Making a generic class that uses interfaces.

I've learned how to do this:
class MyClass<T extends Object> {
}But I want a generic class that takes objects that implement another object like this:
class MyClass<T implements Runnable> {
}Why doesn't this work? Is there any way to do this?

happypigface wrote:
Yes thank you. Thats strange that Java does that. I didn't really test it yet, but I know Runnable is an instance and eclipse doesn't have any compiling errors for it. I'm glad that you can do this. It wouldn't very useful if you could only specify that the generic class had to be a subclass, not an implementation of a class.Generics are about types rather than classes and interfaces. Hence, one specifies the supertype(s) of a Generic Parameter by using the extends keyword.
Note, however, it's possible to extend multiple interface types but only one class type.

Similar Messages

Import java class that uses DbConnection

How to import java class that uses DbConnection? I don't want to write my own PluginDataSource.
It is possible to get Connection in imported java classes?

This is not possible. Even connection used by individual PDS are only limited to those individaul PDS only and are not shared across them.
Thanks
Rohit

Deploying java class that uses oraclexmlsql.jar

Hi,
I'm trying to deploy a java class that I wrote. It simply gets some data out of my oracle database as xml using the oraclexmlsql package. It works fine when I compile it within JDeveloper.
When I try to deploy it JDeveloper gives out the following error:
Errors in xmlquerydb:
ORA-29521: refferenced name oracle/xml/sql/query/OracleXMLQuery could not be found
loadjava: 1 errors
I can't figure out why that happens. I have used other classes and deployed them as Java stored Procedure and they work fine.
Thanks for help
Andre

I tried to load the oraclexmlsql.jar into the shema where I put my class. The classes from this jar were loaded but can not be compiled( apart from 3 ). I also tried loading my class into sys but I still can't compile it or the oraclexmlsql.jar classes.
Is the xsu12.jar a collection of all the classes that are allready in Sys after the database install?
I have noticed that on expanding the oraclexmlsql.jar file has a strange format and two classes classes are in the directory from which I expanded the jar. These two classes do not get a synonym on loading into the db. Could that have anything to do with it?
Thanks for more help
Andre

Extending a class that uses Generics

I'm trying to create a TableModel where the model deals with rows of data. Each row must be of the same "type", hence the need for generics. My RowTableModel is abstract. I'm trying to provide a concrete implentation for a model to use a List (ie. Vector or ArrayList) as the row type. But I can't figure out how to genericize the ListTableModel.
import java.util.*;
import javax.swing.table.*;
abstract class Row2TableModel<T> extends AbstractTableModel
 protected List<T> modelData;
 private List<String> columnNames;
 private Class[] columnClasses;
 private Boolean[] isColumnEditable;
 public Row2TableModel(List<String> columnNames)
 this(new ArrayList<T>(), columnNames);
 public Row2TableModel(List<T> modelData, List<String> columnNames)
 setDataAndColumnNames(modelData, columnNames);
// Implement the TableModel interface
 public Class getColumnClass(int column)
 return Object.class;
 public int getColumnCount()
 return columnNames.size();
 public String getColumnName(int column)
 return "something";
 public int getRowCount()
 return modelData.size();
 public boolean isCellEditable(int row, int column)
 return true;
// Example of custom methods
 public T getRow(int row)
 return modelData.get( row );
 public void insertRow(int row, T rowData)
 modelData.add(row, rowData);
 fireTableRowsInserted(row, row);
 protected void setDataAndColumnNames(List<T> modelData, List<String> columnNames)
 this.modelData = modelData;
 this.columnNames = columnNames;
 columnClasses = new Class[getColumnCount()];
 isColumnEditable = new Boolean[getColumnCount()];
 fireTableStructureChanged();
 class List2TableModel extends Row2TableModel
 public List2TableModel(List<String> columnNames)
 super(columnNames);
 public List2TableModel(List<List> modelData, List<String> columnNames)
 super(modelData, columnNames);
 // Provide implementation of TableModel methods
 public Object getValueAt(int row, int column)
 List rowData = (List)getRow( row );
 return rowData.get( column );
 @SuppressWarnings("unchecked")
 public void setValueAt(Object value, int row, int column)
 List rowData = (List)getRow( row );
 rowData.set(column, value);
 fireTableCellUpdated(row, column);
}

