EXP02-J. Do not use the Object.equals() method to compare two arrays

In Java, arrays are objects and support object methods such as Object.equals(). However, arrays do not support any methods besides those provided by Object. Consequently, using Object.equals() on any array only compares array references rather than their contents. Programmers who wish to compare the contents of two arrays must use the static two-argument Arrays.equals() method.  This method considers two arrays equivalent if both arrays contain the same number of elements, and all corresponding pairs of elements in the two arrays are equivalent, according to Object.equals().  In other words, two arrays are equal if they contain equivalent elements in the same order. To test for reference equality, programmers use the reference equality operators, == and !=.  To test for content equality, use Object.equals().

Because the effect of using Object.equals() to compare two arrays is often misconstrued as content equality, and because a better alternative exists in the use of reference equality operators, the use of the Object.equals() method to compare two arrays is disallowed.

Noncompliant Code Example

This noncompliant code example uses the Object.equals() method to compare two arrays.

int[] arr1 = new int[20]; // initialized to 0
int[] arr2 = new int[20]; // initialized to 0
System.out.println(arr1.equals(arr2)); // prints false

Compliant Solution

This compliant solution compares the content of two arrays using the two-argument Arrays.equals() method.

int[] arr1 = new int[20]; // initialized to 0
int[] arr2 = new int[20]; // initialized to 0
System.out.println(Arrays.equals(arr1, arr2)); // prints true

Compliant Solution

This compliant solution compares the array references using the reference equality operators ==.

int[] arr1 = new int[20]; // initialized to 0
int[] arr2 = new int[20]; // initialized to 0
System.out.println(arr1 == arr2); // prints false 

Risk Assessment

Using the equals() method or relational operators with the intention of comparing array contents produces incorrect results, which can lead to vulnerabilities.

Automated Detection

Static detection of calls to to Object.equals() is straightforward. However, it is not always possible to statically resolve the class of a method invocation's target. Consequently, it may not always be possible to determine when Object.equals() is invoked for an array type.

Related Guidelines

Bibliography

EXP01-J. Never dereference null pointers      02. Expressions (EXP)