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It is because that in JDK 5.0, if the value p being boxed is true, false, a byte, an ASCII character, or an integer 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. In case of that, when we need to do some comparison of these wrapper class, we should use equal instead "=="(see EXP03-J for details):

Compliant solution

 "Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly."

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Noncompliant Code Example

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));    
 }
}

In many times, 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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