Hi,
You mean something like that in your impementation
class List2TableModel extends Row2TableModel<List<Integer>> {
 public List2TableModel(List<String> columnNames) {
 super(columnNames);
 public List2TableModel(List<List<Integer>> modelData, List<String> columnNames) {
 super(modelData, columnNames);
 // Provide implementation of TableModel methods
 public Integer getValueAt(int row, int column) {
 List<Integer> rowData = (List<Integer>) getRow(row);
 return rowData.get(column);
 }Regards,
Alan Mehio
London,UK

Help! - JRE can't find classes that use inner classes!

Whenever I try to launch an app using the JRE, if the class was built with an inner class definition, the JRE says it can't find the class.
For instance:
I created a class called GUI, which contains anonymous inner class definitions for the button handlers: addActionListener(new ActionListener()){, etc.
When I compile this class I get GUI.class and GUI$1.class
When I try to launch this using the JRE:
jre -cp c:\progra~1\myApps GUI
The JRE spits out
'Class not found: GUI'
BUT if I go back and remove the action handlers so that the only compiled class file is GUI.class (not GUI$1.class) then the JRE can find the class no problem.
What in the good Lord's name is going on here?

Thanks for the response. Got the 1.2.2 version of the JRE, where I guess you no longer invoke jre.exe (no such file exists) but java.exe instead (not well documented).
Also, the newer version of the JRE has no problems locating classes, so I guess I'm OK.

Generic classes with parameterized super class and interface

Hello.
I'm trying to write a generic class that looks like the following pseudo-code.
public interface Interface {
 public Interface getSibling(...);
public class Klass {...}
public class MyKlass<T extends Klass, T2 extends Interface>
 extends T implements T2 {
 public T2 getSibling(...) {
 return this;
}where Interface and Klass each have various extensions.
I came across this problem, or challenge, while writing classes for testing my EJBs. I tried and failed various attempts.
Is it possible to write generic classes of this nature in Java?
If so, please tell me and others who are like me.
Thanks in advance.

No. That would not work.
Beside being forbidden by the compiler, to my understanding, it cannot be done in theory either, as the parameterized types get bound at instantiation time and are not available for static reference, which both extends and implements require.

Generic and Numbers - using +,* operators

Hi,
I need to implement a generic class, that does some basic mathematical calculation.
I want it to support Integer, Double, Long, Float etc (supporting all number types).
I thought of writing something like
public class A<T extends Number>
public T a(T v1, T v2, T v3)
return (v1 + v2) * v3;
the problem is that Number does not implement +,* operators.
I don't see other interface that I should implement.
Any other idea?
thanks.

s_roee wrote:
I need to implement a generic class, that does some basic mathematical calculation.
I want it to support Integer, Double, Long, Float etc (supporting all number types).I suspect you're not the first s_roee; I've often wondered why Java doesn't have something like a SimpleArithmeticValue interface that defines add(), subtract(), multiply() and divide(). I guess they thought there might be too many arguments about what functions to support (for example, do you want modulus(), preIncrement()... etc to cover all the standard operators provided by the language?).
Unfortunately, until then I fear your only alternative is to create your own similar interface and create a bunch of wrappers of your own to do it (or wrap Javas own).
Winston
Edited by: YoungWinston on Aug 31, 2009 3:50 AM
Actually, preIncrement is probably not a good example; but plus(), minus(), not(), and() and xor() are certainly possibilities.

