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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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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Oct 17, 2014
While using generics, are there cases when ? wildcard cannot be replaced with letters [A-Z]? So far , I was able to find only one case, it is when you want to have field pointing on generic instance without making class generic.
class OneClass {
private LinkedList<?> myLL;
}
In case above, as I understand, you cannot use [A-Z] without generalize OneClass. Are there any other cases, when there is no way except to use ? wildcard instead of letter [A-Z]?
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Jul 31, 2014
The followings are what I see just now.....
//******************************************
Ambiguities
Wildcard imports have one problem though: they can lead to ambiguities when classes with the same name exist in two packages you import via wildcard.
Imagine the following two imports:
import foo.*;
import bar.*;
Now you want to use the class foo.Node but there is also a class bar.Node. Now you need to use non-wildcard imports to resolve the ambiguity that would happen otherwise.
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Sep 7, 2014
Is there any way to create file refrences so that it look's like there is a file in a directory while it actually is in a different directory?
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Mar 21, 2014
how to do a deep copy of objects that contain references. I am specifically wanting to make a deep copy of a tree. Logically, each tree node contain references to its children nodes. Here is the basics of my node class
public class BSTNode implements Comparable, Serializable {
private String token;
private int count;
private BSTNode leftChild;
private BSTNode rightChild;
I know I must only be making a shallow copy because when I make a copy of tree1, called tree2, and edit tree1 the edits also appear on tree2. Here is my copy method within my BSTNode class
public BSTNode copy()
{
BSTNode obj = null;
try{
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bos);
out.writeObject(this);
out.flush();
out.close();
[code]....
When I wish to copy the entire tree, I call that above copy method from my BSTree class using the methods below. (I have 2 methods because this is a homework assignment that requires a copy method that calls a preorder traversal method)
public BSTNode copy()
{
BSTNode copiedTreeRoot = new BSTNode();
return copyTree(copiedTreeRoot,root);
[code]....
And further along when I make changes to tree1, tree 2 also changes. I have no clue what I'm doing wrong. I believe it must be somewhere in how I return the new tree or something.I tried this edit to my copy method, but it made no difference.
public BSTNode copy() {
BSTNode obj = null;
try{
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bos);
out.writeObject(this);
[code].....
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May 22, 2015
So I set out to write a program that takes two things from user: Name and Age
Then prints out "Name is Age"
I went through using a "launcher" and having a proper object: [URL]
The class names are Practice and Practice Launcher because I just use a Practice file as a sandbox environment so I understand its not correctly named. I also understand my comments aren't great but I'm just trying to make it work.
Practice.java
public class Practice {
//constructor
public Practice (String a, int b) {
[Code]....
My Practice.userName doesnt reference the variable userName. Why is this?
Also y does this line need Practice twice?
Practice Practice = new Practice(userName, 45);
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Apr 19, 2014
I have to implement an object pool that uses phantom references to collect objects abandoned by client threads. This is what I have. I'm really not sure about this implementation.
class ObjectPool<T extends CloneableObject<T>> {
private Queue<T> pool;
private List references = new ArrayList();
private ReferenceQueue rq = new ReferenceQueue();
private CloneableObject<T> prototype
[Code] .....
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Jan 19, 2014
So I have an array of objects, each with their own position, I tried switch the references in the array of two objects, then repainted (immediately), but the two objects aren't switching positions on screen...does this even work?
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Jan 29, 2014
"What happens if you modify the common object references in these lists, myArrList and urArrList ? We have 2 cases here: In the first one, you reassign the object reference using either of the lists. In this case, the value in the second list will remain unchanged.In the second case, you modify the internals of any of the common list elements - in this case, the change will be reflected in both lists."
I have written the following code, which tests the first case mentioned above, and i get the output as expected: myarrList remains unchanged. How can i test the second case ? My thoughts are ....'second case is untestable the following code, because String is immutable. I need to use StringBuilder or something else to write code for test of second case mentioned'.
ArrayList<String> myarrList = new ArrayList<>();
myarrList.add("one");
myarrList.add("two");
ArrayList<String> urarrList = new ArrayList<>();
urarrList.add("three");
urarrList.add("four");
System.out.println("ArrayLists setup");
[code]....
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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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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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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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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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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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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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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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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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Dec 15, 2014
is the Java upcating and downcasting rules are same for general object type or generics types?
