Java Challenge [II]

[[RaIdEn]]

Hey am reopening the thread for the java challenge. Lets not have the mistakes

of the previous thread.

Same rules apply.

1. The person who answers the current question should try to ask the next

question so that it helps the people to learn new stuff and keep the thread

moving.

2. Everyone should cooperate, dont really care if you google the question main purpose is to learn it.

3. NO FLAMING at any costs.

Question:

Try to attach an image to the frame where the application provides a simple

mechanism for the user to choose an image and it should be pasted.

Pretty easy one..just need to know the correct API

[collin389]

Wiat, so like a tree of images and you click on and it displays an image in a JPanel? or a button for each image and it copies it to the clipboard. Iam confused.

[JosAH]

Here's a much more tricky one (no programming involved); if you all love autoboxing so much, explain the output:

import java.util.*;
 
public class Autoboxing {

	public static void main(String[] args) {

		Object foo = new Long(0xcafebabedeadbeefL);
		List<Object> bars = new ArrayList<Object>();
        
		bars.add(true ? (Long)foo : (Long)foo);
		bars.add(true ? (Long)foo : (Number)foo);
		bars.add(true ? (Long)foo : (Double)foo);
		bars.add(true ? (Long)foo : ((Long)foo).longValue());
		
		System.out.print("==    :");
		for (Object bar : bars)
			System.out.print(" "+(foo == bar));
		System.out.println();

		System.out.print("equals:");
		for (Object bar : bars)
			System.out.print(" "+foo.equals(bar));
		System.out.println();
	}
}

kind regards,

Jos

[[RaIdEn]]

Here's a much more tricky one (no programming involved); if you all love autoboxing so much, explain the output:

import java.util.*;
 
public class Autoboxing {

	public static void main(String[] args) {

		Object foo = new Long(0xcafebabedeadbeefL);
		List<Object> bars = new ArrayList<Object>();
        
		bars.add(true ? (Long)foo : (Long)foo);
		bars.add(true ? (Long)foo : (Number)foo);
		bars.add(true ? (Long)foo : (Double)foo);
		bars.add(true ? (Long)foo : ((Long)foo).longValue());
		
		System.out.print("==    :");
		for (Object bar : bars)
			System.out.print(" "+(foo == bar));
		System.out.println();

		System.out.print("equals:");
		for (Object bar : bars)
			System.out.print(" "+foo.equals(bar));
		System.out.println();
	}
}

kind regards,

Jos

the 1st line checks wether both of them relate to the same instance rather than the whole value itself. So, its got the respective truth values
I am not sure of the last value, maybe when you do long.longvalue(), it creates a new instance or something, so it returns false eventhough values are the same.


where the 2nd line checks for the value itself.

[collin389]

Wow, that is very confusing because I though (true ? a : b) would always return a, but when I replace "bars.add(true ? (Long)foo : (Double)foo);" with "bars.add((Long)foo);" it will return true for both instead of false.

[Fubarable]

I'm probably cheating here, but I peeked, and the JLS gives some guidance here. Expressions

The ternary operation:

a ? b : c

always returns something, and regardless if it returns b or c, it can only return one type meaning it has to decide what type to return before it "operates". If the 2nd and 3rd operands are of different types, then some logic must be done to decide the most appropriate return type.

[Fubarable]

Wow, that is very confusing because I though (true ? a : b) would always return a,...

This is correct, and in the example above, it always is returning a, or "a" converted.

[collin389]

Oh, I get it so it always returns foo(because of the true) but it wont always cast it as (Long).

[Fubarable]

See reply #6. The JVM converts the (Long)foo to a double using the rules of "Binary Numeric Promotion". For details please look here: Conversions and Promotions

[[RaIdEn]]

See reply #6. The JVM converts the (Long)foo to a double using the rules of "Binary Numeric Promotion". For details please look here: Conversions and Promotions

How abt the last one(long.Longvalue()). i am little confused over there. why is it showing false in the 1st case.

[JosAH]

How abt the last one(long.Longvalue()). i am little confused over there. why is it showing false in the 1st case.

Here's a bit of help: there are two mechanism working against/for you:

1) the type of the b?t:f expression is the widest type of operands t and f, e.g. if t is a long and f is an int the type of the entire expression is long

2) autoboxing: whenever an object is needed the primitive type is boxed to its corresponding wrapper type, e.g. long --> Long

kind regards,

Jos