I have two classes that make instances of each other, which probably should create an infinite loop, but they're not. class A is created first.
class A {
B[] b = new B[9];
A() {
for (each) { // pseudo code
b[each] = new B(this);
}
}
}
class B {
A a;
B(A a) {
this.a = a;
}
}
My Question: Why is this code NOT making an infinite loop?
You only have one A instance (if you write new A() somewhere) that holds 9 B instances and each of them hold a reference to the same A instance.
This creates a loop in the sense that the memory of A and the Bs cannot be freed because they reference each other. Other than that there is no problem, no "infinite" loop that would cause any major problems, in particular there is no infinite loop creating more and more objects.
If you wrote
class B {
A a;
B() {
this.a = new A();
}
}
then you would get into trouble.
It won't.. You are just capturing a reference of object of Class A with
this.a = a;
and not creating a new one.
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!
public class A {
private String superStr;
public String getSuperStr() {
return superStr;
}
public void setSuperStr(String superStr) {
this.superStr = superStr;
}
}
public class B extends A {
private String subStr;
public String getSubStr() {
return subStr;
}
public void setSubStr(String subStr) {
this.subStr = subStr;
}
}
And I expect result likes below
public static void main(String[] args) {
A a = fuc();
B b = new B();
b = (B) a;
b.setSubStr("subStr");
System.out.println(a.getSuperStr() + b.getSubStr());
}
private static A fuc() {
A a = new A();
a.setSuperStr("super");
return a;
}
but java.lang.ClassCastException is ocuured.
How can I cast this?
I want use subclass variable and super class variable.
thank you
How can I cast this?
You can't. You can only cast when the object in question has an "is a" relationship with the type. In your case, you have an A object (the one from fn), which is not a B. (All B objects are As, because B extends A, but not all A objects are Bs.)
Consider: Let's call B Bird and A Animal: A Bird is an Animal, but not all Animals are Birds. So we can treat all Birds as Animals, but we cannot treat all Animals as Birds. When you're using a variable of a given type to refer to an object, you're treating the object as being of that type (e.g., B b = (B)a tries to treat the Animal a as a Bird).
Side note: There's no point to the indicated part of the code:
B b = new B();
// ^^^^^^^^^^
b = (B) a;
Since the very next line assigns to b (well, it would if it would compile), there's no purpose served by doing new B() and assigning that to b just beforehand. (Unless the B constructor has side-effects, which is generally a Bad Idea™.)
Casting a particular object to another types does not magically convert it into an instance of that class (or at least not in Java); Therefore, the object referenced by variable a does not e.g. have the field subStr to use despite that the object referenced by b after executing B b = new B(); does.
The others have already explained why you can't do that. I'm here to give you a simple alternative. Your B class could have a constructor that had an A as argument and you would simply wrap that A so you could "transform" it to a B. Using that your code would look way more clean, it would actually work and you were following a good design pattern. For more information check the Decorator Pattern
I can't modify either Class A or Class B. And both Class A & B are huge in size (with several nested Classes and hundreds of parameters). And with multiple threads, memory foot print is impacting the performance.I'm checking all ways to reduce memory usage. Basically I'm trying to limit the scope of Class B instance so that GC can work on it at the earliest.
(For Your Information: I already knew that I can do by B b = new B(); b.setS("Calm Down"); a.setB(b);)
Here is the scenario:
Class A{
private B b;
public getB{return b}
public void setB (B b){this.b = b;}
}
Class B{
private String s;
//getters and setters for s}
Class MyNeed{
A a = new A();
// Here I'm trying to create an obj B and set S and then pass that obj to a.setB().
a.setB (new B().setS("Param S Set"));
}
So I guess that new B() is local to setB(). so in the very next line new B() is out of scope.
But this way in eclipse, I'm getting error that setB() can't accept void. I guess it is setS() returning void.
May be I'm missing some concepts. But I want to have something such simple. How to implement this.
You can try to have a constructor within the class A:
class A{
private B b;
public B getB(){
return b;
}
public void setB (B b){
this.b = b;}
}
public A(S s){
this.S = s;
}
}
So when you do a.setB(), you can just do a.setB(s) and that will set the object S for that A object.
a.setB (new B().setS("Param S Set"));
Ok, That statement starts by creating a new B instance, then it calls the setS method on that instance, passing it the string "Param S Set". Finally, it calls a.setB(...) passing the value returned by the setS() call.
As you already know, that doesn't work because setS() returns void.
May be I'm missing some concepts. But I want to have something such simple. How to implement this.
{
B b = new B();
b.setS("Param S Set");
a.setB(b);
}
What you are trying is wrong. setS() does not return B instance.
You will not save any resources by writing one-liners.
GC will also not clean anything if you set B to A as long as A has a reference to B.
I got it using builder design pattern. Thanks to #Builder by lombok.
Thank you all.
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 .