Best way to call a function in a generic class from a base class

Hi,
I have a generic class that works with subclasses of some other baseclass. When I construct the subclass, I call the constructor of the baseclass which I want to call a function in the generic class. e.g. suppose I have the following classes
public class List<T extends BaseClass>
 newTCreated(T t)
}

Sorry, I pressed Tab and Enter last time when typing the code so I posted without meaning to
Hi,
I have a generic class that works with subclasses of some other baseclass. When I construct the subclass, I call the constructor of the baseclass which I want to call a function in the generic class. e.g. suppose I have the following classes
public class List<T extends BaseClass>
 public void newTCreated(T t)
 // add the t to some internal list
 public T getT(int index)
 // get the object from the internal list
public class BaseClass
 public BaseClass(List<?> list)
 list.newTCreated(this);
public class SubClass extends BaseClass
 public SubClass(List<SubCass> list)
 super(list);
}This doesn't compile because of the call to newTCreated in the BaseClass constructor because BaseClass is not necessarily of type T. Is there any way of checking when I call the newTCreated function that the BaseClass is actually of type SubClass? I could either add the call explicitly in each SubClass's constructor or have a function addToList in BaseClass that is called from the BaseClass constructor but overloaded in each subclass but both of those rely on future subclasses doing the same. Or I could change the newTCreated function to take an argument of type BaseClass and then cast it to type T but this doesn't give a compilation error, only a runtime exception.
It seems like there should be solution but having only recently started writing Generic classes I can't find it. Thanks in advance for any help,
Tom

