Hi Rita,
In Example 2,
I search the Integer.java for the answer.
When you are using autoboxing (just like Line 026) , you will get a new
Integer object in fact.
So when you use "==" for compare the reference, it will not true.
But be caution, the implement of autoboxing mechanism in the source do have
set caches.
Like:
public static Integer valueOf(int i) {
if(i >= -128 && i <= IntegerCache.high)
return IntegerCache.cache[i + 128]; //there's the cache
else
return new Integer(i);
}
So you may get true when you do it for a second time.
To solve just using .equals() method instead of "==".
In Example 3
Because String is inmutable, each time you set a new string, you will get a
new String Object.
So use .equals() instead as well.
Hugs,
Eric Cai
On Thu, Jul 30, 2009 at 10:56 PM, RPGoldie <rpgol...@arcor.de> wrote:
>
> Hi all,
>
> I encountered a problem when working with Integer objects which I
> would like to be discussed. Let me first show you my class:
>
> 001 public class Test {
> 002
> 003 int memberVar;
> 004
> 005 public static void main(String[] args) {
> 006
> 007 // Example 1
> 008 Test t1 = new Test();
> 009 t1.memberVar = 7;
> 010 Test t2 = new Test();
> 011 t2 = t1;
> 012
> 013 System.out.println(t1 == t2);
> 014
> 015 t2.memberVar = 55;
> 016 System.out.println(t1.memberVar);
> 017 System.out.println(t1 == t2);
> 018 System.out.println();
> 019
> 020 // Example 2
> 021 Integer i1 = new Integer(1);
> 022 Integer i2 = i1;
> 023 System.out.println(i1 + ", " + i2);
> 024 System.out.println(i1 == i2);
> 025
> 026 i2 = 5;
> 027 System.out.println(i1 + ", " + i2);
> 028 System.out.println(i1 == i2);
> 029 System.out.println();
> 030
> 031 // Example 3
> 032 String s1 = "Hello!";
> 033 String s2 = s1;
> 034 System.out.println(s1 + ", " + s2);
> 035 System.out.println(s1 == s2);
> 036
> 037 s2 = "Bye!";
> 038 System.out.println(s1 + ", " + s2);
> 039 System.out.println(s1 == s2);
> 040
> 041 }
> 042 }
>
> Example 1: I create 2 objects t1 and t2 of class Test. In line 011 t1
> is assigned to t2. Now t2 points to the same reference as t1, so line
> 013 returns true. Now in line 015 I change the property of t2 to 55.
> Because it is the same object as t1, line 016 gives 55. This is what I
> expect objects to do.
>
> Example 2: The same matter but now with wrapper class Integer. In line
> 023 both i1 and i2 have the same contents of 1. Line 024 gives true,
> so they are pointing to the same reference. Ok. In line 026 I change
> object i2 to 5 and expect i1 to change also, because it is the same
> object! But it doesn't, line 027 gives 1 and 5, line 028 gives false!
> Is this a typical behaviour of wrapper classes or why doesn't it
> work???
>
> Example 3: same with Strings. It behaves like the Integers in example
> 2.
> I guess this is because strings are immutable. So in line 037 there is
> in fact created a new object of type String with contents "Bye!" and
> s1/s2 are no longer the same object. Ok.
>
> But why does the wrapper class not do what I'm expecting???
>
> I'm looking forward to your ideas.
> Rita
>
> >
>
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