How to compile two classes that need each other

[Vierda Mila]

Dear all,

I have 2 classes and each class need object from each other as instance member. I have tried to compile these 2 classes and it always fail and compiler tell that cannot found symbol. it make sense because I fail to compile either class A or B.

My question is how to compile class like these according these 2 classes inside in same package, thank you for any kind help.

package com;
import com.B;

class A {
   
     private B b;

}


package com;
import com.A;

class B {
    private A a;
}

regards,
Vierda

[Christophe Verré]

Make sure that "com" is in your classpath.

[Vierda Mila]

Christophe,

Thank you, it works.

[Marty Fried]

Vierda Mila wrote:Dear all,
. . .

package com;
import com.B;

class A {
   
     private B b;

}


package com;
import com.A;

class B {
    private A a;
}

I'm curious about this pattern... I've never tried anything like that, and I'm wondering how it works. It seems like it would be an endless recursion or something; when you construct one class, it would need to construct the other one, which needs to construct the first one, etc.

So, I'm wondering - does this work? Am I making a simple mistake in my thinking (certainly not the first time)?

[Ernest Friedman-Hill]

Hi Marty,

Well as written, neither class actually creates an instance of the other, so it's OK. If each one said something lke

A a = new A();

then yes indeed, it'd be infinite recursion and the program would blow up.

But there is more than one way to end up with a member variable pointing to to an object. For example, consider these two classes:

class A {
     private B b;
     public A() {
         b = new B(this);
     }
}

class B {
    private A a;
    public B(A a) {
        this.a = a; 
    }
}

Here, an instance of A contains an instance of B that refers back to the same "A". If you use "new A()" to create an A object, only two objects are created, one A and one B, and they both know about each other.

[Marty Fried]

Ernest Friedman-Hill wrote:Hi Marty,

Well as written, neither class actually creates an instance of the other, so it's OK. If each one said something lke

A a = new A();

then yes indeed, it'd be infinite recursion and the program would blow up.

But there is more than one way to end up with a member variable pointing to to an object. For example, consider these two classes:

class A {
     private B b;
     public A() {
         b = new B(this);
     }
}

class B {
    private A a;
    public B(A a) {
        this.a = a; 
    }
}

Here, an instance of A contains an instance of B that refers back to the same "A". If you use "new A()" to create an A object, only two objects are created, one A and one B, and they both know about each other.

Thanks, Ernest,

I think I see my mistake now; I'm still thinking in C++, and forgetting a basic difference, although I'm not certain if I'm right about C++. But I guess that in Java, you are only creating the reference to the object when you declare it like that, where in C++ you would actually be constructing the object. I realize that's the case, but I need to keep it in mind all the time.

Thanks for the example - that helped me realize my mistake.

[Ernest Friedman-Hill]

Marty Fried wrote:
I think I see my mistake now; I'm still thinking in C++, and forgetting a basic difference, although I'm not certain if I'm right about C++. But I guess that in Java, you are only creating the reference to the object when you declare it like that, where in C++ you would actually be constructing the object. I realize that's the case, but I need to keep it in mind all the time.

Yep, you're right, the same program in C++ would indeed blow up. But the declaration "A a;" in Java is like "A *a" in C++, which would be safe too.

[Prince Chauda]

Vierda Mila wrote:Dear all,

I have 2 classes and each class need object from each other as instance member. I have tried to compile these 2 classes and it always fail and compiler tell that cannot found symbol. it make sense because I fail to compile either class A or B.

My question is how to compile class like these according these 2 classes inside in same package, thank you for any kind help.

package com;
import com.B;

class A {
   
     private B b;

}


package com;
import com.A;

class B {
    private A a;
}

regards,
Vierda

The thing happening to this code that one class can't found other is due to the classpath.
Since neither of the file was compiled hence the directory having name "com" will not be there and thus your import statement will not work.

You can try this.

> If you want to compile A.java then put B.java in "com" directory and use the following code for compilation.

 > javac B.java 

This code will also compile A.java.
but this will put your B.class in the current directory. Hence it will problematic in future so use this code for best result.

 >javac -d . B.java 

[Campbell Ritchie]

And welcome to JavaRanch, Price Chauda

And thanks to Marty Fried for reminding us that C++ and Java are totally distinct languages.

[harilal ithikkat]

javac -d . *.java


it is working well...............