Let's say I have this Java class:
package com.version1;
class Sample
{ ... }
And in another Sample class, I want to import the Sample class from com.version1 package:
package com.version2; // Now it's a different package
import com.version1.Sample; // From another package
public class Sample
{...}
Please note: I need to import a class with same name as the current class, not 2 classes with same name.
So the question: Importing two classes with same name. How to handle? is not similar.
You simply can't have two classes referenced by the same short name. So the solution is actually the same one as in the case of importing two classes with the same name:
You'll have to reference the other class by its fully qualified class name and your local one by its short name. You could even use the FQCN for both, if that helps readability in your case.
Related
I have started learning Java and I tried to play with import and inheritance in netbeans IDE.
In netbeans I have created a project and two folders 'food' and 'food1'.
food has Fruit.java in it and food1 has Apple.java.
here's the code which is working fine.
Fruit.java
package food;
public abstract class Fruit{
}
Apple.java
package food1;
import food.Fruit;
class Apple extends Fruit{
}
Problem:
1. If i'm not declaring packages(package food and package food1) , it's not working. But i have already put them in the folders manually.
2. If i'm removing the import statement, it's not working. But the class fruit has been declared public.
Your Problem descriptions are correct - means, it works as expected:
If i'm not declaring packages(package food and package food1) , it's not working
Yes, the directory structure must be consistent with the package names. If your directory structure looks like
src
+--food
| +--Fruit.java
+--food1
+--Apple.java
then Fruit.java needs to have the package food statement and Apple.java needs to have the package food1 statement.
If i'm removing the import statement, it's not working. But the class fruit has been declared public.
Yes, if you want to use a class from a different package you must import it (or use a fully qualified class name which includes the package name, like ... extends food.Fruit - the usual approach is to import, though). Declaring the class public is not sufficient - it just defines the class can be used from outside the package, but you still need to import it.
See also
https://docs.oracle.com/javase/tutorial/java/package
http://www.studytonight.com/java/package-in-java.php
Simple question but even though googled it a lot I could not find the answer.
Is it possible to import a class outside a package?
Let's say I have 2 folders A and B with a .java file in each, is it possible by using the clause import to import the class contained in A? import A.Aclass ? or it's mandatory using package syntax whenever there is the keyword import?
Yes it is possible to import the class with the import statement. For better understanding let's assume that you have three folders foldera, folderb and folderc where foldera contains a .java file named "ClassA.java", folderb contains another .java file named "ClassB.java" and folderc contains a .java file named "ClassC.java". Now, if you want to uses the member data and operations of "ClassA.java" in "ClassC.java" you can use the import statement as shown below:
import foldera.ClassA
If you want to use the member data & operations of "ClassB.java" in "ClassC.java" it is also possible with the import statement
import folderb.ClassB
As per the java source file declaration rule, if the class is a part of a package, the package statement must be the first line in the source code file, before any import statements that may be present. In this example, the first line of "ClassC.java" source file must be package folderc since it is located in folderc. Similarly, the first line of "ClassA.java" source file must be package foldera, and the first line of "ClassB.java" source file must be package folderb.
Hope now you are clear with the concept!
Thank you...
Well, if the class is defined to have a package a; then you need to import the class with the package name. If you have two packages which contain a class with the same name, then in your class which needs to invoke each of them, you will need to use a fully-qualified name. For example:
import a.Foo;
import b.Foo;
public class Bar
{
public static void main(String[] args)
{
a.Foo aFoo = new a.Foo();
b.Foo bFoo = new b.Foo();
}
}
Alternatively, if you have two packages with classes of the same name, you can simply skip importing them, but rather -- using them by their fully-qualified names (FQN-s).
If the class does not have a package ...;, then simply import it as:
import Foo;
However, if you have two packages (from different libraries) which contain classes with identical FQN-s, then the first one on the classpath will be picked.
Please, bear in mind that the convention for naming packages is to use lowercase letters and for classes -- the name should start with an upper case letter for each word in the class' name.
Yes it is possible.
If you have the following:
Package: PackA
Class: ClasA
Do:
import PackA.ClassA; //Import the class
OR
import PackA.*; //Import all the classes within the package
yes it is possible just import the package
syntax
import pck.ClassA or import pck.*
Yes, you have to use package syntax.
importing all class inside folder A.
import com.pack.A.*;
importing specific class inside folder A.
import com.pack.ClassName;
A.java
package a;
class A {
void f1()
{
System.out.println("hi");
}
}
B.java
In same folder as of A.java and class files are generated in the same folder.
package b;
class B
{
new A().f1();
}
I am new to package concept. My question here is even though they are not in the same package how can B create the object of A?
If you use public class declaration it goes to public access level. Public classes can be accessed by different packages by using import a.A; or providing the complete class name new a.A().f1().
B will import (or can use fully qualified name) the class A from other package if it has to use it. importing enables the classes in other packages to see the classes from a different package. But this can be further find-tuned by using scoping.
If both are in the same folder and not using any packages explicitly then they both are in the default package. And you no need to explicitly import the classes while using them if they are in the same package.
You should know access modifiers in java. If your class is public ,it can be accessed anywhere simply by importing it.
Please go through below link for better understanding
http://javapapers.com/core-java/access-modifiers-in-java-explain/
There is something called import in Java which you can import into current package and access methods or members as long as they are public(visible across package) or protected (visible to sub-classes)
I am currently learning Java using the Deitel's book Java How to Program 8th edition (early objects version).
