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

Arrays do not override the Object.equals() method; the implementation of the equals() method compares array references rather than their contents. Programmers who wish to compare the contents of two arrays must use the two-argument Arrays.equals() method.  Two arrays are considered equal if both arrays contain the same number of elements, and all corresponding pairs of elements in the two arrays are equal.  In other words, two arrays are equal if they contain the same elements in the same order.  To test for reference equality, programmers can use the reference equality operators, == and !=.  Because the effect of using Object.equals() to compare two arrays is often misunderstood, 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 by this rule.

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)