Failure When Deploying EJB Project That Uses Library Onto Glassfish

Hi,
I have a multi project application that I am developing and I am having issues testing it. I have set up a EAR, WAR, EJB, and two library JARs. The EJB contains classes that use components from the library JARs or implement abstract classes from the library JARs. I have added the library JARs to the compile-time libraries of the EJB. I then also added the library JARs to the libraries of the EAR. However when I then go to deploy the EAR onto the Glassfish server I get this error, displayed in the Glassfish log: 
WARNING: Error in annotation processing: java.lang.NoClassDefFoundError: com/foo/sdk/core/AbstractClass 
WARNING: Error in annotation processing: java.lang.NoClassDefFoundError: com/foo/sdk/core/AbstractClass 
WARNING: Error in annotation processing: java.lang.NoClassDefFoundError: com/foo/sdk/core/AbstractClass 
WARNING: Error in annotation processing: java.lang.NoClassDefFoundError: com/foo/sdk/core/AbstractClas 
SEVERE: Class [ com/foo/sdk/core/InterfaceClass ] not found. Error while loading [ class com.foo2.BeanFooClass ] 
SEVERE: Exception while deploying the app [FooEAR] 
SEVERE: Invalid ejb jar [BeanFoo.jar]: it contains zero ejb. 
Note:
1. A valid ejb jar requires at least one session, entity (1.x/2.x style), or message-driven bean.
2. EJB3+ entity beans (@Entity) are POJOs and please package them as library jar.
3. If the jar file contains valid EJBs which are annotated with EJB component level annotations (@Stateless, @Stateful, @MessageDriven, @Singleton), please check server.log to see whether the annotations were processed properly.
java.lang.IllegalArgumentException: Invalid ejb jar [BeanFoo.jar]: it contains zero ejb.
Note:
1. A valid ejb jar requires at least one session, entity (1.x/2.x style), or message-driven bean.
2. EJB3+ entity beans (@Entity) are POJOs and please package them as library jar.
3. If the jar file contains valid EJBs which are annotated with EJB component level annotations (@Stateless, @Stateful, @MessageDriven, @Singleton), please check server.log to see whether the annotations were processed properly.
 at com.sun.enterprise.deployment.util.EjbBundleValidator.accept(EjbBundleValidator.java:76)
 at com.sun.enterprise.deployment.util.ApplicationValidator.accept(ApplicationValidator.java:128)
 at com.sun.enterprise.deployment.EjbBundleDescriptor.visit(EjbBundleDescriptor.java:730)
 at com.sun.enterprise.deployment.Application.visit(Application.java:1768)
 at com.sun.enterprise.deployment.archivist.ApplicationArchivist.validate(ApplicationArchivist.java:799)
 at com.sun.enterprise.deployment.archivist.ApplicationArchivist.openWith(ApplicationArchivist.java:277)
 at com.sun.enterprise.deployment.archivist.ApplicationFactory.openWith(ApplicationFactory.java:240)
 at org.glassfish.javaee.core.deployment.DolProvider.load(DolProvider.java:170)
 at org.glassfish.javaee.core.deployment.DolProvider.load(DolProvider.java:93)
 at com.sun.enterprise.v3.server.ApplicationLifecycle.loadDeployer(ApplicationLifecycle.java:826)
 at com.sun.enterprise.v3.server.ApplicationLifecycle.setupContainerInfos(ApplicationLifecycle.java:768)
 at com.sun.enterprise.v3.server.ApplicationLifecycle.deploy(ApplicationLifecycle.java:368)
 at com.sun.enterprise.v3.server.ApplicationLifecycle.deploy(ApplicationLifecycle.java:240)
 at org.glassfish.deployment.admin.DeployCommand.execute(DeployCommand.java:370)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl$1.execute(CommandRunnerImpl.java:355)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl.doCommand(CommandRunnerImpl.java:370)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl.doCommand(CommandRunnerImpl.java:1067)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl.access$1200(CommandRunnerImpl.java:96)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl$ExecutionContext.execute(CommandRunnerImpl.java:1247)
 at com.sun.enterprise.v3.admin.CommandRunnerImpl$ExecutionContext.execute(CommandRunnerImpl.java:1235)
 at com.sun.enterprise.v3.admin.AdminAdapter.doCommand(AdminAdapter.java:465)
 at com.sun.enterprise.v3.admin.AdminAdapter.service(AdminAdapter.java:222)
 at com.sun.grizzly.tcp.http11.GrizzlyAdapter.service(GrizzlyAdapter.java:168)
 at com.sun.enterprise.v3.server.HK2Dispatcher.dispath(HK2Dispatcher.java:117)
 at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:234)
 at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:822)
 at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:719)
 at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1013)
 at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
 at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
 at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
 at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
 at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
 at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
 at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
 at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
 at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
 at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
 at java.lang.Thread.run(Thread.java:679)
SEVERE: Exception while deploying the app [FooEAR] : Invalid ejb jar [FooBean.jar]: it contains zero ejb. 
Note:
1. A valid ejb jar requires at least one session, entity (1.x/2.x style), or message-driven bean.
2. EJB3+ entity beans (@Entity) are POJOs and please package them as library jar.
3. If the jar file contains valid EJBs which are annotated with EJB component level annotations (@Stateless, @Stateful, @MessageDriven, @Singleton), please check server.log to see whether the annotations were processed properly. 
I am using these technologies:
NetBeans IDE 7.0 for Java EE
Glassfish 3.1
I did read that putting the libraries in this directory: <glassfish_home>/domains/domain1/lib may resolve the problem, however there must be a better way.
Any help would be greatly appreciated!
Edited by: 866180 on Jun 15, 2011 2:32 PM

First of all, use a normal font when posting a question. This is just terrible to read.
You have multiple failures here. Did you actually read and try to understand the error?
java.lang.NoClassDefFoundError: com/foo/sdk/core/AbstractClassJava is not going to lie, this class is not on the classpath of the application. Either the jar is missing, the class is missing from the jar or you put the jar in the wrong place. Open up the ear using your favorite zip tool and check out its structure. What is the path to the jar inside it?
Also, open up the META-INF/manifest.mf file inside the EJB jar. Is there a class-path line in there? There shouldn't be!
A valid ejb jar requires at least one session, entity (1.x/2.x style), or message-driven bean. Apparently your EJB jar contains not a single EJB or MDB class. Did you forget some annotations perhaps?
Seems to me that your application compiles, but other than that it is very much broken.

