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 compares only array references, not 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, use the reference equality operators, == and !=.  

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 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.

Tool	Version	Checker	Description
CodeSonar		JAVA.COMPARE.EQ JAVA.COMPARE.EQARRAY	Should Use equals() Instead of == (Java) equals on Array (Java)
Coverity	7.5	BAD_EQ FB.EQ_ABSTRACT_SELF FB.EQ_ALWAYS_FALSE FB.EQ_ALWAYS_TRUE FB.EQ_CHECK_FOR_OPERAND_NOT_ COMPATIBLE_WITH_THIS FB.EQ_COMPARETO_USE_OBJECT_ EQUALS FB.EQ_COMPARING_CLASS_NAMES FB.EQ_DOESNT_OVERRIDE_EQUALS FB.EQ_DONT_DEFINE_EQUALS_ FOR_ENUM FB.EQ_GETCLASS_AND_CLASS_ CONSTANT FB.EQ_OTHER_NO_OBJECT FB.EQ_OTHER_USE_OBJECT FB.EQ_OVERRIDING_EQUALS_ NOT_SYMMETRIC FB.EQ_SELF_NO_OBJECT FB.EQ_SELF_USE_OBJECT FB.EQ_UNUSUAL	Implemented
Parasoft Jtest		CERT.EXP02.UEIC	Do not use '==' or '!=' to compare objects
SonarQube		S2159	Silly equality checks should not be made

Related Guidelines

Bibliography

[API 2006]	Class Arrays
[Seacord 2015]	EXP02-J. Do not use the Object.equals () method to compare two arrays LiveLesson