EXP05-J. Be careful about the wrapper class and autoboxing

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]. 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 always be careful about this process, especially while performing comparisons. Section 5.1.7 of JLS 3rd 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 Code Example

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. We thus deduce that the results of using the == operator here will be misleading.

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

public class TestWrapper1 {
 public static void main(String[] args) {
  //Create an array list of integers, where 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, where each element
  //is the same as 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++;
 
     System.out.println(counter);
 }
}

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

Risk Assessment

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

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

Chapter 5, Core Javaâ„¢ 2 Volume I - Fundamentals, Seventh Edition by Cay S. Horstmann, Gary Cornell
Publisher:Prentice Hall PTR;Pub Date:August 17, 2004.
Section 5.1.7, The Javaâ„¢ Language Specification,Third Edition by James Gosling, Bill Joy, Guy Steele, Gilad Bracha
Publisher:ADDISON-WESLEY;Pub Date:May 2005.

EXP04-J. Be wary of invisible implicit casts      02. Expressions (EXP)      03. Scope (SCP)