Comparison of Object References Instead of Object Contents

Description

For example, in Java, comparing objects using == usually produces deceptive results, since the == operator compares object references rather than values; often, this means that using == for strings is actually comparing the strings' references, not their values.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In the example below, two Java String objects are declared and initialized with the same string values. An if statement is used to determine if the strings are equivalent.

String str1 = new String("Hello");
String str2 = new String("Hello");
if (str1 == str2) {
  System.out.println("str1 == str2");
}

However, the if statement will not be executed as the strings are compared using the "==" operator. For Java objects, such as String objects, the "==" operator compares object references, not object values. While the two String objects above contain the same string values, they refer to different object references, so the System.out.println statement will not be executed. To compare object values, the previous code could be modified to use the equals method:

if (str1.equals(str2)) {
  System.out.println("str1 equals str2");
}

Example Two

In the following Java example, two BankAccount objects are compared in the isSameAccount method using the == operator.

public boolean isSameAccount(BankAccount accountA, BankAccount accountB) {
  return accountA == accountB;
}

Using the == operator to compare objects may produce incorrect or deceptive results by comparing object references rather than values. The equals() method should be used to ensure correct results or objects should contain a member variable that uniquely identifies the object.

The following example shows the use of the equals() method to compare the BankAccount objects and the next example uses a class get method to retrieve the bank account number that uniquely identifies the BankAccount object to compare the objects.

public boolean isSameAccount(BankAccount accountA, BankAccount accountB) {
  return accountA.equals(accountB);
}

See Also