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A Boxing conversion converts the values of a primitive type to the corresponding values of the reference type, for instance, from {{int}} to the type {{Integer}} \[[JLS 5.1.7 Boxing Conversion|http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.7]\].  It can be convenient in many cases where an object parameter is desired, such as with collection classes like {{Map}} and {{List}}. It also helps avoid clutter in code.

Autoboxing can automatically wrap the primitive type to the corresponding wrapper object. But one must Autoboxing can automatically wrap the primitive type to the corresponding wrapper object, which can be convenient in many cases and avoid clutter in code. But you should always be careful about this process, especially when doing while performing comparisons. Section 5.1.7 of JLS 3rd Edition explains this point clearly:

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This code uses == to compare two integer objects. From EXP03-J we 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 We thus deduce that the results of using the == operator in this code will be falsemisleading. However,

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);
 
 }
}

Output of This Code

Here the These comparisons generate the sequence: true, false, false and true. 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 output of the above code. To avoid making such mistakes, when you need to compare wrapper classes, use equal equals instead of == (see EXP03-J for details).

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Compliant Solution

Using object1.equals(object2) only compares their values. Now, the results will be true, as we expected.

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));

 }
}

Noncompliant Code Example

Sometimes you may want to create a dynamic array of integers is desired. 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> ArrayList<Integer> can be used to achieve this goal.

import java.util.ArrayList;
public class TestWrapper1 {
 public static void main(String[] args) {
  //createCreate an array list of integers, whichwhere each element
  //is moregreater than 127
     ArrayList<Integer> list1 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
       &#xA0;list1.add(i+1000);
  //createCreate another array list of integers, whichwhere 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);
 }
}

In JDK 1.6.0_10, the output of this code snippet 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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public class TestWrapper1 {
 public static void main(String[] args) {
  //createCreate an array list of integers, whichwhere each element
  //is more than 127
     ArrayList<Integer> list1 = new ArrayList<Integer>();
     for(int i=0;i<10;i++)
      list1.add(i+1000);
  //createCreate another array list of integers, whichwhere each element
  //is the same withas 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.)Notice that equals has been used for comparisons in this case.

Risk Assessment

Using array lists with primitive types causes a potential security risk.

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