list
   *
   *@param    The student
   */
  public void removeFromList(Student s){
      list.remove(s);
      System.out.println("--Begin--");
      for (int i = 0; i < list.size(); i++) {
          System.out.println(list.get(i) +"has been removed from the list");
      }
      System.out.println("--End--");
  
  }
  
  /**
   *  A main method to test
   */
  public static void main(String[] args) {
  
      // Create an instance of the class 
      StudentList studentList = new StudentList();
  
      //create 3 student objects
      Student s1 = new Student("John Smith", "js@qmul.ac.uk", 2008);
      Student s2 = new Student("Tom Will", "tw@qmul.ac.uk", 2007);
      Student s3 =  new Student("Cameron Young","Cammyoung@live.co.uk",2018);
  
      //add the three students to the list
      studentList.addToList(s1);
      studentList.addToList(s2);
      studentList.addToList(s3);
  
      // Print the list
      studentList.printList();
  
      // Remove the student "Tom Will"
      studentList.removeFromList(s2);
  
      // Print the list again
      studentList.printList();
  }
  

  }
• Android GridLayout in popup does not display data

  We are currently working on a small game for Android devices, and i ran into a minor problem: I want to open a popup, which displays all the tiles which are remaining. But for some Reason, the popup does not display anything, what am i missing out? Note, that the listForAdapter is an ArrayList including testdata, an object having a Tile and the amount. It does not seem to cause any problems.

    /**Inflate and position the bag's Layout*/
              LayoutInflater inflater = (LayoutInflater) this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
              View layout = inflater.inflate(R.layout.bag,null);
  
              float density = this.getResources().getDisplayMetrics().density;
              final PopupWindow pw = new PopupWindow(layout, (int)density*350, (int)density*450, true);
  
              /**Set up button and listener to close the bag*/
              ((Button) layout.findViewById(R.id.close)).setOnClickListener(new View.OnClickListener() {
                  public void onClick(View v) {
                      pw.dismiss();
                  }
              });
  
              /**set up the bag's RecyclerView*/
              pw.showAtLocation(layout, Gravity.CENTER, 0, (int) density*10);
              RecyclerView recyclerView  = (RecyclerView) layout.findViewById(R.id.my_recycler_view);
              recyclerView.setLayoutManager(new GridLayoutManager(this, 3));
              BagRecyclerAdapter adapter = new BagRecyclerAdapter(this, listForAdapter);
              recyclerView.setAdapter(adapter);
  

  The BagRecyclerAdapter, our idea is to open it up in a recyclerview which includes a gridlayout, but currently it just simply stays white:

  public class BagRecyclerAdapter extends RecyclerView.Adapter<BagRecyclerAdapter.ViewHolder> {
  
      private Context mContext;
      private List<GameBoardActivity.TileCount> bData;
      private LayoutInflater bInflater;
  
      /**
       * fetched bag is fed into the constructor
       */
      BagRecyclerAdapter(Context context, List<GameBoardActivity.TileCount> data) {
          mContext=context;
          this.bInflater = LayoutInflater.from(context);
          this.bData = data;
      }
  
      public class ViewHolder extends RecyclerView.ViewHolder {
          public TextView counter;
          public ImageView tileInfo;
  
          public ViewHolder(View itemView) {
              super(itemView);
              counter = itemView.findViewById(R.id.info_number);
              tileInfo= itemView.findViewById(R.id.info_image);
          }
      }
  
      /**
       * inflates the cell layout from xml when needed
       */
      @NonNull
      public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
          View view = bInflater.inflate(R.layout.bag_item, parent, false);
          return new ViewHolder(view);
      }
  
