Generics Super Keyword

Feb 14, 2014

Suppose I have

class A {
public void speak() {
System.out.println("I am class A");
}
}

class B extends A{
public void speak() {
System.out.println("I am class B");
}
}

class C extends B{
public void speak() {
System.out.println("I am class C");
}
}

Why this doesn't work while A is a super type of B ?

public static void insertElements(List<? super B> list){
list.add(new A()); //not OK, why?
}

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Java Generics Super And Ext

Jun 17, 2014

Set<? super TreeMap> s = new HashSet<SortedMap>();

SortedMap<String,String> sm = new TreeMap<String,String>();
TreeMap<String,String> tm = new TreeMap<String,String>();
s.add(sm); //This fails
s.add(tm);

Why does adding sorted map to a Set that allows ? super TreeMap and instantiated as such fail?

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Why Use Extends Keyword Instead Of Implements In Generics

Feb 22, 2015

Why java uses the keyword extends when setting the bound of a type parameter(Generic) to an interface. I think using the keyword implements is more intuitive.

public static <T extends Comparable<T>>

why use extends? and not implements.

int countGreaterThan(T[] anArray, T elem) {
int count = 0;
for (T e : anArray)
if (e.compareTo(elem) > 0)
++count;
return count;
}

I know if I want to set multiple bounds I will use extends keyword, and I will concatenate the bounds using & operator.

Is this a design decision to always use extends keyword to set bounds?

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Super Keyword When Used Explicitly In A Subclass Constructor

Jul 9, 2014

The super keyword when used explicitly in a subclass constructor must be the first statement but what about if i have a this(parameters) statements ? As the this one must also be the first statement... Does this means that i can have only one or the other ? What about when the super constructor is not explicit (aka implicit ) , can i use the this( parameters) in the same constructor ?

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Program Shows Error While Using Super Keyword

Jan 3, 2015

//constructor
class Base
{
Base(int a) {
System.out.println("in base"+a);;
}
}
class Cons extends Base

[Code] .....

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Difference Between Static Keyword And Transient Keyword

Oct 7, 2014

What is the difference between the static keyword and transient keyword?

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Integer Sum In Generics

Mar 20, 2015

What is wrong in this

class Gen<k,v>
{
K a;
V b;
Gen(K s, V f) {
a=s;
b=f;

[Code] .....

a is object of type k,then how can I retrieve content of a?

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Swing/AWT/SWT :: Relationship Between Super And Child?

Aug 27, 2014

I want to make an application and must use strategy pattern my idea is to create a super class in this case Movie Player and three sub classer and they'll komminesera with each other using strattegy pattern, one of the sub classes is Button Panel and I want to add it to Movie Player and it was to be its child,so how can I add the butt panel to Movie Player and it shall be its children?

MoviePlayer:
import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import com.github.sarxos.webcam.WebcamPanel;

[code]....

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Super Call / Subclasses And Inheritance

Feb 12, 2014

what does super(); do in the following method, I understand its uses to access variables belonging to the superclass but i am unsure of what that one line does. Here is a sample constructor..

public CreditCard()
{
// fill in the default constructor and use the super call
super();
id = "000000";
year = 0;
}

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Casting Exception In Generics

Jan 29, 2014

I have studied that Generics are used to shift the Class Cast Exception into Compile time errors , So that we get errors at compile time error and we do correct them before executing ,but Here is a program in which i am getting Class Cast Exception

class Animal
{
}
class Dog extends Animal
{
}
class Cat extends Animal

[code]..

Getting Exception at line no 29 which i know why it occurs but just wanna ask that isn't it should be caught at compile time According to Generics ?

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Clarify Generics For Classes

Jun 19, 2014

I just want to clarify generics for classes.

Referring to [URL] ....

Generics means all methods in class Box will return an Integer if their parameters are defined as T when the instantiation is :

Box<Integer> integerBox = new Box<Integer>();

Are those Boxes only can contain Integers? Can I write

Box <Toys> toybox = new Box <> ();

To show that the Box only contain toys?

