I have some major issue here. This will be a little bit complex situation. I have 3 java project namely A, B and C. Projects A and C have B as the .jar file, means that, B act as a library for these A and C projects. What I want to do is, I want to get a function from any java file in A project and use it in java file in C project without import the java file to C. And the only way to connect between these two project is via project B. can some one help me solve this? let me give an example:
there is aa.java in project A, and the function in it is:
private static boolean keyBlocking = false;
public static boolean iskeyBlocking() {
return keyBlocking;
}
i want to use this function in cc.java in the C project without import this aa.java to C project. I hope this example helps to understand my question.
If I understand your question correctly, you want to use a class from another project without linking the project as dependent project/jar.
It is not possible.
In your scenario, you have common project(helper/utility) which is linked to both the projects A and C.
May be you can move the current class 'aa.java' to the project B.
**"I want to get a function from any java file in A project and use it in java file in C project without import the java file to C" **
Taking the statement literally "without import the java file to C", you can directly use it without import as below.
If the package is 'com.app.test', class is AA.java.
Using import:
import com.app.test.AA
public void someMethod()
{
AA.iskeyBlocking();
}
Without using import:
// NO IMPORT
public void someMethod()
{
com.app.test.AA.iskeyBlocking();
}
Related
I need to use a .jar library, given by my teacher, to code for my Java class.
I am using VS Code, with the Java Extension Pack installed, for Java Project Management.
Can someone please explain me step by step how is it possible to import the .jar library, in order to use the classes defined by my teacher.
I have tried to copy the .jar in the lib folder and then add the reference, but it still did not work. I also know that I have to declare the classpath, but when I create the Java Project the .classpath file is not created automatically.
Thanks already!
First you should examine the classes in .jar file. Then you should load that class as,
Class<?> c1 = Class.forName("java.lang.String");
Then after you can use that class by calling that Class reference type variable.
See this example as well,
public class Test {
public static void main(String[] args) throws ClassNotFoundException {
// get the Class instance using forName method
Class c1 = Class.forName("java.lang.String");
System.out.print("Class represented by c1: "+ c1.toString());
} }
Try to understand the code and implement proper solution to your project.
Good Luck.
I have two questions:
1) Is there a way to extend of class A from external file ? how?
2) I am building one class of my custom methods ( to use globally, in all my projects). Here is phseudo-code:
package MyFunctions;
import Twitter.profile;
public class MyFuncs{
public String externalProfile1() { return Twitter.profile.TwitterUrl(); }
}
I want Is there a way to include that file in all my projects, and avoid IDE errors, as I should be able to use any when one of the above functions in my projects... The problem is that the Twitter.Profile classes are not included in all my projects, and whenever happens so - it see error in IDE("cannot find symbol method")...
how to solve the problem?
Question 2:
Just make sure all of your functions in your library are written statically:
public class MyFuncs{
public static String externalProfile1(String link) { return TwittUrl() + "/profile"; }
public static String externalProfile2(String link) { return YahooUrl() + "/profile"; }
}
And then import that class in your project files that you'll be using your library in. Then you can easily call the functions in your library. Alternatively, you can avoid importing the library in every other file and instead call the functions in a static way:
MyFuncs.externalProfile1("link");
As for TwittUrl(), if it doesn't require to be in a separate Class, then refactor it and put it in MyFuncs class; otherwise you can make TwittUrl() and YahooUrl() methods static in their own class.
I tried to search for the solution, but what I found I don't know how to apply in this situation. Please help me correct my code.
package Exercise;
public class Ex11_11 {
public static void main(String[] args) {
A a = new A(3);
}
}
class A extends B { // type A is already defined, A has a red underline
public A (int t) {
System.out.println("A's constructor is invoked");
}
}
class B { // type B is already defined, B has a red underline
public B () {
System.out.println("B's constructor is invoked");
}
}
Eclipse sometimes gets confused. If you choose Clean from the Project menu, it might fix these errors.
Well, the first thing to check is obviously whether or not you have another class called A in your file or in the same package.
I had the same problem. My computer was restarted remotely by I.T, and Eclipse did not shut down gracefully. I noticed there was an extra java file in my project that I didn't add. Deleted it, and now the error is gone.
In Project-> Clean, select "Clean projects selected below", select my project(s) and check "Start a build immediately" with "Build only selected projects".
Then problem will resolve.