      @Override
      public void onBindViewHolder(@NonNull ViewHolder viewHolder, int i) {
          GameBoardActivity.TileCount curr = bData.get(i);
          switch(curr.tile.getShape()){
              case 0: viewHolder.tileInfo.setImageResource(R.drawable.circle); break;
              case 1: viewHolder.tileInfo.setImageResource(R.drawable.frame); break;
              case 2: viewHolder.tileInfo.setImageResource(R.drawable.plus); break;
              case 3: viewHolder.tileInfo.setImageResource(R.drawable.domino); break;
              case 4: viewHolder.tileInfo.setImageResource(R.drawable.point); break;
              case 5: viewHolder.tileInfo.setImageResource(R.drawable.pointhollow); break;
              case 6: viewHolder.tileInfo.setImageResource(R.drawable.ring); break;
              case 7: viewHolder.tileInfo.setImageResource(R.drawable.spade); break;
              case 8: viewHolder.tileInfo.setImageResource(R.drawable.cross); break;
              case 9: viewHolder.tileInfo.setImageResource(R.drawable.sun); break;
              case 10: viewHolder.tileInfo.setImageResource(R.drawable.triangle); break;
              case 11: viewHolder.tileInfo.setImageResource(R.drawable.rectangle); break;
          }
          switch(curr.tile.getColor()){
              case 0: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xffe6194B, 0)); break;
              case 1: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xff3cb44b, 0)); break;
              case 2: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xffffe119, 0)); break;
              case 3: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xff4364d8, 0)); break;
              case 4: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xfff58231, 0)); break;
              case 5: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xffaaffc3, 0)); break;
              case 6: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xff469990, 0)); break;
              case 7: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xff8904b1, 0)); break;
              case 8: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xfff032e6, 0)); break;
              case 9: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xffa9a9a9, 0)); break;
              case 10: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xff9A6324, 0)); break;
              case 11: viewHolder.tileInfo.setColorFilter(new LightingColorFilter(0xffffffff, 0)); break;
          }
          viewHolder.counter.setText(Integer.toString(curr.getCount()));
      }
  
      @Override
      public int getItemCount() {
          return bData.size();
      }
  
  
      }
  

  and the two necessary XMLs:

  <?xml version="1.0" encoding="utf-8"?>
  
  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:id="@+id/popup_element"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:background="@color/colorWhite"
      >
  
      <ScrollView
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:layout_above="@+id/close"
          android:layout_marginBottom="462dp">
  
          <LinearLayout
              android:id="@+id/recLayout"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:paddingLeft="16dp"
              android:paddingRight="16dp"
              android:orientation="vertical">
  
              <android.support.v7.widget.RecyclerView
                  android:id="@+id/my_recycler_view"
                  android:layout_width="wrap_content"
                  android:layout_height="match_parent"
                  android:scrollbars="none" />
          </LinearLayout>
      </ScrollView>
  
      <Button
          android:id="@+id/close"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_alignParentBottom="true"
          android:layout_centerHorizontal="true"
          android:text="@string/close_bag"
          />
  </RelativeLayout>
  

  Number 2:

  <?xml version="1.0" encoding="utf-8"?>
  <LinearLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="horizontal"
      android:padding="5dp"
      android:layout_width="50dp"
      android:layout_height="50dp">
  
      <ImageView
          android:id="@+id/info_image"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:gravity="center"
          android:background="@color/colorAccent"/>
      <TextView
          android:id="@+id/info_number"
          android:layout_width="10dp"
          android:layout_height="10dp"
          android:gravity="bottom|right"
          android:background="@color/colorGrey"/>
  
  </LinearLayout>
  

  Also, i am sorry for not using the proper XML style here, i could not figure out how to include that :(
• Having a problem adding 2 employees to a string

  im creating log in and password using string it work fine with one employee but cant get it working if i want to add a second employee to the string

  String Password = "2234"; String Username = "Employee1";
• How can I express this method in a way for generic enums

  I cant find a way to express this method into a generic method. Target is to use any flagged enum. Would be great, if someone could help.

  class Program
  {
    [Flags]
    public enum MyRights
    {
        Read, Write, Delete, CreateChild, FullRights
    }
  
    static void Main(string[] args)
    {
        MyRights myRights = new MyRights();
        myRights = MyRights.Read | MyRights.Delete;
  
        var kvpList = GetList(myRights);
        Console.ReadKey();
    }
  
    private static List<KeyValuePair<string, int>> GetList(MyRights myRights)
    {
        return Enum.GetValues(typeof(MyRights)).Cast<MyRights>()
            .Where((enumValue) => myRights.HasFlag(enumValue))
            .Select((enumValue) => 
               new KeyValuePair<string, int>(enumValue.ToString(), (int)enumValue))
            .ToList();
    }
  }
  

  Best regards Tzwenni

  PS: Not regarding NULL check etc. at the moment.

