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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}}. Another use case is to pass object references to methods, as opposed to primitive types that are always passed by value. The resulting wrapper types also help reduce clutter in code.

Autoboxing can automatically wrap the primitive type to the corresponding wrapper object.

 Some care should be taken during this process, especially when performing comparisons. The Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] 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 Code Example

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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);
 
 }
}

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 output of the above code. To avoid making such mistakes, when you need to compare wrapper classes, use equals instead of == (see EXP03-J for details).

Compliant Solution

Using object1.equals(object2) only compares their values. Now, the results will be true, as 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 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, ArrayList<Integer> can be used to achieve this goal.

import java.util.ArrayList;
public class TestWrapper1 {
 public static void main(String[] args) {
  //Create an array list of integers, where each element
  //is greater 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, where 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.

Compliant Solution

In JDK 1.6.0_10, the output of this code is 10. Notice that equals has been used for comparisons in this case.

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Noncompliant Code Example

This noncompliant code snippet \[[Techtalk 07|AA. Java References#Techtalk 07]\] prints {{100}} as the size of the {{HashSet}} while it is expected to print {{1}}. The combination of a {{short}} and an  {{integer}} value in the operation {{i-1}} leads to autoboxing into an {{Integer}} object. The {{HashSet}} contains only {{short}} values whereas (distinctly typed) {{Integer}} objects are being removed successively. The remove operation is as a result equivalent to a _No Operation_ (NOP). The compiler enforces type checking so that only {{short}} values are inserted, however, a programmer is free to remove an object of any type without triggering any exceptions since {{Collections<E>.remove}} takes an Object parameter and not {{E}}. Such behavior can result in unintended object retention or memory leaks.

public class ShortSet {
  public static void main(String[] args) {
    HashSet<Short> s = new HashSet<Short>();
      for(short i=0; i<100;i++) {
        s.add(i);
        s.remove(i - 1);
      }
    System.out.println(s.size());
  }
}

Compliant Solution

Avoid mixing different types together with an integer type. If inevitable, cast the autoboxed Integer object to a Short to remedy the issue described in the noncompliant code.

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