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|---|---|---|
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public class TestWrapper2 {
  public static void main(String[] args) {
  
  
Integer i1 = 100;
     Integer i2 = 100;
     Integer i3 = 1000;
     Integer i4 = 1000;
     System.out.println(i1==i2);
     System.out.println(i1!=i2);
     System.out.println(i3==i4);
     System.out.println(i3!=i4);
    
 
}
}
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Output of This Code
Here the cache in the Integer class can only make the number from -127 to 128 refer to the same object, which clearly explains the result of the above code. To avoid making such mistakes, when you need to compare wrapper classes, use equal instead of == (see EXP03-J for details).
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| Code Block | ||
|---|---|---|
| ||
public class TestWrapper2 {
  public static void main(String[] args) {
  
  
Integer i1 = 100;
     Integer i2 = 100;
     Integer i3 = 1000;
     Integer i4 = 1000;
     System.out.println(i1.equals(i2));
     System.out.println(i3.equals(i4));    
 
}
} 
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Noncompliant Code Example
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| Code Block | ||
|---|---|---|
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import java.util.ArrayList;
public class TestWrapper1 {
  public static void main(String[] args) {
   //create an array list of integers, which each element
   //is more than 127
     ArrayList<Integer> list1 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
      list1.add(i+1000);
   //create another array list of integers, which each element
   //is the same with the first one
     ArrayList<Integer> list2 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
       list2.add(i+1000);    
        
    
int counter = 0;
     for(int i=0;i<10;i++)
       if(list1.get(i) == list2.get(i)) counter++;
     //output the total equal number
     System.out.println(counter);
  }
} 
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In JDK 1.6.0_10, the output of this code is 0. In this code, we want to count the same numbers of array list1 and array list2. Undoubtedly, the result is not what we expect. Integer can only cache from -127 to 128, so when an int number is beyond this range, it will be autoboxed into different objects, and == will return false. But if we can set more caches inside Integer (cache all the integer values -32K-32K, which means that all the int values could be autoboxed to the same Integer object), then the result may be different.
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| Code Block | ||
|---|---|---|
| ||
public class TestWrapper1 {
  public static void main(String[] args) {
   //create an array list of integers, which each element
   //is more than 127
     ArrayList<Integer> list1 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
       list1.add(i+1000);
   //create another array list of integers, which each element
   //is the same with the first one
     ArrayList<Integer> list2 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
       list2.add(i+1000);    
        
    
int counter = 0;
     for(int i=0;i<10;i++)
       if(list1.get(i).equals(list2.get(i))) counter++;
     //output the total equal number
     System.out.println(counter);
  }
}
|
In JDK 1.6.0_10, the output of this code is 10. (The reason is the same as the above code example.)
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