Why cannot create instance to an abstract class?
Dear Developers....
abstract class can create be its own constructor but why they didn't create object
We cann't create an object for abstract class
because they have abstract methods which depends on
its subclass for its implementation. if it is
possible to create the object for abstract class
means then you can call a method which has no
implementation(abstract methods) which is a illegal
one thats what the ruleWhat about classes with no abstract members?
public abstract class A {}No danger of accessing anything illegal here.
The rule is far more simple. By declaring a class as abstract, the creator defines that no instances of this class may exist. Period. If you could create instances, the class wouldn't be abstract anymore, per definition. It's like asking why a green light doesn't shine red. If it would, it wouldn't be green anymore.
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2. Add the full class name @ Admin server --> ssl --> Advanced --> custom hostname verifier -- also choose custom hostname verifier in the host name verification field above it.
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osb_domain.log00932:####<Aug 3, 2012 12:01:41 PM EST> <Error> <Security> <server host name> <osb_server1> <[ACTIVE] ExecuteThread: '
1' for queue: 'weblogic.kernel.Default (self-tuning)'> <<anonymous>> <> <6cf4c9e4b2f5468a:-4668cc77:138ea27406f:-8000-000000000000095d> <134
3959301805> <BEA-090563> <Cannot create instance of Hostname Verifier <package name>:WildcardHostnameVerifier.>
Can somebody let me know if there are any additional settings required for this to work ?
JAVA CLASS
package <package name>;
import java.io.ByteArrayInputStream;
import java.security.cert.Certificate;
import java.security.cert.CertificateEncodingException;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import javax.net.ssl.SSLPeerUnverifiedException;
import javax.net.ssl.SSLSession;
import weblogic.security.SSL.HostnameVerifier;
* @author Ravi_Shah01
public class WildcardHostnameVerifier implements HostnameVerifier {
/* (non-Javadoc)
* @see com.certicom.net.ssl.HostnameVerifier#verify(java.lang.String, javax.net.ssl.SSLSession)
public WildcardHostnameVerifier() {
System.out.print("Creating instance of WildcardHostnameVerifier");
public boolean verify(String hostname, SSLSession session) {
try {
Certificate cert = session.getPeerCertificates()[0];
byte [] encoded = cert.getEncoded();
CertificateFactory cf = CertificateFactory.getInstance("X.509");
ByteArrayInputStream bais = new ByteArrayInputStream(encoded);
X509Certificate xcert = (X509Certificate)cf.generateCertificate(bais);
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Pattern validHostPattern = Pattern.compile("\\*\\.[^*]*\\.[^*]*");
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if(validHostMatcher.matches())
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e.printStackTrace();
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} catch (CertificateEncodingException e) {
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* @return
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if(matcher.find())
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}Hi Ravi,
3. Package the class in the jar file using eclipse export java project and place the file in server-lib folder.Instead of placing the jar in server-lib, please modify setDomainEnv.cmd/setDomainEnv.sh as below to add jar in the classpath-
set CLASSPATH=<Your_Jar_PATH>;%CLASSPATH%
Restart WebLogic Server and re-test.
From Weblogic 10.3.4 onwards there is a patch available to enhance the existing default BEA hostname verifier to include wildcard certificates and wildcard host name verifier is available in Weblogic 10.3.6
Regards,
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".. is an abstract class. It can't be instantiated"
Does anyone have an idea of how I can get rid of the above error message? Here is a bit of my code:
import java.io.*;
import java.util.Vector;
class Project3 {
private BiTree company = new BiTree();
public static void main(String[] args) throws IOException {
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run.Command();
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public int compare(Object object1,Object object2);
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}It is not possible to create instances of an abstract class.
I hope this example helps,
abstract class Vehicle
public static void main(String arg[])
Vehicle a;
a = new Automobile(); // ok, since Automobile is a kind of Vehicle
// The class Automobile, a special case of Vehicle
class Automobile extends Vehicle -
Why interfaces why not abstract classes?
why they are using interfaces concept in JDBC or struts2.0 why not abstract classes?
thanks in advance.user5287726 wrote:
jverd wrote:
user5287726 wrote:
Interfaces are more flexible. For certain definitions of flexible, yes, but not for others.
A Java class can only inherit one class, but it can implement multiple interfaces.Which has absolutely nothing to do with why JDBC declares interfaces rather than abstract classes.How does your post even attempt to answer the OP's question?OP asked: "why they are using interfaces concept in JDBC or struts2.0 why not abstract classes?" I'm pointing out that your post does not address the question asked. And of course your answer is wrong anyway.
As for addressing the OP's question, I did that in my first response. -
Geting subclass info from abstract class
Does ne one know how i can retreive informtion from the subclasses of an abstract class by creatinig an instance of the abstract class.
are Methods such as getClass used?Does ne one know how i can retreive informtion from
the subclasses of an abstract class by creatinig an
instance of the abstract class.You can never create an instance of the abstract class. That is why they are called abstract.
>
are Methods such as getClass used?
Coming to your actual question, a superclass will not know anything about its children. So you will have to do something else get that information.
Maybe inspecting all the classes in a classloader and checking its parent will be an option. I do not think this is possible, because ClassLoader does not provide a mechanism to get all the classes.
Somebody else might give you a better answer. I am just trying to answer one part of your question concerning abstract classes. -
I have read that you cannot create and instance of an abstract class. If that is correct how is it possiable that I can code this ?
File Myfile = new File('c:/abc.txt);
What am I missing can anyone help ?Ohh, wait, got it.
java.io.File An abstract representation of file and directory pathnames.
