I am new to Java. I have the following package structure.
Package 1
AppName
Main
SomeOtherClass
Package 2
API
Load
Callback
From the Main class of AppName Package I want to create an object of Load class of the API package.
Now I can easily do that by import API.* and then using new Load();.
But what I want the full connotation of the Load class. I want to make a new API call using new API.Load();
So What I want is to be able to use API. prefix with Load class's construction. Is it possible at all? What kind of import will work? Simply using new API.Load(); doesn't work. Android studio doesn't recognize the API keyword.
Any ideas?
Edit:
Ok I sort of have what I want. com.achshar.testapp.API is the complete name. So
new com.achshar.testapp.API.Load(); works fine. Now how can I make it new API.Load();?
If you have a doubt, you can use Class#getCanonicalName to print the full path of a class.
Here, an example with ArrayList
List<String> str = new ArrayList<>();
System.out.println(str.getClass().getCanonicalName()); // java.util.ArrayList
And now if I want I can write my lists like the following
java.util.List<String> list = new java.util.ArrayList<>();
You can create your API (some what like Proxy layer). In it you can create your Load.java class, Callback.Java class. Provide some different name to your package.
If classes in API package of package 2 are not declared as final and also have public constructors, you can extend each class to create a new class in your created new API package.
In the new class you create, you can override the methods (functions) and give your behavior. You can use super.methodOfAPI to get the behavior provided by the parent class and then can write your code to add the behavior you want.
You need to take care as when you wish to use original API package classes and when from your created layer. Use your imports accordingly. Another way out is to append some common word like 'Proxy' in the end of the classes you creae in the new API package you create, it will resolve the conflicts.
Related
I am coming from Java and C++ background. However, I am doing a C# application at the moment and one thing made me confused.
In Java when I import a package, it means I can access the classes that are placed in that package. For example, here I imported the ArrayList class from package java.util.
import java.util.ArrayList;
class ArrayListUtilization {
public static void main(String[] args) {
ArrayList<Integer> myList = new ArrayList<>(3);
myList.add(3);
myList.add(2);
myList.add(1);
System.out.println(myList);
}
}
Recently I had a problem in my c-sharp app, that I asked here. The funny part in my point of view is, when I added the following snippet of code:
var accountDbContext = services.GetRequiredService<AccountDbContext>();
accountDbContext.Database.EnsureCreated();
var accountDbCreator = accountDbContext.GetService<IRelationalDatabaseCreator>();
accountDbCreator.CreateTables();
I saw an error as following:
Error CS1929 'AccountDbContext' does not contain a definition for
'GetService' and the best extension method overload
'ServiceProviderServiceExtensions.GetService(IServiceProvider)'
requires a receiver of type 'IServiceProvider'
and from the error text, I understood accountDbContext object does not have GetService function. But when I press on show potential fixes, then it suggests me to add a using statement.
And it was the real fix. However, my question is what is the effect of this using statement on my object? The object is an instantiation of its class. How can adding a using statement effect on my object and add a function to it?
Note that what you are actually calling an extension method here:
accountDbContext.GetService<IRelationalDatabaseCreator>();
accountDbContext does not have a method called GetService. GetService is declared in AccessorExtensions, and the above line is just syntactic sugar for:
AccessorExtensions.GetService<IRelationalDatabaseCreator>(accountDbContext);
Now it should make sense that you need to add a using directive for the class in which the extension method is declared, in order to access the extension method.
can anyone give me an idea of how the class loading flow works?,
What i'm trying to say is that i need to understand what happend when a developer create a new object eg:
User user = new User();
where java goes to load the class, I try excluding a class from my build and setting the current thread class loader with a jar that contains the excluded class and run it with no good results.
ok, i found it. Thanks!.
Java looks for the classLoader of the class that contains it.
There are some classes used in a java program referred from abc package from xyz.jar. The package is imported in the java file.
Also the same class is in other lmn.jar.
So if I delete the jar file from the project, i should be getting the error.
But the class compiles and takes the class from the other lmn.jar.
Eg.
weblogic.jdbc.oci.Blob is a class in weblogic.jar
But if i delete weblogic.jar, it takes it from java.sql.Blob.
I don't want this to happen, the program should display error.
In such a case you can use fully qualified name of class, which will include package name. For example, instead of:
Blob blob = new Blob();
You can write:
weblogic.jdbc.oci.Blob blob = new weblogic.jdbc.oci.Blob();
When importing a class you are also defining the package. So when you import "weblogic.jdbc.oci.Blob" and remove this class from classpath it will not automatically import it from a different package unless you change the import statement as well.
Some IDEs might automatically try to resolve classes and add the missing import statements. Maybe check that.
For one of the case try to use the entire path, ie
For example if you need Blob from weblogic.jar
then try to call weblogic.jdbc.oci.Blob bl = ...
I'm developing an android test app and i'm going to access all internal class of android.view package. android.view is a package that is present in jar file. I tried by loading package name but it doesn't display the classes if any one tried
this already, please help.
Here's what I tried so far:
public static void main() throws ClassNotFoundException{
Class o =Class.forName("android.view");
Class[] C=o.getDeclaredClasses();
for(int i=0;i<C.length;i++) {
Classname = C[i].getName();
ClassesDisplayActivity.your_array_list3.add(Classname);
Log.i("Ramu","classname "+ C[i].getName());
}
}
}
It is not possible to determine at runtime all of the classes that are in a package using a standard class loader.
You might have some luck with this library though:
https://code.google.com/p/reflections/
Package is not a class. You cannot call Class.forName() for package and access classes that belong to class using getDelcaredClasses().
I do not know what do you really need, so I'd recommend you to explain this in separate question. probably you will receive better solutions.
However if you really need this you have to do the following:
Get your classpath by calling System.getProperty(java.class.path)
split this property to its elements by colon
iterate over the list and read each resource. If resource is jar you can use ZipInputStream, if it is a directory use File class.
filter list of resources you got at #3.
Fortunately you can use 3rd party library named Reflections that helps you to do all this without writing code.
Have got two projects javaapplication2 and javaapplication1. The same being their package names. In javaapplication2 ive imported javaapplication1 using
import javaapplication1.*;
i need to list all classses in the packeage. How to achieve this? I tried a simple code but it gets a null exception.
Package pck;
pck = Package.getPackage("javaapplication1");
System.out.println(pck.getClass());
I don't have enough rep to comment on this question or to mark it as such, but it is a duplicate of:
Getting all Classes from a Package
There are many good answers listed on that question - for instance the top is looking for classes that implement ICommand, so to implement this all you need to do is remove:
if (ICommand.class.isAssignableFrom(cls)) {
commands.add((Class<ICommand>) cls);
}
from the for loop and you have what you want.
I guess the jar containing package javaapplication1 is not in classpath as
Package.getPackage("javaapplication1");
is returning null for you.
Also,
You can have a look at the following link:
http://dzone.com/snippets/get-all-classes-within-package
or can explore the Reflection library from Google in order to get the required information, below is a sample code.
e.g.
Reflections reflections = new Reflections("javaapplication1");
Set<Class<? extends Object>> allClasses = reflections.getSubTypesOf(Object.class);
If you go throgh the object class API it is written as:
//Object API
getClass
public final Class getClass()Returns the runtime class of an object. That Class object is the object that is locked by static synchronized methods of the represented class.
Returns:
the object of type Class that represents the runtime class of the object.
It's clearly written as it represents the run time class of an object.
Suppose if you have class called as "Helloworld" in side "javaapplication1" package
create object of the class as:
Helloworld world=new Helloworld();
and then try to run as
System.out.println(wolrld.getClass());
It will return the class path of current object.