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Returning Int Array Using Generics

Apr 10, 2014

public int[] allIndicesOf(E itemSought) {
ArrayList<Integer> toUse = new ArrayList<>();
for (E anArray : container) {
if (anArray.equals(itemSought)) {
toUse.add(container.indexOf(itemSought));

[Code] ....

I have an array list of strings. I want to be able to return an array of integers telling me which indexes in the string array list contain the itemSought object.

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Static Keyword For Arraylist Containing Objects

Jun 14, 2014

I've come across something that i'm not overall sure about regarding the static keyword in Java.I'm making a vertical scrolling game where the player simply shoots enemies and they shoot back as they fall, dropping items if they die such as power ups and coins. I have an enemy called Bat and this is the bullet creation code in the update method:

if(oldPlayerY + 220 > posY && getBulletDelay > 0.90f){
batBullets.add(new Bullet(posX + 10, posY - 10));
getBulletDelay = 0;
}

The method is creating a new bullet object and it then adds that to the arraylist called batBullets, which is simple enough. I then need to access this arraylist in the main game update class so I can render those bullets on the screen, even if the bat dies. I was always taught that you use the static keyword when you need to access something from the class that doesn't require an object. Because of this, I have the following code.

for(Bullet bullet : Bat.batBullets){
bullet.setY(bullet.getY - 5); // Set the bullet to fall
renderMap.getSpriteBatch().draw(bullet.batBullet(), bullet.getX(), bullet.getY()); // render the bullets
}

This seems perfectly fine to me because I need to access the batBullet arraylist and it doesn't make sense to create a new bat object as I already have random spawning in place for them.

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How Registry Can Look Before Java Install - Keyword

Dec 2, 2014

I am attaching a document which shows the current state of my registry.What I want to know is if I can [safely] delete the JavaSoft folder with all lower subfolders, then re-install jdk1.6/0_31 which, I am told, is the current version being used here by developers.According to others on the development team (not my team), there COULD be something in the registry that is preventing both the installation of java jdk AND its uninstallation.Since I cannot seem to attach any kind of document.

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Super Class - Getting Error Identifier Expected

Aug 9, 2014

I created a superclass Ships and under that a class CivilShips. Under that HumanBulkFreighter.

I declared variables in Ships and CivilShips and wanted to have them set in HBF to a specific value. When I know try to compile them I get the following:

HumanBulkFreighter.java:2: error: <identifier> expected
cargo=1500;
^
HumanBulkFreighter.java:3: error: <identifier> expected
size=200;

[Code] ....

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Super Constructor Call - Creating Object

Sep 18, 2014

Its written that every constructor calls its super class constructor. And we know, constructors are called to create an object. Does it mean that if I am creating an object of a class, I am actually creating objects of all its super class???

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How To Get Access From Variables In Super Class Or Subclass

Dec 2, 2014

how to get access from variables in a super class or a subclass. Here is what I got:

1) I have a super class that is in Jar file, I created a link in Eclipse, I know that the link is created correctly, I am going to concentrate just in one variable, so I don’t have to put all the code here firstName; in the super class(the one that is define in the path)

public class CommissionEmployee {
// Field descriptor #6 Ljava/lang/String;
private java.lang.String firstName;

in my class i have 6 argument constructor

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Set Methods In Super And Subclass By Using Dialog Boxes

Nov 7, 2014

I am creating a set of 3 subclasses, 1 superclass, and an application. In my instructions it says to make set methods in my super and subclass by using dialog boxes. In the application you have 3 different arrays where you create objects and are supposed to call the methods from the subclasses to be used in the application. I don't know how to make the dialog boxes from my subclasses to show up in my application.

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Generics Wildcard Array Of References?

Jul 27, 2014

An array of references to a specific generic type is not allowed in Java.

e.g.,

ArrSpec<String> arrs[] = new ArrSpec<String>[10];

is not allowed though the type checkng and memory allocation can be done at the compile time itself.

Instead of this, Java allows to use Wildcard type array of references to a generic type.

e.g.,

ArrSpec<?> arrs[] = new ArrSpec<?>[10];

is allowed though the type checking, memory allocation and any type of values to be stored would be decided at the runtime.

