Need Help

[Madan Mohan]

Could someone please explain me how boxing and var - args work...

[Kenneth Lomvey]

Hi Madan,

Welcome to JavaRanch. First of all, please use a Meaningful Subject Line when posting a question again.

Now read the topics below. You will get an answer for your question:

http://www.coderanch.com/t/417852/java-programmer-SCJP/certification/Var-Args

http://www.coderanch.com/t/269788/java-programmer-SCJP/certification/boxing-vs-var-args

http://www.coderanch.com/t/264482/java-programmer-SCJP/certification/var-arg-Boxing-Widening-ambiguous

[Rajshekhar Paul]

Just to give an idea -

1. through var-args you can send zero as well as more than zero argument of same type to a method which has only one parameter(), i.e. var-arg parameter. a method can have only one var-arg parameter and it should be the last parameter if there are more than one parameter for that method.

2. boxing is used to have the primitive specific behavior in a wrapper class object.

Boolean b = new Boolean(true);
 
/* This if block will fail if boxing would not have been done here.
 * Boxing unwraps the b object to the primitive boolean type and executes 
 * the if block.
 */
if(b) {
    System.out.println("Magic done by boxing");
}

[Madan Mohan]

Hi Raj,

Could you please explain me this paragraph with some example..I am not able to get it

"through var-args you can send zero as well as more than zero argument of same type to a method which has only one parameter(), i.e. var-arg parameter. a method can have only one var-arg parameter and it should be the last parameter if there are more than one parameter for that method."

[Rajshekhar Paul]

Sure.

package com.rajshekharpaul.scjp.practice.src.classes;
 
public class VarArgsTest {
 
public void test(int...is) {
System.out.println("inside test");
}

public void test1(int i, int...js) {
System.out.println("inside test1");
}

public static void main(String[] args) {
VarArgsTest vat = new VarArgsTest();

/*
 * Here I am passing zero parameter though original method 
 * expects one var-arg parameter. But still the code compiles 
 * as with var-arg parameter methods, we can send zero parameter if 
 * the method has only one parameter. In this case, test() method has 
 * only one parameter which is a var-arg parameter.
 */
vat.test();

/*
 * In this case, we passing values for all the two parameters. 
 * But the first argument that is 1 is for the first parameter and 
 * next two arguments 4 and 5 are for the var-arg parameter.
 */
vat.test1(1, 4, 5);

/*
 * Here we are passing value for the first int parameter and 
 * not passing any value for the var-arg parameter.
 */
vat.test1(1);
}
}

Hope, it helps you.

[Ankit Garg]

Let me also try to explain you what raj meant

void method(String... args, int... i) //not allowed, cannot have more than one var-args in a method

-------------------------------------------------------------------------

void method(String... args, int i) //not allowed, var-arg must be the last parameter so the correct code would be

void method(int i, String... args)

-------------------------------------------------------------------------
void method(String... args)

now you can call this method as

method(); //no arguments
method("a"); //1 argument
method("a", "b"); //2 argument

and so on. i.e. you can pass any number of arguments to this method...

[Edit: Oops! I was not quick enough]
[ December 05, 2008: Message edited by: Ankit Garg ]