Igor Lichs

Welcome,

New to Java?

http://java.sun.com/docs/books/tutorial/

Have fun =]

Two answers? I think it's always pre in post.

Yea I called about 5 bookshops and noone has the book.

I suggest you to buy it in www.amazon.com .

A dollar is R$1,6 I think and you can buy it in amazon just paying the post (10 dollars I think). In my opinion, it worth it!

Abra�o!

I have them (Study Guide and Mock), but I bought in London and brought them to Brazil.

I'll try to find them here in Curitiba and then I tell you.

Do you need it in Portuguese or can it be in English?

Sure.

package example; public class MyObject1 { private int number; MyObject1( int number ) { this.number = number; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + number; return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; final MyObject1 other = (MyObject1) obj; if (number != other.number) return false; return true; } }

package example; public class MyObject2 { private int time; MyObject2(int time) { this.time = time; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + time; return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; final MyObject2 other = (MyObject2) obj; if (time != other.time) return false; return true; } }

package example; public class Main { public static void main(String[] args) { Main main = new Main(); MyObject1 obj1 = new MyObject1(5); MyObject2 obj2 = new MyObject2(5); System.out.println(main.fun(obj1, obj2)); } public String fun(Object x,Object y) { return (x==y)+ " " + x.equals(y) + " " + (x.hashCode()==y.hashCode()); } }

If you run this code, the answer will be false false true

in letter c, it's false true true

hasCode is NOT the memory space where you allocate the object, so x==y is NOT in the very same memory space.

if x is dog1 and y is dog2, they are Dog objects, same properties, but they are NOT in the same memory space.

equals were correctly overriden, so it check the dog1 properties with the dog2 properties and its hashcodes. Does it match? Great! They are equals. If they are equals, their hashCodes MUST be equals, because the question said they were appropriately overriden, and it means the Dog class has overriden the hashCode with the number 123456789 for exemple, so every dog will have the hashCode 123456789.

Can a Cat class have the hashCode 123456789? YES. It's not memory position.

The only possible way to x==y be true, is when you call fun like this:

String a = "test"; fun(a, a);

x = a
y = a

so they are the very same object, so it is TRUE, they are equals, so it is TRUE and their hashCode is the same, so it is TRUE.

true true true is one possible answer, but its NOT in the options. I think I have confused you with my previous post, sorry about that.

Ok, check this out: http://java.sun.com/javase/6/docs/api/java/lang/Object.html#hashCode()

It says: "It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results."

So even if the objects are different, they can have the same hashCode, however, if they are equals, they MUST have the same hashCode, if the equals and the hashCode were appropriately overridden, which it was, according to the question title.

x==y just compares memory spaces. To be true, x and y MUST have to be the SAME object, not just the same type.

Therefore, 2 different object will be false on x==y, false on x.equals(y), but comparing hashCodes they can the same or not. But it's a possibility. So letter a is correct.

If I have 2 Objects CAR and both are MERCEDES, they are not in the same memory space, so x==y is false, but they are equals, so their hashCodes must be the same. So letter c is also correct.

This is the way I see this, at least.

Hey hermano, bad thing what your team did with mine in this Olympics.. 3x0?!?!
hahaha

ok, serious now, if you aim J2EE, go straight forward to web component developer...

i'm thinking now, why letter b is wrong? why the hashCodes MUST be the same? cant I implement a method which gives me different hashCodes?

I think it has 4 correct answers:

- false false true
- false true true

and other two which are not in the options:

- false false false
- true true true

imagine the following situation in the main method, assuming the fun method is in the TestClass:

String a = "test; System.out.println(new TestClass().fun(a, a));

that's why i suppose its 4 correct answers and not just 3.

please guys, correct me if i'm wrong, i'm here to learn

but aren't you overriding the equals method?

Is its output really "true" ?

As far as I know, the equals melhod defined in the Object class compares hashCodes.

I just made some implements in your code to print the hashCode:

class Dog{ String name; public static void main(String []args){ Dog d1=new Dog(); Dog d2=new Dog(); d1.name="aiko"; d2.name="aiko"; if(d1.equals(d2)) System.out.println("true"); System.out.println("hC1: " + d1.hashCode() + "\nhC2: " + d2.hashCode()); } }

and the output is:

hC1: 25669322 hC2: 14978587

so their hashCodes are different and this code will never output a "true" message.

are you sure this is all the code?

Hey guys,

This is my first topic

I would like to know what is the utility of bit shift operators.

<<
>>
>>>
~

I know what they are, but I have no idea in what situation should I use them.

Cheers!!

Hello mate,

You will be fine, and you ARE ready.

Take the exam!!