What is the use of Generic class in java

hi everyone,
i want to know that
what is the use of Generic class in java ?
regards,
dhruvang

Simplistically...
A method is a block of code that makes some Objects in the block of code abstract (those abstract Objects are the parameters of the method). This allows us to reuse the method passing in different Objects (arguments) each time.
In a similar way, Generics allows us to take a Class and make some of the types in the class abstract. (These types are the type parameters of the class). This allows us to reuse the class, passing in different types each time we use it.
We write type parameters (when we declare) and type arguments (when we use) inside < >.
For example the List class has a Type Parameter which makes the type of the things in the list become abstract.
A List<String> is a list of Strings, it has a method "void add(String)" and a method "String get(int)".
A List<File> is a list of Files, it has a method "void add(File)" and a method "File get(int)".
List is just one class (interface actually but don't worry about that), but we can specify different type arguments which means the methods use this abstract type rather than a fixed concrete type in their declarations.
Why?
You spend a little more effort describing your types (List<String> instead of just List), and as a benefit, you, and anyone else who reads your code, and the compiler (which also reads your code) know more accurately the types of things. Because more detail is known, the compiler is able to tell you when you screw up (as opposed to finding out at runtime). And people understand your code better.
Once you get used to them, its a bit like the difference between black and white TV, and colour TV. When you see code that doesn't specify the type parameters, you just get the feeling that you are missing out on something. When I see an API with List as a return type or argument type, I think "List of what?". When I see List<String>, I know much more about that parameter or return type.
Bruce

Using static .values() method of Enum in Generic Class

Hi *,
I tried to do the following:
public class AClass<E extends Enum<E> > {
 public AClass() {
 E[] values = E.values(); // this DOESN'T work
 for (E e : values) { /* do something */ }
}This is not possible. But how can I access all Enum constants if I use
an Enum type parameter in a Generic class?
Thanks for your help ;-) Stephan

Here's a possible workaround. The generic class isn't adding much in this case; I originally wrote it as a static method that you simply passed the class to:
public class Test21
public static enum TestEnum { A, B, C };
public static class AClass<E extends Enum<E>>
 private Class<E> clazz;
 public AClass(Class<E> _clazz)
 { clazz = _clazz; }
 public Class<E> getClazz()
 { return clazz; }
 public void printConstants()
 for (E e : clazz.getEnumConstants())
 System.out.println(e.toString());
public static void main(String[] argv)
 AClass<TestEnum> a = new AClass<TestEnum>(TestEnum.class);
 a.printConstants();
}

Cannot use user classes in webservices interface of appmodule custom method

Hi,
I need to expose an application module's custom method that uses user defined classes as parameters and return types as a web service interface. I have not been able to do that. Below are more details.
In particular, I have an app module that has a client interface with a custom method that takes and returns a serializable Emp object (user defined classes/objects). I need to expose this app module as a web service through the use of a service interface.
When i run the "Create Service Interface" wizard, under service custom methods (Create Service Interface Step 2 of 4 wizard), i don't see my custom method under "Available."
In addition, the JDev message log says: Business Components: Method "yourCustomMethod(Emp):Emp" cannot be exposed via service interface because the return type "com.mypackage.Emp" is not supported.
From what i understand by reading the docs below, it seems that in an application module's service interface (Webservice) you only can use simple data type or java.util.list or AttributeList (which is a wrapper for any viewRow). In other words, i cannot use my user defined object in the custom method parameters and return types.
This seems to be an artificial limitation of webservice creation through the use of app module because one can use complex/user defined objects in methods of a java class and expose them through a webservice.
I would appreciate if of the tooling PMs can make a well thought out comment.
11.2.3 What You May Need to Know About Method Signatures on the Service Interface
http://docs.oracle.com/cd/E12839_01/web.1111/b31974/bcextservices.htm#CJAEHFJD
9.8.4 What You May Need to Know About Method Signatures on the Client Interface
http://docs.oracle.com/cd/E12839_01/web.1111/b31974/bcservices.htm#CHDFHGII
Thanks,
AJ
ps: I am using Build JDEVADF_11.1.1.5.0_GENERIC_110409.0025.6013.