1) Dog dog = new Animal();
Type mismatch can't covert Dog to Animal - complie time error
2) Animal animal = new Animal();
Dog dog = (Dog) animal;
java.lang.ClassCastException: Animal cannot be cast to Dog at runtime.
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Oct 25, 2014
I'm working with Doubly Linked Lists and using Java Generics..
My nodes looks like this:
class DNode<E> {
DNode<E> previous;
DNode<E> next;
E element;
//and all methods inside
}
My list of Nodes looks like this:
class DLL<E>{
private DNode<E> head;
private DNode<E> tail;
private int size;
[code]....
As you can see, as arguments they get "E o"...I need to write a program, which from the main function asks the users how long is the list, and after they type it's length, I ask them to start typing the elements (integers)...and this is how my main method is written, but I can't seem to make it work, specialy when I call the "insLast" method,I guess it's because the arguments i'm giving to the function...how to read the elements and write them into the list?
public static void main(String[] args) throws IOException {
DLL<Integer> lista=new DLL<Integer>();
BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
String s = stdin.readLine();
int N = Integer.parseInt(s);
s = stdin.readLine();
String[] pomniza = s.split(" ");
for (int i = 0; i < N; i++) {
lista.instLast(Integer.parseInt(pomniza[i]));
}
}
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Aug 27, 2014
i am interested to add integer objects and String objects into any collection object ..... while iterating the collection object i am not interested to do any type cast in java
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Aug 29, 2014
I am trying to make a generic method that will replace the data type T with those number types usable with a Scanner object. However, whenever I try to compile, I get errors saying that a Byte/Integer/Double etc are found when only a type T is allowed. This is the beginning of my method. I can;t understand what is wrong with it.
Java Code:
public <T extends Number> T nextRanged(T lowerBound, T upperBound, boolean inclusive, String errorMessage){
// Holds program execution until user inputs a numeric value between the bounds. Prevents all other input without exception.
// Output data type determined by the type of the bounds.
T input = null;
try{
if(input instanceof Byte){
input = new Byte(internalScanner.nextByte());
[Code] ....
The purpose of the method, in the end, will be to provide the nextXXX() functionality of a Scanner object but with built in validation procedures. I could easily do this by making a nextIntRanged(), nextDoubleRanged() etc methods, but this seems wasteful to me.
I meant "incompatible type errors"!
Error example:
ValidatedScanner.java:57: error: incompatible types
input = new Byte(internalScanner.nextByte());
^
required: T
found: Byte
where T is a type-variable:
T extends Number declared in method <T>nextRanged(T,T,boolean,String)
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Jun 14, 2014
The erasures of all constituent types of a bound must be pairwise different, or a compile-time error occurs.
Well I know what type erasure is, and I think I kind got what this statement means. My understanding from it is that if your type parameter has more than one bound and those bounds occurs to be the same type after erasure that is a compile-time error. Is that it?
The only thing I could found related is something like this:
class A<T extends List<Integer> & List<Integer>>{
}
Which as you might know gives the Duplicated bound error.
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Feb 21, 2014
I have doubt in generics,
List<int> c=new ArrayList<int>();
why we cannot use primitive data type like int,double.
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Dec 27, 2014
I am reading Head First: Java and got to Object References. In the book I got a little bit confused on what happens when two object reference's point at the same object so I wrote a small crude test, the below code. This of course clarified what happens but what I am interested in knowing is in what circumstances would you want to have two separate references for the same object when you could just use the original? Eg. v1
class ObjectValue{
int objVal = 1;
}
class ObjectValueTestDrive{
public static void main(String [] args){
// "Value of v# should be" refers to if it copied the given object values, instead of referencing the same object
ObjectValue v1 = new ObjectValue();
System.out.println("Value of v1 should be 1:" + " "+ v1.objVal);
[code]....
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Nov 8, 2014
So I need to generate 10 random integers in the range 1-20 but i have to store them in an array called numbers. Then I have to call a method called displayArray which displays the contents of the array and for the assignment i have to use a for loop to traverse the array. The method header for the displayArray method is:
public static void displayArray(int[] array)
This is what I have done
public class RandomIntegers {
static int numbers = 0;
public static void displayArray(int[] array) {
System.out.println(numbers + "Numbers Generated");
[Code] .....
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