  UPDATE Solution

  thx to @Flydog57

  initialize the values

  1st important hint ;)

      public enum MyRights
      {
          Read = 0x01, Write = 0x02, Delete = 0x04, CreateChild = 0x08
      }
  

  Then: I couldn't cast value to int. But using HashCode works fine:

  private static List<KeyValuePair<string, int>> GetListGeneric<TEnum>(TEnum myRights) 
          where TEnum : Enum 
  {
      return Enum.GetValues(typeof(TEnum)).Cast<TEnum>()
          .Where((enumValue) => myRights.HasFlag(enumValue))
          .Select((enumValue) =>  
              new KeyValuePair<string, int>(enumValue.ToString(), enumValue.GetHashCode())
          ).ToList();
  }
  
• Typescript: infer keyof base

  Lets say I have code like this:

  interface KeysA {
    'propA1': number;
    'propA2': string;
  }
  
  class ClassA {
    public set<K extends keyof KeysA>(prop: K, value: KeysA[K]): void {}
  }
  
  interface KeysB {
    'propA1': HTMLDivElement;
    'propA2': HTMLButtonElement;
  }
  
  class ClassB {
    public set<K extends keyof KeysB>(prop: K, value: KeysB[K]): void {}
  }
  

  KeysA and keysB works like guards for set method. So new ClassA().set('propA1', 'somestr') throw error because type of propA1 in KeysA is number. That's fine.

  Now lets say I want to write following class:

  class UniversalWrapper<
      TTarget,
      TItem extends TTarget extends {
        set<TK extends keyof infer TItem>(prop: infer TK, value: infer TProp): void
      } ? TItem : never > {
    public prop?: TItem;
    public constructor({target}: {target: TTarget}) {
    }
  }
  
  const w = new UniversalWrapper({
    target: new ClassA()
  });
  w.prop = '111'; // No type guard here!!!
  

  Type of w.prop would be just {}. Why can't I infer base of keyof interface?

  Let's assume I can't rewrite ClassA, ClassB, KeysA and KeysB because they come from third-party library.
• Constructing a n-ary product with all the values of a simple sum type

  I'm working with the generics-sop library. I want to write a value with the following type:

  values :: forall r. IsEnumType r => NP (K r) (Code r)
  

  That is, for sum types whose constructors don't have any arguments (IsEnumType) I want to produce an n-ary product (NP) which contains the corresponding constructor value at each point.

  For example, for the type

  {-# LANGUAGE DeriveGeneric #-}
  
  import qualified GHC.Generics as GHC
  import Generics.SOP
  
  data Foo = Bar
           | Baz
           deriving (GHC.Generic)
  
  instance Generic Foo
  

  I want to produce the n-ary product

  K Bar :* K Baz :* Nil 
  

  I believe the solution will involve transforming an n-ary product carrying generic representations of each constructor, so I wrote this:

  values :: forall r. IsEnumType r => NP (K r) (Code r)
  values = liftA_NP (mapKK (to . SOP))  _
  

  Using liftA_NP and mapKK. But I'm not sure how to produce the generic representations themselves.
• Laravel collection grouped by property array

  My boss has asked me for a report, and I don't want to do many loops to obtain the information if I can do it in a single Eloquent sentence. I have this model:

  {
     "id":1,
     "persona_id":75,
     "localidad_id":1,
     "datos":{
        "web":null,
        "lider":{
           "persona":"18"
        },
        "redes":{
           "email":null
        },
        "activa":"true",
        "nombre":"Comisión Ambiental Local - Usaquén",
        "asistente":"true",
        "secretaria":"true"
     },
     "created_at":"2018-06-15 20:01:40",
     "updated_at":"2018-11-02 20:37:02"
  }
  

  The model is called Instancia and it has a Json field called datos which is casted to an array every time the model is created using the protected attribute $casts.

