I am thinking about this java oop problem . I don't exactly know exactly what is really happening there . Can someone make me understand ?
abstract class A {
public int proc (A p){
return 98;
}
}
class B extends A {
public int proc(A p) {
return 17;
}
}
class C extends A {
public int proc (C p) {
return 65;
}
}
public class HelloWorld{
public static void main(String []args){
C x = new C(); // here x is C type and is an instance of C ?
A y=new B(); // here y is A type and is an instance of B?
C z=new C(); // here z is C type and is an instance of C ?
System.out.println(y.proc(x)+z.proc(x)); /* y is A type so it is looking for proc function in A ,but doesn't return 98
, z is C type and it is looking for proc function in C and return 65 .*/
}
}
Can someone tell me how should I tackle theese instances ?
Y is an A but contains a reference to a B so when we call y.proc it is the proc in B that is called, and returns 17, not 98.
You need to look at the instance that is created and not the type of variable.
In Java, you can create base class objects which hold the child classes objects except for abstract classes.
A is an abstract class, you cannot instantiate it, but you can assign an object of the child class to it which holds the child class object's features.
You can think, we are assigning the reference of B to y which has a type of A.
System.out.println(y.proc(x));
The code above will print 17, which is the return value of the proc in class B returns.
Whenever you call the methods of y, the compiler will give you class B's methods automatically.
So, you are creating an instance of class B in the memory and assigning it to y which points to the same memory location.
For that reason, you can use the methods of class B.
Same goes for z as well.
I'm trying to generalise some code by iterating over all constants of an enum to receive the same specific argument from each one.
Specifically I have an enum P with some constants A,B,C.
Each of these constants is itself an enum and implements an interface I that defines a function f.
P.values() gives me an array P[] A = {A,B,C}, however I can't call A[i].f() since A[i] is of course of type P which doesn't implement I.
Now in my understanding a function can return an interface, but I can not instantiate it and therefore can't cast to it.
Should I overwrite values() for P to return I[]? If so, how would I do that since I can't cast to I? Or is there another solution?
I am working in eclipse but assuming that it's complaints are indicative of a true mistake, not just eclipse not recognising types.
Since I'm somewhat new to Java I would also appreciate any links to resources that explain the underlying rules of type matching/checking.
This seems to do what you describe - perhaps I have misunderstood your question though. If so please explain further.
interface I {
void f ();
}
enum P implements I{
A,
B,
C {
// Demonstrate enum-specific implementation.
#Override
public void f () {
System.out.println("SEEEEEE!");
}
};
// By default `f` prints the name of the enum.
#Override
public void f () {
System.out.println(name());
}
}
public void test() throws Exception {
for ( I i : P.values()) {
i.f();
}
}
I am facing some problems about inheritance in Java.
I can't understand why the following two programs have those outputs!
Could anyone help me? :)
1)
public class A {
int foo() {
return 1;
}
}
public class B extends A {
int foo() {
return 2;
}
}
public class C extends B {
int bar(A a) {
return a.foo();
}
}
C x = new C();
System.out.println(x.bar(x));
// OUTPUT:2
2)
public class A {
int e=1;
}
public class B extends A {
int e=2;
}
public class C extends B {
int bar(A a){
return a.e;
}
}
C x= new C();
System.out.println(x.bar(x));
// OUTPUT:1
In both cases, you're passing in an object of type C into the print function. The bar function asks for an object of type A, but it's still acceptable for you to pass in an object of type C since it is a subclass of A. So first of all, it's important to keep in mind that a.foo() and a.e are being called on a C object.
So what is happening in both cases is that it's searching for the lowest attribute or method in the list. Here is a very simplified version of what Java is doing in part 1:
Hey, you've passed in an object of type C to the bar method! Now let's call its foo method.
Whoops! C doesn't have a foo method! Let's take the next step up to the B class to see if it has a foo method.
Yay! B has a foo method, so let's call it. No need to work our way up to the A class because we've already found what we need in B.
It's all about understanding that the parameter was downcast from A to C. The exact same sort of logic is used in part 2. It notices that an object of type C was passed in, so it gets the e attribute from object B since its the lowest class in the hierarchy that contains that attribute.
Hopefully that answers your question!
Upcasting is allowed in Java, however downcasting gives a compile error.
The compile error can be removed by adding a cast but would anyway break at the runtime.
