EXP03-J. Avoid the equality operators when comparing values of boxed primitives

The values of boxed primitives cannot be directly compared using the == and != operators because they compare object references, not object values. Programmers could find this surprising because autoboxing memoizes the values of some primitive variables. Consequently, reference comparisons and value comparisons produce identical results for the subset of values that are memoized.

Autoboxing automatically wraps a value of a primitive type with the corresponding wrapper object. The Java Language Specification Section 5.1.7 Boxing Conversion explains which primitive values are memoized during autoboxing:

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 noncompliant code example ([[Bloch 2009]]), defines a Comparator with a compare() method. The compare() method accepts two boxed primitives as arguments. The problem is the use of the == operator to compare the two boxed primitives; in this context, it compares the references to the wrapper objects rather than comparing the values held in those objects.

static Comparator<Integer> cmp = new Comparator<Integer>() {
  public int compare(Integer i, Integer j) {
    return i < j ? -1 : (i == j ? 0 : 1);
  } 
};

Note that primitive integers are also accepted by this declaration because they are appropriately autoboxed at the call site.

Compliant Solution

This compliant solution uses the comparison operators, <, >, <=, or >=, because these cause automatic unboxing of the primitive values. The == and != operators should not be used to compare boxed primitives.

public int compare(Integer i, Integer j) {
  return i < j ? -1 : (i > j ? 1 : 0) ;
}

Noncompliant Code Example

This noncompliant code example uses the == operator to compare two Integer objects. According to the guideline EXP01-J. Do not confuse abstract object equality with reference equality, in order for the == operator to return true for two object references, they must point to the same underlying object.

public class Wrapper {
  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);

  }
}

This program prints the output sequence: true, false, false and true. The cache in the Integer class memoizes integer values from -127 to 128 only, which accounts for the output of the above code. Avoid this problem by using the equals() method instead of the == operator to compare wrapper classes.

Compliant Solution

This compliant solution uses the equals() method to compare the values of the objects. The program now prints true, false, true and false, as expected.

public class Wrapper {
  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(!i1.equals(i2));
    System.out.println(i3.equals(i4));
    System.out.println(!i3.equals(i4));
  }
}

Noncompliant Code Example

Java Collections contain only objects; they cannot contain primitive types. Further, the type parameters of all Java generics must be object types rather than primitive types. That is, attempting to declare an ArrayList<int> (which would presumably contains values of type int) fails at compile time because type int is not an object type. The appropriate declaration would be ArrayList<Integer>, which makes use of the wrapper classes and autoboxing.

This noncompliant code example attempts to count the number of indices in arrays list1 and list2 that have equivalent values. Recall that class Integer must memoize only those integer values in the range -127 to 128; it might return non-unique objects for all values outside that range. Consequently, when comparing autoboxed integer values outside that range, the == operator might return {{false}, and the output of this example could be 0.

public class Wrapper {
  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
    // has the same value as the first list
    ArrayList<Integer> list2 = new ArrayList<Integer>();
    for (int i = 0; i < 10; i++) {
      list2.add(i + 1000);
    }

    // Count matching values.
    int counter = 0;
    for (int i = 0; i < 10; i++) {
      if (list1.get(i) == list2.get(i)) { 
        counter++;
      }
    }

    // print the counter: 0 in this example
    System.out.println(counter);
  }
}

If the particular JVM running this code memoized integer values from -32768 to 32767, all of the int values in the example would have been autoboxed to singleton Integer objects and the example code would have operated as expected. Using reference equality instead of object equality requires that all values encountered fall within the interval of values memoized by the JVM. The JLS does not specify this interval; it only provides a minimum range. Consequently, successful prediction of this program's behavior would require implementation-specific details of the JVM.

Compliant Solution

This compliant solution uses the equals() method to perform value comparisons of wrapped objects. It produces the correct output 10.

public class Wrapper {
 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
   // has the same value as the first one
   ArrayList<Integer> list2 = new ArrayList<Integer>();
   for (int i = 0; i < 10; i++) {
     list2.add(i + 1000);
   }
 
   // Count matching values
   int counter = 0;
   for(int i = 0; i < 10; i++) {
     if (list1.get(i).equals(list2.get(i))) { 
       counter++;
     }
   }
 
   // print the counter: 10 in this example
   System.out.println(counter);
 }
}

Exceptions

EXP03-EX1: The values of autoboxed Boolean variables may be compared using the reference equality operators because the Java language guarantees that the autoboxing yields either Boolean.True or Boolean.False (as appropriate). These objects are guaranteed to be singletons.

Boolean b1 = true; // Or Boolean.True
Boolean b2 = true; // Or Boolean.True
b1 == b2;          // always equal

Note, however, that the constructors for class Boolean return distinct newly-instantiated objects. Using the reference equality operators in place of value comparisons will yield unexpected results in this case.

Boolean b1 = new Boolean("true");
Boolean b2 = new Boolean("true");
b1 == b2;          // never equal

Risk Assessment

Using the equivalence operators to compare values of boxed primitives can lead to erroneous comparisons.

Automated Detection

Detection of all uses of the reference equality operators on boxed primitive objects is straightforward. Determining the correctness of such uses is infeasible in the general case.

Related Vulnerabilities

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

Bibliography

[[Bloch 2009]] 4. "Searching for the One"
[[JLS 2005]] Section 5.1.7, "Boxing Conversion"
[[Pugh 2009]] Using == to compare objects rather than .equals

EXP02-J. Use the two-argument Arrays.equals() method to compare the contents of arrays      04. Expressions (EXP)      EXP04-J. Beware of invisible implicit casts when using compound assignment operators