• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Jeanne Boyarsky
  • Ron McLeod
  • Paul Clapham
  • Liutauras Vilda
Sheriffs:
  • paul wheaton
  • Rob Spoor
  • Devaka Cooray
Saloon Keepers:
  • Stephan van Hulst
  • Tim Holloway
  • Carey Brown
  • Frits Walraven
  • Tim Moores
Bartenders:
  • Mikalai Zaikin

scjp1.5

 
Greenhorn
Posts: 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
public class MyClass221
{
public Integer startingI;
public void methodA()
{
Integer i=new Integer(25);
startingI=i;
methodB(i);
}
private void methodB(Integer i2)
{
i2=i2.intValue();
System.out.println(i2==startingI);//aa
System.out.println(i2.equals(startingI));

Integer ss=new Integer(8);
int s=ss.intValue();
System.out.println(s==ss);//bb
}
public static void main(String[] args)
{
new MyClass221().methodA();
}
}
请问为什么aa输出的是false,但是bb那里输出的是true?
希望赐教,不胜感激!
 
Ranch Hand
Posts: 332
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator


line 1: you are using "new", so the "ss" will not point to some constant literal in pool
line 2: here the value taken as int, and then boxed, i2 will point to constant literal in pool
aa: here you are comparing object explicitly created with new, and object created by autoboxing, with value within (-128,127), which is cached in constant pool
bb: here ss is unboxed before comparison, so you are comparing just two ints

Hint: next time, try to post your question in English :-)
提示: 下次, 尝试张贴您的问题用英语:-)
[ May 27, 2007: Message edited by: John Stone ]
 
Ranch Hand
Posts: 329
Oracle Java Linux
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I hope Babelfish translated your question correctly...

Originally posted by SmileFish Fish:
System.out.println(i2==startingI);//aa


You are comparing two different Integer objects.
One (startingI) constructed as new Integer(25) the other boxed from an integer primitive (the primitive returned from Integer.intValue()), as in i2 = i2.intValue();

Originally posted by SmileFish Fish:
System.out.println(s==ss);//bb


You are comparing two integer primitives here.


Now, for an interesting variation try the following: change Integer i=new Integer(25) for Integer i=25;
Do you understand the result?

[ May 27, 2007: Message edited by: Sergio Tridente ]
[ May 27, 2007: Message edited by: Sergio Tridente ]
 
Ranch Hand
Posts: 7729
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Hello "SmileFish Fish"-

Welcome to JavaRanch.

On your way in you may have missed that we have a JavaRanch Naming Policy for displayed (screen) names. Your displayed name must consist of a first name (or an initial), a space, and a family name (in that order) and not be obviously fictitious. Since yours "SmileFish Fish", does not conform with it, please take a moment to change it, which you can do right here.

Posters with nonconforming displayed names will be locked out of JavaRanch after a few posts using those names.

Thanks
-Barry

(NR) - search tag
 
Barry Gaunt
Ranch Hand
Posts: 7729
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
In future post in english please.
 
Z Weitao
Greenhorn
Posts: 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Thanks,next time .I will use in english.
 
Barry Gaunt
Ranch Hand
Posts: 7729
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Originally posted by Z Weitao:
Thanks,next time .I will use in english.



Thankyou.
 
With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.
reply
    Bookmark Topic Watch Topic
  • New Topic