Check if all your class files are saved. I've had this problem a few times: i define a class in a class file then move it in it's own one. Java gets confised, because it reads from the old version of the original file. Once you save it with the missing class definition in it and define the class in the new file all should be ok.
In your project you might have test directory with the same package structure and the same class name (for example copied without changing class name to *Test).
If none of the above solutions worked for you then it possible that Build Path is messed up. When you add a src to the build path make sure the src is not in the exclusion list. Place *(wild card) in the inclusion list and nothing in the exclusion list.
Make sure
Project | Build Automatically
is checked.
The main reason for this is that somewhere in same package you have already defined a class with name A.
which causes type A is already defined error.
check if there is any subclass or inner class named A
Have you added another project to the build path?
I had the same issue on my development environment (Eclipse).
My Maven application consumed other applications. On Eclipse, those projects were just added to the build path. One of them had the same package structure and class filename. Eclipse ends up thinking both the files which are physically different are in the same directory since the package structure and filename are same.
For example, let's say there are two files as below:
/Users/uname/home/proj1/com/app/proj/main/java/util/file1.java
and
/Users/uname/home/proj2/com/app/proj/main/java/util/file1.java
and lets say both have the package name
com.app.define.proj.util
If you add one of the project to the other, Eclipse would consider both the files to be in the same location.
I resolved by creating a JAR file out of the consumed application, adding it to the build path and removing the Eclipse project from build path.
rename the class name A to Aa or something and try to run.
I love the Intellij IDEA but i have been stacked on one little problem with Java imports.
So for example there is a package with name "example" and two different classes in it: A.java and B.java.
And i wanna get access to class "A" from class "B" without imports. Like this:
class A:
package example;
public class A{ ... some stuff here ...}
class B:
package example;
public class B{
public static void main(String[] args){
A myVar = new A();
}
}
This code may not work, but it's doesn't matter. Trouble just with IDE and with its mechanism of importing classes.
So, problem is that i can't see A class from B. Idea says 'Cant resolve symbol' but i actually know that class A exists in package. Next strange is that complier works fine and there are no exceptions. Just IDEA can't see the class in the same package.
Does anybody has any ideas?
If they are in the same package, you can access A in class B without import:
package example;
public class B{
public static void main(String[] args){
A myA = new A();
}
}
Maybe this will help you, or somebody else using IntelliJ that is getting a "Cannot resolve symbol" error but can still compile their code.
Lets say you have the two files that buymypies wrote up, the standard Java convention is that the two files would exist in an Example directory in your source code area, like /myprojectpath/src/Example. But it is technically not a requirement to reflect the package structure in the source directory structure, just a best practice sort of thing.
So, if you don't mimic the package structure, and the two files are just in /myprojectpath/src, IntelliJ will give you the "Cannot resolve symbol" error because it expects the source code structure to reflect the package structure, but it will compile okay.
I'm not sure if this is your problem, but I do use IntelliJ and have seen this, so it's something to look at.
I have the same problem as this: 2 classes in the same package, yet when one tries to call the other, Intellij underlines it in red and says Cannot resolve symbol 'Classname', e.g. Cannot resolve symbol 'LocalPreferencesStore'.
It then wants to add the fully qualified name in situ - so it clearly knows the path - so why can't it just access the class?
The module still compiles and runs, so it's just the IDE behaving oddly - and all that red is very distracting, since it isn't actually an error, it's just IDEA throwing a weird wobbly.
This is also recent. Two weeks ago I wasn't having this problem at all, and now suddenly it's started up. Of course, it could go away again on its own soon, but it's really annoying.
Same issue and I just fixed it.
I don't know your folder structure.
However, if your package example was added manually.That's the problem.
The package should be the same as your folder structure,which means if you want your class file to be stored in package example,you must store you java file in the src's subfolder named example.
You need to learn the basics about Java i think.
Here is a basic example of what i think you are trying:
package Example;
public class A
{
String myVar;
public String getMyVar()
{
return myVar;
}
public void setMyVar(String myVar)
{
this.myVar = myVar;
}
}
You need to create an instance of A.
package Example;
public class B
{
/**
* #param args
*/
public static void main(String[] args)
{
// TODO Auto-generated method stub
A myA = new A();
myA.setMyVar("Hello World!");
System.out.println(myA.getMyVar);
}
}
Look up java 'getters' and 'setters'.
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.