Which of these methods of class String is used to compare two String objects for their equality

Weakness ID: 597

The product uses the wrong operator when comparing a string, such as using "==" when the .equals() method should be used instead.

In Java, using == or != to compare two strings for equality actually compares two objects for equality rather than their string values for equality. Chances are good that the two references will never be equal. While this weakness often only affects program correctness, if the equality is used for a security decision, the unintended comparison result could be leveraged to affect program security.

Nature	Type	ID	Name
ChildOf	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	480	Use of Incorrect Operator
ChildOf	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	595	Comparison of Object References Instead of Object Contents

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	133	String Errors

Nature	Type	ID	Name
ChildOf	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	595	Comparison of Object References Instead of Object Contents

Example 1

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.

(bad code)

Example Language: Java 

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 2

In the example below, three JavaScript variables are declared and initialized with the same values. Note that JavaScript will change a value between numeric and string as needed, which is the reason an integer is included with the strings. An if statement is used to determine whether the values are the same.

(bad code)

Example Language: JavaScript 

<p id="ieq3s1" type="text">(i === s1) is FALSE</p>
<p id="s4eq3i" type="text">(s4 === i) is FALSE</p>
<p id="s4eq3s1" type="text">(s4 === s1) is FALSE</p>

var i = 65;
var s1 = '65';
var s4 = new String('65');

if (i === s1)
{

document.getElementById("ieq3s1").innerHTML = "(i === s1) is TRUE";

}

if (s4 === i)
{

document.getElementById("s4eq3i").innerHTML = "(s4 === i) is TRUE";

}

if (s4 === s1)
{

document.getElementById("s4eq3s1").innerHTML = "(s4 === s1) is TRUE";

}

However, the body of the if statement will not be executed, as the "===" compares both the type of the variable AND the value. As the types of the first comparison are number and string, it fails. The types in the second are int and reference, so this one fails as well. The types in the third are reference and string, so it also fails.

While the variables above contain the same values, they are contained in different types, so the document.getElementById... statement will not be executed in any of the cases.

To compare object values, the previous code is modified and shown below to use the "==" for value comparison so the comparison in this example executes the HTML statement:

(good code)

Example Language: JavaScript 

<p id="ieq2s1" type="text">(i == s1) is FALSE</p>
<p id="s4eq2i" type="text">(s4 == i) is FALSE</p>
<p id="s4eq2s1" type="text">(s4 == s1) is FALSE</p>

var i = 65;
var s1 = '65';
var s4 = new String('65');

if (i == s1)
{

document.getElementById("ieq2s1").innerHTML = "(i == s1) is TRUE";

}

if (s4 == i)
{

document.getElementById("s4eq2i").innerHTML = "(s4 == i) is TRUE";

}

if (s4 == s1)
{

document.getElementById("s4eq2s1").innerHTML = "(s4 == s1) is TRUE";

}

Example 3

In the example below, two PHP variables are declared and initialized with the same numbers - one as a string, the other as an integer. Note that PHP will change the string value to a number for a comparison. An if statement is used to determine whether the values are the same.

(bad code)

Example Language: PHP 

var $i = 65;
var $s1 = "65";

if ($i === $s1)
{

echo '($i === $s1) is TRUE'. "\n";

}
else
{

echo '($i === $s1) is FALSE'. "\n";

}

However, the body of the if statement will not be executed, as the "===" compares both the type of the variable AND the value. As the types of the first comparison are number and string, it fails.

While the variables above contain the same values, they are contained in different types, so the TRUE portion of the if statement will not be executed.

To compare object values, the previous code is modified and shown below to use the "==" for value comparison (string converted to number) so the comparison in this example executes the TRUE statement:

(good code)

Example Language: PHP 

var $i = 65;
var $s1 = "65";

if ($i == $s1)
{

echo '($i == $s1) is TRUE'. "\n";

}
else
{

echo '($i == $s1) is FALSE'. "\n";

}

Phase: Implementation

Within Java, use .equals() to compare string values.

Within JavaScript, use == to compare string values.

Within PHP, use == to compare a numeric value to a string value. (PHP converts the string to a number.)

Effectiveness: High

[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Typos", Page 289. 1st Edition. Addison Wesley. 2006.

Submissions
Submission Date	Submitter	Organization
2006-12-15	CWE Content Team	MITRE
Modifications
Modification Date	Modifier	Organization
2008-07-01	Eric Dalci	Cigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08	CWE Content Team	MITRE
updated Description, Relationships
2008-10-14	CWE Content Team	MITRE
updated Relationships
2009-05-27	CWE Content Team	MITRE
updated Demonstrative_Examples
2011-03-29	CWE Content Team	MITRE
updated Demonstrative_Examples, Description, Potential_Mitigations
2011-06-01	CWE Content Team	MITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2012-05-11	CWE Content Team	MITRE
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings
2014-07-30	CWE Content Team	MITRE
updated Relationships, Taxonomy_Mappings
2017-11-08	CWE Content Team	MITRE
updated Taxonomy_Mappings
2019-01-03	CWE Content Team	MITRE
updated Relationships, Taxonomy_Mappings
2020-02-24	CWE Content Team	MITRE
updated Relationships
2020-08-20	CWE Content Team	MITRE
updated Relationships
2021-03-15	CWE Content Team	MITRE
updated Demonstrative_Examples, Description, Potential_Mitigations, Relationships
Previous Entry Names
Change Date	Previous Entry Name
2008-04-11	Erroneous String Compare

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Which of these method of class string is used to compare two strings?

Which method is used to compare two strings equality?

Which of the following methods compare two strings?