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Generics In Java - E Is A Subtype Of Comparable

Jun 5, 2015

My book defines this generic method

Java Code:

public static <E extends Comparable<E>> void sort(E[] list... mh_sh_highlight_all('java');

Comparable is an interface and from how i look at this piece of code is that I can only use a class that implements the Comparable interface; however, this is the context my book uses when explaining the following code

First, it specifies that E is a subtype of Comparable.

Second, it specifies that the elements to be compared are of the E type as well.

What does it mean when it says E is a subtype.

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Generics And Inheritance - Specialized Methods

Mar 6, 2014

I've an interface with generic methods in it. I would like to have specialized methods in the sub types. While doing that I'm seeing the following warnings in eclipse.

class Sorter {
<E> void sort(E[] elements);
};
 
class StringSorter {
 
// This gives me a warning 'hiding' to 'sort'
<String> void sort(String[] elements) {
}
 
// Gives me an error "The method someCrap(String[]) in the type StringSorter is not applicable for the arguments (String[])"
void someCrap(String[] elements) {
}
};

I would like to understand why eclipse gives the above warnings and errors.

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Enciphering And Deciphering Message Using Provided Keyword

May 23, 2014

I need creating a java keyword program that can encipher and decipher a message using the provided keyword.

The keyword is :javbean

I have attached the message text as well...

public class Caesarcipher
{
public static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz";
public static void main(String[] args)
{
String theKey = "JAVBEANDEFGHIJKLMNOPQRSTUVWXYZ";
String text= "message.txt";

[Code] ....

message.txt (763bytes)

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Formatting Exception Output - Keyword Not Found

Nov 7, 2014

I've been trying for a while to get my exception output to print in a particular form to System.err.

What I'm looking for as output is

KeywordException: edu.cofc.csci221.KeywordException: **Keyword Not Found**

I'm getting

Keyword Exception: edu.cofc.csci221.KeywordException
at edu.cofc.csci221.CheckLine.checkForInvalidKeyword(CheckLine.java:101)
at edu.cofc.csci221.ReadLogFile.main(ReadLogFile.java:47)

The code:

line = scan.nextLine();
try { check.checkForInvalidSymbols(line); } catch (SymbolException sEx) { System.err.print("Symbol Exception: "); sEx.printStackTrace(); }
try { check.checkForInvalidKeyword(line); }catch (KeywordException kEx) { System.err.print("Keyword Exception: "); kEx.printStackTrace(); }
if(check.checkFirstKeyword(line) && line.split(" ")[0].equals(keywords[0])) { System.out.println(line); }

[Code] ....

I'm just unsure of where to go/what to do.

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Display Name Of All People In ArrayList Containing Passed In Keyword

Dec 8, 2014

i need to write a method, that passes in an arraylist and a keyword,and display the name of all the people in the arrayList whose name contain the keyword (irrespective of uppercase or lowercase). how to write such a method ??

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Inheritance Super Variables - Fail To Return Null

Aug 13, 2014

I'm writing a simple program in which I have a super class Person, inherited by the subclasses Customer and Employee (they inherit the variables ID, name and surname).

Java Code:

public class Person {
int id;
String name;
String surname;
public Person () {

[Code] .....

However the problem is here: when I try to get the variables ID, name and surname through my main class, they fail to return (0,null,null). Why is this? I have get-Methods in my subclasses which should return the super variables, but they are not.

Java Code:

public String getUser() {
return username;
}

public String getName() {
return super.name;
} mh_sh_highlight_all('java');

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Why Super Classes Always Declared As Interfaces And Not Abstract Class

Dec 1, 2014

While reading the design patter book, i got one doubt ,There is a List an interface having sub classes ArrayList, LinkedList etc.,

Q1) My question is Why they declared the List as interface rather than Abstract class?

Q2) i read some site -

List l = new ArrayList(); Why it is GOOD line?
ArrayList l = new ArrayList() ; Why it is BAD line?

Answer required with detailed information for Q1 and Q2.

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