  Inside that the datos array, I have a key called lider and inside that one, another called persona. I want if possible to group my collection by that one parameter persona.

  I have tried several approaches, but non has worked:

  $instancias = Instancia::where( 'datos->lider->persona', '<>', null )->get()->groupBy( 'datos->lider' );
  

  That one returns the entire collection now grouped by a "" parent.

  $instancias = Instancia::where( 'datos->lider->persona', '<>', null )->get()groupBy( 'persona' );
  

  Any help will be appreciated.
• Intersection of two lists maintaining duplicate values in Kotlin

  I want to find the number of common elements between two lists without eliminating duplicates.

  For example:

  ---

  input: [1, 3, 3] & [4, 3, 3]

  output: 2, since the common elements are [3, 3]

  ---

  input: [1, 2, 3] & [4, 3, 3]

  output: 1, since the common elements are [3]

  ---

  If I were to use the Kotlin collections intersect, the result is a set, which will prevent me from counting duplicate values.

  I found (for Python) this, which handles duplicates differently and this, which led me to use this implementation, where a and b are the lists:

  val aCounts = a.groupingBy { it }.eachCount()
  val bCounts = b.groupingBy { it }.eachCount()
  var intersectionCount = 0;
  for ((k, v) in aCounts) {
      intersectionCount += Math.min(v, bCounts.getOrDefault(k, 0))
  }
  

  However, being new to Kotlin I'm wondering if there's a more "Kotlin-y" way to do this--something taking advantage of all Kotlin's collections functionality? Maybe something that avoids explicitly iterating?
• How to add elements to an arraylist in java inside a for loop without concurrentmodificationexception

  I have a Java Spring MVC Web application. I am trying to iterate through an ArrayList and add new elements to the list based on certain conditions. I use the following code:

  List<LocationHourListHB> locationHoursList = new ArrayList<LocationHourListHB>();
  List<HoursTO> hourList = listHoursByEntityId(applicationId, siteId, locationId);
  for (HoursTO hoursTO : hourList) 
  {
      if(locationHoursList.size() == 0)
      {
          LocationHourListHB locationHourListHB = new LocationHourListHB();
          locationHourListHB.setStartTIme(hoursTO.getStartTime());
          locationHourListHB.setEndTime(hoursTO.getEndTime());
          locationHoursList.add(locationHourListHB);
          break;
      }
      for (LocationHourListHB locationHour : locationHoursList) 
      {
          if(hoursTO.getStartTime().equalsIgnoreCase(locationHour.getStartTIme()) && hoursTO.getEndTime().equalsIgnoreCase(locationHour.getEndTime()))
          {
              break;
          }
          else
          {
              LocationHourListHB locationHourListHB = new LocationHourListHB();
              locationHourListHB.setStartTIme(hoursTO.getStartTime());
              locationHourListHB.setEndTime(hoursTO.getEndTime());
              locationHoursList.add(locationHourListHB);
          }
      }
  }
  }
  

  But executing this throws a concurrent modification exception. I have done some research and found that it can be solved using an iterator or list iterator. Is there a good example to follow for using the list iterator to solve my issue. Or is there any better solution that could save me.

  I have tried the following code:

  for (HoursTO hoursTO : hourList) 
  {
      if(locationHoursList.size() == 0)
      {
          LocationHourListHB locationHourListHB = new LocationHourListHB();
          locationHourListHB.setStartTIme(hoursTO.getStartTime());
          locationHourListHB.setEndTime(hoursTO.getEndTime());
          locationHoursList.add(locationHourListHB);
          }
          ListIterator<LocationHourListHB> iter = locationHoursList.listIterator();
          while(iter.hasNext())
          {
          if(!(iter.next().getStartTIme().equalsIgnoreCase(hoursTO.getStartTime()) && iter.next().getEndTime().equalsIgnoreCase(hoursTO.getEndTime())))
          {
              LocationHourListHB locationHourListHB = new LocationHourListHB();
              locationHourListHB.setStartTIme(hoursTO.getStartTime());
              locationHourListHB.setEndTime(hoursTO.getEndTime());
              iter.add(locationHourListHB);
          }
      }
  }
  

  But getting a NoSuchElementException