In this case why Java allows downcasting if it cannot be executed at the runtime?
Is there any practical use for this concept?
public class demo {
public static void main(String a[]) {
B b = (B) new A(); // compiles with the cast,
// but runtime exception - java.lang.ClassCastException
}
}
class A {
public void draw() {
System.out.println("1");
}
public void draw1() {
System.out.println("2");
}
}
class B extends A {
public void draw() {
System.out.println("3");
}
public void draw2() {
System.out.println("4");
}
}
Downcasting is allowed when there is a possibility that it succeeds at run time:
Object o = getSomeObject(),
String s = (String) o; // this is allowed because o could reference a String
In some cases this will not succeed:
Object o = new Object();
String s = (String) o; // this will fail at runtime, because o doesn't reference a String
When a cast (such as this last one) fails at runtime a ClassCastException will be thrown.
In other cases it will work:
Object o = "a String";
String s = (String) o; // this will work, since o references a String
Note that some casts will be disallowed at compile time, because they will never succeed at all:
Integer i = getSomeInteger();
String s = (String) i; // the compiler will not allow this, since i can never reference a String.
Using your example, you could do:
public void doit(A a) {
if(a instanceof B) {
// needs to cast to B to access draw2 which isn't present in A
// note that this is probably not a good OO-design, but that would
// be out-of-scope for this discussion :)
((B)a).draw2();
}
a.draw();
}
I believe this applies to all statically typed languages:
String s = "some string";
Object o = s; // ok
String x = o; // gives compile-time error, o is not neccessarily a string
String x = (String)o; // ok compile-time, but might give a runtime exception if o is not infact a String
The typecast effectively says: assume this is a reference to the cast class and use it as such. Now, lets say o is really an Integer, assuming this is a String makes no sense and will give unexpected results, thus there needs to be a runtime check and an exception to notify the runtime environment that something is wrong.
In practical use, you can write code working on a more general class, but cast it to a subclass if you know what subclass it is and need to treat it as such. A typical example is overriding Object.equals(). Assume we have a class for Car:
#Override
boolean equals(Object o) {
if(!(o instanceof Car)) return false;
Car other = (Car)o;
// compare this to other and return
}
We can all see that the code you provided won't work at run time. That's because we know that the expression new A() can never be an object of type B.
But that's not how the compiler sees it. By the time the compiler is checking whether the cast is allowed, it just sees this:
variable_of_type_B = (B)expression_of_type_A;
And as others have demonstrated, that sort of cast is perfectly legal. The expression on the right could very well evaluate to an object of type B. The compiler sees that A and B have a subtype relation, so with the "expression" view of the code, the cast might work.
The compiler does not consider the special case when it knows exactly what object type expression_of_type_A will really have. It just sees the static type as A and considers the dynamic type could be A or any descendant of A, including B.
In this case why Java allows downcasting if it cannot be executed at the runtime?
I believe this is because there is no way for the compiler to know at compile-time if the cast will succeed or not. For your example, it's simple to see that the cast will fail, but there are other times where it is not so clear.
For instance, imagine that types B, C, and D all extend type A, and then a method public A getSomeA() returns an instance of either B, C or D depending on a randomly generated number. The compiler cannot know which exact run-time type will be returned by this method, so if you later cast the results to B, there is no way to know if the cast will succeed (or fail). Therefore the compiler has to assume casts will succeed.
# Original Poster - see inline comments.
public class demo
{
public static void main(String a[])
{
B b = (B) new A(); // compiles with the cast, but runtime exception - java.lang.ClassCastException
//- A subclass variable cannot hold a reference to a superclass variable. so, the above statement will not work.
//For downcast, what you need is a superclass ref containing a subclass object.
A superClassRef = new B();//just for the sake of illustration
B subClassRef = (B)superClassRef; // Valid downcast.
}
}
class A
{
public void draw()
{
System.out.println("1");
}
public void draw1()
{
System.out.println("2");
}
}
class B extends A
{
public void draw()
{
System.out.println("3");
}
public void draw2()
{
System.out.println("4");
}
}
Downcast works in the case when we are dealing with an upcasted object.
Upcasting:
int intValue = 10;
Object objValue = (Object) intvalue;
So now this objValue variable can always be downcasted to int because the object which was cast is an Integer,
int oldIntValue = (Integer) objValue;
// can be done
but because objValue is an Object it cannot be cast to String because int cannot be cast to String.