Hi,
question 2 doesn't apply as the client interface is for Java access to public methods. For the Service Interface, the document lists the supported data types you can use in arguments and return values. To expose a custom methods, what about using a POJO that accesses ADF BC (using create root application module) and then expose your POJO as a JAWS service. This will allow you to expose the data queried from ADF BC through your custom classes.
Frank

When to use abstract classes instead of interfaces with extension methods in C#?

"Abstract class" and "interface" are similar concepts, with interface being the more abstract of the two. One differentiating factor is that abstract classes provide method implementations for derived classes when needed. In C#, however,
this differentiating factor has been reduced by the recent introduction of extension methods, which enable implementations to be provided for interface methods. Another differentiating factor is that a class can inherit only one abstract class (i.e., there
is no multiple inheritance), but it can implement multiple interfaces. This makes interfaces less restrictive and more flexible. So, in C#, when should we use abstract classes
instead of interfaces with extension methods?
A notable example of the interface + extension method model is LINQ, where query functionality is provided for any type that implements IEnumerable via
a multitude of extension methods.

Hi
Well I believe Interfaces have more uses in software design. You could decouple your component implementing against interfaces so that
you have more flexibility on changing your code with less risk. Like Inversion of Control patterns where you can use interfaces and then when you decide you can change the concrete implementation that you want to use. Or other uses for interfaces is you could
use Interceptors using interfaces (Unity
Interceptor) to do different things where not all of these is feasible or at least as straightforward using abstract classes.
Regards
Aram

When should I use abstract classes and when should I use interfaces?

Can any body tell me in which scenario we use /we go for Interface and which scenario we go for abstract class, because as per my knowledge what ever thing we can do by using Interface that thing can also done through abstract class i mean to say that the
behavior of the two class.
And other thing i also want to know that which concept comes first into the programming abstract class or Interface.
S.K Nayak

The main differences between an abstract class and an interface:
Abstract
An abstract class can contain actual working code (default functionality), and can have either virtual or abstract method.
An abstract class must be sub-classed and only the sub-classes can be instantiated. Abstract methods must be implemented in the sub-class. Virtual methods may be overridden in the sub-class (although virtual methods typically contain code, you still may
need/want to override them). A good use for an abstract class is if you want to implement the majority of the functionality that a class will need, but individual sub-classes may need slightly different additional functioality.
Interface
An interface only contains the method signatures (method name and parameters), there is no code and it is not a class.
An interface must be implemented by a class. An interface is not a class and so it cannot be sub-classed. It can only be implemented by a class. When a class implements an interface, it must have code in it for each method in the interface's definition.
I have a blog post about interfaces:
http://geek-goddess-bonnie.blogspot.com/2010/06/program-to-interface.html
(sorry, I have no blog posts specific to abstract classes)
~~Bonnie DeWitt [C# MVP]
http://geek-goddess-bonnie.blogspot.com

Create session bean using interfaces, not implementation classes

Hi,
I'm using interfaces and session beans for my persistence/data layer in my adf application.
I've created a data control for my session bean and during creation of this data control, javabean.xml files are created for the different interfaces that are used in my session bean.
If I create bindings on these methods, interfaces in jspx-documents I will get errors because he can't find the impl-classes that implement these interfaces.
When using interfaces in your session bean (as return values or parameters) instead of classes you need to manually create javabean.xml files for the implementation classes.
It's required by the databindings/datacontrol that the implementation-classes are described in a javabean-format as well? Is this a spec we need to adhere to?
Could someone verify that you need to create javabean.xml-files for as well the interfaces as the classes when you're working with interfaces in your session beans?
regards,
Nathalie

A patch was released by Oracle to be able to work with Interfaces and so it's not required to work with implementation-classes.
Patch for 5726754(Base bug 5657179)

Making a generic class that uses interfaces.

Similar Messages

Maybe you are looking for