EXP03-J. Do not compare String objects using equality or relational operators

Java defines equality operators == and != and relational operators such as <=,>=,>,<. When it comes to String object reference comparisons, these manifest as traps that an amateur programmer may unintentionally fall victim to.

Noncompliant Code Example

For == to return true for two String references, they must point to the same underlying object. This noncompliant example declares two different String objects with the same values, however, they compare unequal since they constitute different object references.

public class BadComparison {
  public static void main(String[] args) {
    String one = new String("one");
    String two = new String("one");
    if(one == two)
      System.out.println("Equal"); //not printed
  }
}

Compliant Solution

To be compliant, use the object1.equals(object2) method when comparing string values.

public class GoodComparison {
  public static void main(String[] args) {
    String one = new String("one");
    String two = new String("one");
    boolean result;
    if (one == null){
    	result = two == null;
    }
    else{
    	result = one == two || one.equals(two);
    }
   System.out.println(result);
  }
}

The mentioned operators now seemingly work while dealing with string literals that have constant values (such as in String one = "one" and String two = "two". or when the intern method has been used on both strings to compare pointer references. (See Compliant Solution 2)

Compliant Solution

If it is desired to keep only one copy of the string in memory, perform quick repeated comparisons and ensure that string1.equals(string2) is true, the following Compliant Solution may be used.

public class GoodComparison {
  public static void main(String[] args) {
    String one = new String("one");
    String two = new String("one");
    boolean result;
    if (one != null){
    	one = one.intern();
    }
    if (two != null){
    	two = two.intern();
    }
    result = one == two;

   System.out.println(result);
  }
}

Note however, that the performance gains achieved by doing so may be meeker than the benefits of having more robust code that also takes non-constant and non-interned values. Moreover, such behavior encourages ambiguity that hinders selection of proper methods for comparing String objects.

Exceptions

EXP03: In general, for any two objects, it is permissible to compare their elements provided that the class is a singleton. The use of static factory methods over constructors facilitates instance control which in turn limits the effective number of instances of an immutable class to one. As a result, for two objects a and b, a.equals(b) is true when a==b [[Bloch 08]]. The String class does not meet these requirements and consequently, does not possess these characteristics.

Risk Assessment

Using the equality or relational operators to compare objects may lead to unexpected results.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Section 3.10.5, String Literals
[[FindBugs 08]] ES: Comparison of String objects using == or !=
[[MITRE 09]] CWE ID 595 "Incorrect Syntactic Object Comparison", CWE ID 597 "Use of Wrong Operator in String Comparison"

EXP02-J. Do not ignore values returned by methods      03. Expressions (EXP)      EXP04-J. Be wary of invisible implicit casts when using compound assignment operators