Downcasting is very useful in the following code snippet I use this all the time. Thus proving that downcasting is useful.
private static String printAll(LinkedList c)
{
Object arr[]=c.toArray();
String list_string="";
for(int i=0;i<c.size();i++)
{
String mn=(String)arr[i];
list_string+=(mn);
}
return list_string;
}
I store String in the Linked List.
When I retrieve the elements of Linked List, Objects are returned. To access the elements as Strings(or any other Class Objects), downcasting helps me.
Java allows us to compile downcast code trusting us that we are doing the wrong thing.
Still if humans make a mistake, it is caught at runtime.
Consider the below example
public class ClastingDemo {
/**
* #param args
*/
public static void main(String[] args) {
AOne obj = new Bone();
((Bone) obj).method2();
}
}
class AOne {
public void method1() {
System.out.println("this is superclass");
}
}
class Bone extends AOne {
public void method2() {
System.out.println("this is subclass");
}
}
here we create the object of subclass Bone and assigned it to super class AOne reference and now superclass reference does not know
about the method method2 in the subclass i.e Bone during compile time.therefore we need to downcast this reference of superclass to subclass reference so as the resultant reference can know about the presence of methods in the subclass i.e Bone
To do downcasting in Java, and avoid run-time exceptions, take a reference of the following code:
if (animal instanceof Dog) {
Dog dogObject = (Dog) animal;
}
Here, Animal is the parent class and Dog is the child class.
instanceof is a keyword that is used for checking if a reference variable is containing a given type of object reference or not.
Downcasting transformation of objects is not possible.
Only
DownCasting1 _downCasting1 = (DownCasting1)((DownCasting2)downCasting1);
is posible
class DownCasting0 {
public int qwe() {
System.out.println("DownCasting0");
return -0;
}
}
class DownCasting1 extends DownCasting0 {
public int qwe1() {
System.out.println("DownCasting1");
return -1;
}
}
class DownCasting2 extends DownCasting1 {
public int qwe2() {
System.out.println("DownCasting2");
return -2;
}
}
public class DownCasting {
public static void main(String[] args) {
try {
DownCasting0 downCasting0 = new DownCasting0();
DownCasting1 downCasting1 = new DownCasting1();
DownCasting2 downCasting2 = new DownCasting2();
DownCasting0 a1 = (DownCasting0) downCasting2;
a1.qwe(); //good
System.out.println(downCasting0 instanceof DownCasting2); //false
System.out.println(downCasting1 instanceof DownCasting2); //false
System.out.println(downCasting0 instanceof DownCasting1); //false
DownCasting2 _downCasting1= (DownCasting2)downCasting1; //good
DownCasting1 __downCasting1 = (DownCasting1)_downCasting1; //good
DownCasting2 a3 = (DownCasting2) downCasting0; // java.lang.ClassCastException
if(downCasting0 instanceof DownCasting2){ //false
DownCasting2 a2 = (DownCasting2) downCasting0;
a2.qwe(); //error
}
byte b1 = 127;
short b2 =32_767;
int b3 = 2_147_483_647;
// long _b4 = 9_223_372_036_854_775_807; //int large number max 2_147_483_647
long b4 = 9_223_372_036_854_775_807L;
// float _b5 = 3.4e+038; //double default
float b5 = 3.4e+038F; //Sufficient for storing 6 to 7 decimal digits
double b6 = 1.7e+038;
double b7 = 1.7e+038D; //Sufficient for storing 15 decimal digits
long c1 = b3;
int c2 = (int)b4;
//int 4 bytes Stores whole numbers from -2_147_483_648 to 2_147_483_647
//float 4 bytes Stores fractional numbers from 3.4e−038 to 3.4e+038. Sufficient for storing 6 to 7 decimal digits
float c3 = b3; //logic error
double c4 = b4; //logic error
} catch (Throwable e) {
e.printStackTrace();
}
}
}
I will tell you why this happened. First of all you have to understand how JVM supports when we assign parent class into the child class using downcasting, because of reference . For example consider in the following code.
A is the super type any class that extends from it and can store the reference B class.
A a =new B();
When you assign a reference variable into the child class jvm will understand that since A can store the reference of B class that is why you can do it.