It is not an abstract class, if you look at an abstract class AbstractList for example, it will say "abstract class AbstractList" in the API.
File however takes a concept common (but not necessary identical) to all operating Systems (like Windows, Linux and MacOS X), but "abstracts" it, so you the developer are not exposed to the differences between OSes. -
Dynamically invoke methods of abstract class?
Hi,
I am using reflection to write a class (ClassA) to dynamically invoke methods of classes. I have an abstract class (ClassB) that has some of the methods already implemented, and some of the methods that are declared abstract. Is there any way that I can:
(a) invoke the methods that are already implemented in ClassB;
(b) I have another class (ClassC) that extends ClassB, some of the methods are declared in both classes. Can I dynamically invoke these methods from ClassB?
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Matt.Ok, the program is quite long, as it does other things as well, so I'll just put in the relevant bits.
What I have is a JTree that displays classes selected by the user from a JFileChooser, and their methods.
// I declare a variable called executeMethod
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Object createdObject = objectClass.newInstance();
// methodName is the method selected by the user. objectClassMethods is an array containing all the methods in the chosen class.
executeMethod = objectClassMethods[j].getDeclaringClass().getMethod(methodName, null);
Object executeObject = executeMethod.invoke(createdObject, new Object[]{});Ok, here are the test classes:
public abstract class ClassB{
private int age;
private String name;
public ClassB(){ age = 1; name="Me";}
public int getAge(){ return age; }
public String getName(){ return name; }
public void PrintAge(){System.out.println(age);}
public void PrintName(){System.out.println(name);}
public abstract void PrintGreeting();
public class ClassC extends ClassB{
public ClassC(){super();}
public void PrintAge(){
System.out.println("I am " + getAge() + " years old.");
public void PrintGreeting(){
System.out.println("Hello");
}Now, I can print out the PrintAge method from ClassC (i.e. have it output "Hello" to the command line, how can I, say, get it to output the result of PrintName from ClassB, this method does not appear in ClassC. As you can see at the top, I can create a new instance of a normal method (in this case, ClassC), and have it output to the command line, but I know that I can't create a new instance of an abstract class. And since PrintName is implemented in abstract class ClassB, how do I get it to output to the command line?
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Matt. -
When we will go for an abstract class and when we will go for an interface?
it's always some what confusing to choose an abstract class and an interface,can anybody post a suitable answer for this.
jwenting wrote:
with experience and the insight it brings, you will know which to use when.
Without it, it can't be explained.
More often than not there's no X OR Y anyway.It's fortunate that there are posters here who possess the insight and experience necessary to explain this. The principal differences between an abstract class and an interface are,
1. An abstract class can carry implementation, whereas an interface cannot.
2. An abstract class is singly inherited, wheras an interface is multiply inherited.
So use an abstract class when the implementation it can carry outweights the fact that it cannot be multiply inherited That's the gist of it.
The inheritance relationship where this happens is when the supertype is a general concept of which all potential subtypes are special cases. This is called a specialization (or sometimes a generalization) relationship. For example Apple and Banana are Fruit. Or Car and Bike are Vechicle. The Fruit and Vechicle supertypes are general concepts of which their subtypes are special cases. In this case make Fruit and Vechicle abstract classes because the subtypes will benefit from a shared implementation.
If you don't have a clearcut specialization/generalization relationship make the supertype an interface. An example could be the Comparable supertype. The potential subtypes aren't supposed to be specializations of the Comparable concept, they're suppose to become Comparable (and make this property an integral part of their being). This is not a specialization/generalization relationship. Instead the supertype is intended to add character to the subtypes. The subtypes are unlikely to benefit from an inherited implementation. So make Comparable an interface. -
Can we create object of an abstract class ?
As Richard Brünning mentioned in his reply, we can't instantiate the object with reference to the Abstract Class. i.e.
DATA: O_OBJ TYPE REF TO ZCL_ABS_CLASS.
CREATE O_OBJ. " << This will give syntax error
But you can surely use the abstract class to reference the object. At run time, based on your required you can instantiate the object with actual reference type to the subclass of the abstract class. Like:
DATA: O_OBJ TYPE REF TO ZCL_ABS_CLASS.
* ZCL_ABS_CLASS_SUB is sub class of the ZCL_ABS_CLASS
CREATE O_OBJ TYPE ZCL_ABS_CLASS_SUB.
Regards,
Naimesh Patel -
BO4 - cannot create new universe connections
Hi,
In Universe Design Tool, trying to create new universe connections but not allowing this.
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Why cannot create new universe connections
Many ThanksHi,
there are some notes about this in the knowldge base. Here are a couple:
For ODBC, Note 1272557 - Data Access Drivers missing while creating a new connection in Designer
for JDBC, Note 1481410 - JDBC Drivers do not appear in the List Of Drivers when Creating a New Universe Connection in Universe Designer
Can you hit www.service.sap.com/notes to look them up?
Regards,
H -
What is the difference between Abstract class and Interface ?
Hi,
Could u plz tell me the difference between Abstract class and Interface?
Thanks in advance.
GopiLots.
An abstract class can contain some method implementations, or indeed all the method implementations. It may contain methods with all the various access modifiers. It cannot be instantiated. A class may inherit from only a single abstract class.
An interface contains only public method stubs and constants. A class may implement multiple interfaces. An interface cannot (obviously) be instantiated.
Abstract classes are particularly useful when you need to provide a semi-complete implementation for reuse. Interfaces are used more like types.
Look at java.util.* for some good examples of the use of both.
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