I am on the chapter on creating classes and methods.
However, I got really confused by the example provided there because it consists of two separate .java files and when one of them uses a method from the other one, it did not import the class. It just created an object of that class from the other .java file without importing it first.
How does that work? Why don't I need to import it?
Here is the code from the book (I removed most comments, to save typing space/time...):
.java class:
//GradeBook.java
public class GradeBook
{
public void displayMessage()
{
System.out.printf( "Welcome to the grade book!" );
}
}
The main .java file:
//GradeBookTest.java
public class GradeBookTest
{
public static void main( String[] args)
{
GradeBook myGradeBook = new GradeBook();
myGradeBook.displayMessage();
}
}
I thought I had to write
import GradeBook.java;
or something like that.
How does the compiler know where GradeBook class and its methods are found and how does it know if it exists at all if we dont import that class?
I did lots of Googling but found no answer.
I am new to programming so please tolerate my newbie question.
Thank you in advance.
It is because both are in same package(folder). They are automatically imported no need to write import statement for that.
You don't have to import classes that are in the same package as the current class.
Also, note that GradeBook.java is the name of the file. The (simple) name of the class is GradeBook. Every class should be in a package. If it is in the package com.foo.bar, the class name is com.foo.bar.GradeBook, and this is the name you must use when importing this class.
Read http://download.oracle.com/javase/tutorial/java/package/packages.html to learn more about packages.
The classes located in the same package do not have to be imported, as they are visible to each other. You simply import a class that is in another package:
import java.util.ArrayList;
Note that you are not importing the file, but the class.
It's all about packages. You are trying to use a class from the default package which does not need explicit import of the java file, ie GradeBook inside GradeBookTest
Here is where you can start with learning about packages :
Java Package Tutorial
and :
Creating and Using Packages
Java doesn't use includes the way C does. Instead java uses a concept called the classpath, a list of resources containing java classes. The JVM can access any class on the classpath by name so if you can extend classes and refer to types simply by declaring them.
From: Include one java file in another java file
Imports are for importing classes that are in a different package. Since you didn't declare a package for either they are both put in the default package. The compiler can find it because the class lives in the same directory.
You don't have to import classes which are in the same package.
Well, classes in the same package are automatically imported.
They're in the same package. This tutorial will do more justice than I will.
In java parent package class accessible from child packge class? please explain me any one?
example
package A.A1.A2 contains class sub
package A contains class sup
Is there anyway to access sup from sub?
pls explain.
i tried import it won't work
Example:
before the program Directory Structure is
package1 contains package1.java --> package2 --> package3 contains PCheck.java
//package1.java
package package1;
public class package1{
public static void main(String[] args) {
}
}
class phelo{
phelo(){
int a;
System.out.println("hai fun freom package 1");
}
}
//PCheck.java;
package package1.package2.package3;
import package1.*; //to import package1.java
public class PCheck {
public static void main(String[] args) {
phelo obj=new phelo();
}
}
class helo{
helo(){
int a;
System.out.println("hai fun from package 3");
}
}
output:
compile time error:package package1.package2.package3 doesnot exist;
for import class from different directory we use import statements but here we need access parent package from subpackage.i tried import it won't work pls explain with an example.
Java does not recognize the notion of a subpackage1. As far as Java is concerned packages a and a.b and a.b.c are unrelated. They are just names.
So, if you want to access a.b.SomeClass from a.b.c.SomeOtherClass, you must either use a fully qualified class name, or add an import to SomeOtherClass
1 - From Java 9 onwards you can use modules to implement abstraction boundaries that are larger than a single package. This doesn't address this question which is about package-private access, but it could be viewed as an alternative to package-private.
As for your example that doesn't compile, I think we need a proper MCVE to understand that. My guess is that you have gotten the file organization for your source tree wrong ...
It is also possible that the problem is that the visibility of the class you are trying to import is wrong (package private), but that wouldn't cause the compiler to say that the package doesn't exist, like you said it does.
In java parent package class
accessible from child packge class?
please explain me any one?
Not the way you're thinking. The directories are hierarchical, but the packages are just distinguishing names.
If a child needs a parent package, or any other outside its hierarchy, it simply needs to import it.
That's why import foo.* doesn't give you access to all sub-package names - packages aren't hierarchical the way directories are.
All answers seem to miss OP's point on package class as everyone seem to suggest importing the class as a workaround.
The answer is: package-level classes (i.e., without explicit access level modifier), are visible ONLY for the EXACT same package.
https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
If a class has no modifier (the default, also known as package-private), it is visible only within its own package
This effectively means that neither parent/child/external packages can view the class.
In object of a.b.c.SomeOtherClass:
List<String> tmp=new ArrayList<String>(Arrays.asList(this.getClass().getPackage().getName().split("\\.")));
tmp.remove(tmp.size()-1);
String parent_package_name=tmp.toString().replace(", ", ".").replaceAll("[\\[\\]]", "");
Class cls=Class.forName(parent_package_name+".SomeClass");
Simply import it:
import A.sup;
Yes I have encountered the same error whenever I tried to access a class in the same package of the source file or the parent package of the source file it is generating a compiled time error , so by trial and error method I came to the conclusion that the packages are not built in a way to support main methods in the classes and they are not built in a way to support importing their parent packages or child packages
default class can be only used within package except subpackage