Autoboxing can automatically wrap the primitive type to the corresponding wrapper object, which can be convenient in many cases and avoid clutters clutter in your own code. But you should always be careful about this process, especially when comparisondoing comparisons. Section 5.1.7 of JLS 3rd Ed can explain Edition explains this point clearly:
"If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2."
Noncompliant
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Code Example
This code uses "==" to compare 2 Integer object, from two integer objects. From EXP03-J we can know that for "==" to return true for two object references, they must point to the same underlying object. So we can simply draw the conclusion that the results of using "the ==" operator in this code will be false. However,
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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
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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. In case of To avoid making such mistakes, when we you need to do some comparisons of these wrapper class, we should compare wrapper classes, use equal instead "of ==" (see EXP03-J for details):.
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true false false true |
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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.equals(i2));
     System.out.println(i3.equals(i4));    
 }
} 
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Noncompliant Code Example
In many times, Sometimes you may want to create a dynamic array of integers. Unfortunately, the type parameter inside the angle brackets cannot be a primitive type. It is not possible to form an ArrayList<int>. Thanks to the wrapper class, now you can use ArrayList<Integer> to achieve this goal.
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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 the same as our expectation. Owning to the fact that the what we expect. Integer can only caches cache from -127 to 128, so when an int number is beyond this range, it will be autoboxed into different objects, then the "and ==" will return false. But if we can set more caches inside Integer (cache all the integer value (values -32K-32K), which means that all the int value values could be autoboxed to the same Integer object), then the result may be different!.
Compliant
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Solution
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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);
 }
}
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In JDK 1.6.0_10, the output of this code is 10. (the The reason is the same as the above code example.)
Risk Assessment
We often use Using array list lists with primitive type, so it will exert types causes a potential security risk.
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