...
This noncompliant code example defines a CaseInsensitiveString class that includes a String and overrides the equals() method. The CaseInsensitiveString class knows about ordinary strings, but the String class has no knowledge of case-insensitive strings. Consequently, CaseInsensitiveString.equals() method should not attempt to interoperate with objects of the String class.
...
By operating on String objects, the CaseInsensitiveString.equals() method violates the second contract requirement (symmetry). Because of the asymmetry, given a String object s and a CaseInsensitiveString object cis that differ only in case, cis.equals(s)) returns true, while s.equals(cis) returns false.
...
| Wiki Markup |
|---|
Unfortunately, it is impossible to extend an instantiable class (as opposed to an {{abstract}} class) by adding a value or field in the subclass while preserving the {{equals()}} contract. Use composition rather than inheritance to achieve the desired effect \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. This compliant solution adopts this approach by adding a private {{card}} field to the {{XCard}} class and providing a {{public}} {{viewCard()}} method. |
| Code Block | ||
|---|---|---|
| ||
class XCard {
private String type;
private Card card; // Composition
public XCard(int number, String type) {
card = new Card(number);
this.type = type;
}
public Card viewCard() {
return card;
}
public boolean equals(Object o) {
if (!(o instanceof XCard)) {
return false;
}
XCard cp = (XCard)o;
return cp.card.equals(card) && cp.type.equals(type);
}
public static void main(String[] args) {
XCard p1 = new XCard(1, "type1");
Card p2 = new Card(1);
XCard p3 = new XCard(1, "type2");
XCard p4 = new XCard(1, "type1");
System.out.println(p1.equals(p2)); // Prints false
System.out.println(p2.equals(p3)); // Prints false
System.out.println(p1.equals(p3)); // Prints false
System.out.println(p1.equals(p4)); // Prints true
}
}
|
...
| Wiki Markup |
|---|
A Uniform Resource Locator (URL) specifies both the location of a resource and also a method to access it. According to the Java API documentation for Class URL \[[API 2006|AA. Bibliography#API 06]\],: |
Two URL objects are equal if they have the same protocol, reference equivalent hosts, have the same port number on the host, and the same file and fragment of the file.
Two hosts are considered equivalent if both host names can be resolved into the same IP addresses; else if either host name can't be resolved, the host names must be equal without regard to case; or both host names equal to null.
...
| Code Block | ||
|---|---|---|
| ||
public class Filter {
public static void main(String[] args) throws MalformedURLException {
final URL allowed = new URL("http://mailwebsite.com");
if (!allowed.equals(new URL(args[0]))) {
throw new SecurityException("Access Denied");
}
// Else proceed
}
}
|
Compliant Solution (
...
Strings)
This compliant solution compares two URLs' string representations, thereby avoiding the pitfalls of URL.equals().
...
| Wiki Markup |
|---|
A Uniform Resource Identifier (URI) contains a string of characters used to identify a resource; this is a more general concept than an URL. The {{java.net.URI}} class provides string-based {{equals()}} and {{hashCode()}} methods that satisfy the general contracts for {{Object.equals()}} and {{Object.hashCode()}}; they do not invoke hostname resolution and are unaffected by network connectivity. {{URI}} also provides methods for normalization and canonicalization that {{URL}} lacks. Finally, the {{URL.toURI()}} and {{URI.toURL()}} methods provide easy conversion between the two classes. It is recommended to use URIs instead of URLs whenever possible. According to the Java API \[[API 2006|AA. Bibliography#API 06]\] {{URI}} class documentation,: |
A
URImay be either absolute or relative. AURIstring is parsed according to the generic syntax without regard to the scheme, if any, that it specifies. No lookup of the host, if any, is performed, and no scheme-dependent stream handler is constructed.
...
The method java.lang.Object.equals() by default , is unable to compare composite objects such as cryptographic keys. Most Key classes lack an equals() implementation that overrides Object's default implementation. In such cases, the components of the composite object must be compared individually to ensure correctness.
...
| Wiki Markup |
|---|
This compliant solution uses the {{equals()}} method as a first test, and then compares the encoded version of the keys to facilitate provider-independent behavior. For example, this code can determine whether a {{RSAPrivateKey}} and {{RSAPrivateCrtKey}} represent equivalent private keys \[[Sun 2006|AA. Bibliography#Sun 06]\]. |
...
| Wiki Markup |
|---|
*MET08-EX0:* Requirements of this rule may be violated provided that the incompatible types are never compared. There are classes in the Java platform libraries (and elsewhere) that extend an instantiable class by adding a value component. For example, {{java.sql.Timestamp}} extends {{java.util.Date}} and adds a nanoseconds field. The {{equals}} implementation for {{Timestamp}} violates symmetry and can cause erratic behavior if {{Timestamp}} and {{Date}} objects are used in the same collection or are otherwise intermixed \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. |
...
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="600f4ca3d6e4c747-ce6e5769-41af4db7-9074b63e-ad4bc932ef05299b59c2ca09"><ac:plain-text-body><![CDATA[ | [[API 2006 | AA. Bibliography#API 06]] | [method equals() | http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#equals(java.lang.Object)] | ]]></ac:plain-text-body></ac:structured-macro> |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="3af2988d6ca26faa-7fa8d780-461948e3-beb0a5f8-14873910df1528e3f6dd2dbc"><ac:plain-text-body><![CDATA[ | [[Bloch 2008 | AA. Bibliography#Bloch 08]] | Item 8: Obey the general contract when overriding equals | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="afc0f56178680573-16f8895c-421b4462-8be58a99-6a705c21bb633eeb7f602fa2"><ac:plain-text-body><![CDATA[ | [[Darwin 2004 | AA. Bibliography#Darwin 04]] | 9.2 Overriding the equals method | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="c90918409d0a49d5-2aa871a1-4d6740e4-a40f8a83-0c62418b703aca24766c2ae4"><ac:plain-text-body><![CDATA[ | [[Harold 1997 | AA. Bibliography#Harold 97]] | Chapter 3: Classes, Strings, and Arrays, The Object Class (equality) | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="d4182e981bf85773-bbc0f85c-439d44da-9b84a429-92f84940502650c08944cade"><ac:plain-text-body><![CDATA[ | [[Sun 2006 | AA. Bibliography#Sun 06]] | [Determining If Two Keys Are Equal | http://java.sun.com/javase/6/docs/technotes/guides/security/crypto/CryptoSpec.html#Determining%20If%20Two%20Keys%20Are%20Equal] (JCA Reference Guide) | ]]></ac:plain-text-body></ac:structured-macro> |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="0da53930bee181d2-adf83966-4d984d47-86b691e8-25400e4196459c8895ce38db"><ac:plain-text-body><![CDATA[ | [[Techtalk 2007 | AA. Bibliography#Techtalk 07]] | "More Joy of Sets" | ]]></ac:plain-text-body></ac:structured-macro> |
...