Just Java IT
          

          西門町學士關于Java的隨便一說而已……
          

          

          
	
					
	
          
		
          
			Tiger pitfall
		
          
		在SDN踩到一個Tiger的pitfall：
          package sdn;

          import java.util.ArrayList;

          public class BoxingEquality {
              
              /** Creates a new instance of BoxingEquality */
              public BoxingEquality() {
              }
              
              /**
               * @param args the command line arguments
               */
              public static void main(String[] args) {
                  // TODO code application logic here
                  int i = 1;
                  int j = 1;
                  ArrayList<Integer> list = new ArrayList<Integer>();
                  list.add(i);
                  list.add(j);
                  System.out.printf("It is %b that i == j.\n", (i==j));
                  System.out.printf("It is %b that list.get(0) == list.get(1).\n", (list.get(0)==list.get(1)));
                  System.out.printf("It is %b that list.get(0).equals(list.get(1)).",            list.get(0).equals(list.get(1)));
              } 
          }

          輸出結果出乎意料：
          It is true that i == j.
          It is true that list.get(0) == list.get(1).    // WHY???
          It is true that list.get(0).equals(list.get(1)).

          然而，僅僅將 i 和 j 的值改成1000：
          int i = 1000;
          int j = 1000;
          就這一個改動后輸出結果雖然符合了java的思維方式，但在這個context中卻更顯得怪異：
          It is true that i == j.
          It is false that list.get(0) == list.get(1). // Oops
          It is true that list.get(0).equals(list.get(1)). 

          Sun給出的解釋：
          
The primitives are equal and the values of the boxed ints are equal. But this time the ints
point to different objects. What you have discovered is that for small
integral values, the objects are cached in a pool much like Strings.
When i and j are 2, a single object is referenced from two different locations. When i and j
are 2000, two separate objects are referenced. Autoboxing is guaranteed
to return the same object for integral values in the range [-128, 127],
but an implementation may, at its discretion, cache values outside of
that range. It would be bad style to rely on this caching in your code.

          呵呵，要不注意說不定還真中招了！          
		
          
			posted on 2006-03-16 22:03 西門町學士 閱讀(336) 評論(4)  編輯  收藏  所屬分類: Java 
		
          
	
          
	
	





          
	    
    


          

          Feedback
          
	
	
			
          
				# re: Tiger pitfall
					
						2006-10-21 09:56
					
				11
			
          
			
          
				11111111111111111  回復  更多評論
				  
			
          
		
			
          
				# re: Tiger pitfall
					
						2006-10-21 09:58
					
				11
			
          
			
          
				不好意思!剛才發錯了!  回復  更多評論
				  
			
          
		
			
          
				# 你好!
					
						2006-10-21 10:04
					
				11
			
          
			
          
				由于最近在做一個在線購物系統!所以看到你的主頁后就來想想你這個留言版是怎么實現的~!:)  回復  更多評論
				  
			
          
		
			
          
				# 關于你的留言版的實現原理
					
						2006-10-21 10:07
					
				11
			
          
			
          
				由于最近在做一個在線購物系統!所以看到你的主頁后就來想想你這個留言版是怎么實現的~!:)    回復  更多評論
				  
			
          
		

          




          








          

				

          







    







          
        
	




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