Can we extend multiple classes ?

[nirjari patel]

public class ChildClass extends ParentClass {}

this class extends ParentClass. Can ChildClass extend multiple classes ?

public class ChildClass extends ParentClass_1 and ParentClass_2{}
Is this possible ? If not then to use the methods of both classes, one class can be extended and other class can be imported in the ChildClass ?

thanks

[ankur rathi]

nirjari patel wrote:public class ChildClass extends ParentClass {}

this class extends ParentClass. Can ChildClass extend multiple classes ?

public class ChildClass extends ParentClass_1 and ParentClass_2{}
Is this possible ? If not then to use the methods of both classes, one class can be extended and other class can be imported in the ChildClass ?

thanks

No. You can't.

If you want your class to use methods of other classes, as you said, import them and use their methods. Do extend when you want to override a method. If you want a class (A) to override methods of two classes (B & C) then you can go for multi-level inheritance.

Example:

public class B extends C
public class A extends B

Now methods of both B & C will be available in A for override.

[Campbell Ritchie]

What do you mean by "import?" You can't "use" methods of other classes, unless they are static.

[ankur rathi]

Campbell Ritchie wrote:What do you mean by "import?" You can't "use" methods of other classes, unless they are static.

If it's question for me then I meant import the package containing class and then use it's method.

[shivendra tripathi]

import doesn't serve any purpose but readability. If you dont want to write fully qualified name of a class then you can import it. In java 1.5 static import has been addded where after doing static import you can directy call static method in the class.

[nirjari patel]

<%@ page import="ca.rogers.evolution.presentation.content.model.CommonImage"%>

If I import a page as coded above in jsp, then I can only use static methods of the page.
instead of that, can I import a class or package in jsp as it is imported in java class ? If not, then how to use non-static methods of a java class in jsp ?

[Campbell Ritchie]

You can't use any instance methods of that class. You can statically import static public members of a class and call them as if they were private static members of your class. You cannot use instance members of another class inside your class, except by inheritance.

import static java.lang.String.*;

public class Foo
{
    . . .
    String s = format("A foo with %d %ss.", count, name); // Use of static import.
    String ss = valueOf(123); // There is an overridden valueOf static method in String: also static import.
    String sss = substring(3, 6);
    /*
     *    Use of an instance method of the String class. Won't compile unless you have a substring(int, int) method in your class
     */
    String ssss = s.substring(3, 6); // Invocation of a method of a different class on an object of that other class

You can invoke the valueOf and format methods from inside your class, but the instance methods of String can only be invoked on a String object. That is what you can do in Java.

No idea about JSP: I shall move you to the JSP forum where that sort of question is usually discussed.

[Aurelian Tutuianu]

nirjari patel wrote:public class ChildClass extends ParentClass {}
this class extends ParentClass. Can ChildClass extend multiple classes ?
public class ChildClass extends ParentClass_1 and ParentClass_2{}
Is this possible ? If not then to use the methods of both classes, one class can be extended and other class can be imported in the ChildClass ?
thanks

Thanks to God, James Gosling, whoever it was that did not let that happen.

[shivendra tripathi]

Hi Nirjari,

I will take one example to explain you....

package a;
public class B{
public static void printName(){}

public void printFullName(){}

}

now supppose in class C i want use class A method without import

    
class C{

void myMethood(){
   a.B.printName(); //direct call since printName is static
   new a.B.printFullName(); // call after creating instance of B, since printFullName is not static
}
}

now same scenario with inport

    
import a.B;
class C{

void myMethood(){
   B.printName(); //direct call since printName is static
   new B.printFullName(); // call after creating instance of B, since printFullName is not static
}
}

same is valid for jsp as well

[Campbell Ritchie]

shivendra tripathi wrote: new a.B.printFullName();
new B.printFullName();

Surely it's

new B("Campbell Ritchie").printFullName();

assuming there is an appropriate constructor?

[Himanshu Kansal]

Not that it matters, just wanted to add... I think he just missed the "()"

One doubt that came to me while going through this thread... an import in jsp is a static import?

Regards

[David Newton]

No; it's an import. Just look at the generated code.

[David Newton]

(And as a CLOS user I really miss multiple inheritance. Java's lack of mixins is a real PITA.)

[ankur rathi]

Campbell Ritchie wrote:You can't use any instance methods of that class.

You cannot use instance members of another class inside your class, except by inheritance.

Why can't? I think I am missing something.
Why can't I create an instance and use it's instance method in my class?

[David Newton]

You can, but that's not what it sounded like you were asking.