B b=(B)b;
The reason which is called compile time error and why you couldn't directly assign Parent class into the Child class because there is not any extends relationship. Note that casting only occurring with the key which is called extends, that is why you receive the compile time error.
Another reason which is called ClassCastException by the runtime because of jvm it directly accept the rule which is okay I accept that it is true but jvm after that will understand that by the runtime it is not store any referance of Child class when code was writing by the programmer who write coding in the syntax .
I am learning Java using the book Java: The Complete Reference. Currently I am working on the topic of abstract classes.
Please Note: There are similar questions on stackoverflow. I searched them but I couldn't understand the concept.
If I run the below program, it produces the correct output, but I didn't understand the concept.
What is the need of reference variable of an Abstract class here. I can get the output without the reference variable of an abstract class.
First I ran the below program and got the desired output.
abstract class Figure {
double dim1;
double dim2;
Figure(double a, double b) {
dim1 = a;
dim2 = b;
}
// area is now an an abstract method
abstract double area();
}
class Rectangle extends Figure {
Rectangle(double a, double b) {
super(a, b);
}
// override area for rectangle
double area() {
System.out.println("Inside Area for Rectangle.");
return dim1 * dim2;
}
}
class Triangle extends Figure {
Triangle(double a, double b) {
super(a, b);
}
// override area for right triangle
double area() {
System.out.println("Inside Area for Triangle.");
return dim1 * dim2 / 2;
}
}
class AbstractAreas {
public static void main(String args[]) {
Rectangle r = new Rectangle(9, 5);
Triangle t = new Triangle(10, 8);
Figure figref;
figref = r;
System.out.println("Area is " + figref.area());
figref = t;
System.out.println("Area is " + figref.area());
}
}
And I tried the below code without creating/using abstract class reference.
class AbstractAreas {
public static void main(String args[]) {
Rectangle r = new Rectangle(9, 5);
Triangle t = new Triangle(10, 8);
// Figure figref;
// figref = r;
System.out.println("Area is " + r.area());
// figref = t;
System.out.println("Area is " + t.area());
}
}
It also gave the same output as the first program.
Can anyone please explain what is the need of calling "area method" using abstract class reference.
It's meant simply as a demonstration that even though you declared the variable as the abstract type, you can assign an instance of a concrete subclass to it and get the overriden behavior from the subclass.
Practical use example would be if you needed a collection of them:
List<Figure> figureList = new ArrayList<Figure>();
figureList.add(new Rectangle(9, 5));
figureList.add(new Triangle(10, 8));
for (Figure f : figureList) {
System.out.println(f.area());
}
Or if you want to pass any subclass of Figure to a method that used the area():
public void printArea(Figure f) {
System.out.println("Area is: " + f.area());
}
...
myObject.printArea(new Rectangle(9, 5));
myObject.printArea(new Triangle(10, 8));
In Abstract classes you can define abstract as well as non abstract methods. However the 1st concrete subclass of the Abstract class must implement those abstract methods. You cannot create instance of Abstract classes and they must be subclassed to some concrete class.
Also note JLS states if abstract classes have all abstract method it is better to use interface.
Can anyone please explain what is the need of calling "area method" using
abstract class reference.
Concept is same as inheritance. We use abstract classes to avoid duplicate.
What is the need of reference variable of an Abstract class here. I can get the
output without the reference variable of an abstract class.
Abstract class is used as a reference because you can take advantage of polymorphism here. If you call area() on the reference variable at runtime it will call the corresponding implementation of Traingle or Rectangle based on the actual instance type.
Hey here you're using a concept of UPCASTING which is also known as Parent reference to the child object . And the above code program which u have written is performing UPCASTING. Let us look what is UPCASTING.
Upcasting is a mechanism of using parent class reference to refer the child class objects.
Whenever you use Upcasting you can access only the parents class members ( both variables and methods) and the overridden methods of parent class.
In your example the method area() has been overidden in the child classes Rectangle and Triangle so they can be accessed using parent reference figref .
One of the advantage of UPCASTING is we can achieve Dynamic Method Dispatch or Dynamic Polymorphism which is very much necessary in writing complex applications having complex class hierarchies.
Since u mentioned you're using Complete reference Check out the section Dynamic Method Dispatch which comes after method overriding .
Hope this answer Helps :)
Yes you can get the same answer But it is always preferred to use abstract classes or intefaces to call any api.
area() is an api which is overridden in Rectangle or Triangle.