EXP03-J. Do not use the equality operators when comparing values of boxed primitives

The values of boxed primitives cannot be directly compared using the == and != operators because these operators compare object references rather than object values. Programmers can find this behavior surprising because autoboxing memoizes, or caches, 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 (JLS) §5.1.7, "Boxing Conversion," [JLS 2005], 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.

Use of the == and != operators for comparing the values of fully memoized boxed primitive types is permitted.

Use of the == and != operators for comparing the values of boxed primitive types that are not fully memoized is permitted only when the range of values represented is guaranteed to be within the ranges specified by the JLS to be fully memoized.

Use of the == and != operators for comparing the values of boxed primitive types is not allowed in all other cases.

Note that JVM implementations are allowed, but not required, to memoize additional values:

Less memory-limited implementations could, for example, cache all characters and shorts, as well as integers and longs in the range of -32K to +32K.

Code that depends on implementation-defined behavior is non-portable.  In the unusual case where a program is guaranteed to execute only on a single implementation, it is permissible to depend on implementation-specific ranges of memoized values.

Noncompliant Code Example

This noncompliant code example defines a Comparator with a compare() method [Bloch 2009]. The compare() method accepts two boxed primitives as arguments. The == operator is used to compare the two boxed primitives. In this context, however, 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 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 in an attempt to compare the values of pairs of Integer objects. However, the == operator compares object references rather than object values.

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);
  }
}

The Integer class is only guaranteed to cache integer values from -128 to 127, which can result in equivalent values outside this range comparing as unequal when tested using the equality operators. For example, a Java Virtual Machine (JVM) that did not cache any other values when running this program would output

true
false
false
true

Compliant Solution

This compliant solution uses the equals() method instead of the == operator to compare the values of the objects. The program now prints true, false, true, false on all platforms, 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, contain 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 is required to memoize only those integer values in the range -128 to 127; it might return a nonunique object for any value outside that range. Consequently, when comparing autoboxed integer values outside that range, the == operator might return false and the example could deceptively output 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)) {  // uses '=='
        counter++;
      }
    }

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

}

However, if the particular JVM running this code memoized integer values from -32,768 to 32,767, all of the int values in the example would have been autoboxed to the corresponding 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 lacks a specification of this interval; rather, it specifies a minimum range that must be memoized. 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
    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))) {  // uses 'equals()'
        counter++;
      }
    }
 
    // Print the counter: 10 in this example
    System.out.println(counter);
  }
}

Noncompliant Code Example (new Boolean)

In this noncompliant code example, constructors for class Boolean return distinct newly-instantiated objects. Using the reference equality operators in place of value comparisons will yield unexpected results.

public void exampleEqualOperator(){
  Boolean b1 = new Boolean("true");
  Boolean b2 = new Boolean("true");

  if (b1 == b2) {           // never equal
    System.out.println("Never printed");
  }
}

Compliant Solution (new Boolean)

In this compliant solution, the values of autoboxed Boolean variables may be compared using the reference equality operators because the Java language guarantees that the Boolean type is fully memoized. Consequently, these objects are guaranteed to be singletons.

public void exampleEqualOperator(){
  Boolean b1 = true; // Or Boolean.True
  Boolean b2 = true; // Or Boolean.True
	
  if (b1 == b2) {        // always equal
    System.out.println("Always print");
  }
}

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 Guidelines

Bibliography