The theory (for the language lawyers and the mathematically inclined):
equals()
(javadoc) must define an equivalence relation (it must be reflexive, symmetric, and transitive). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value). Furthermore, o.equals(null)
must always return false.
hashCode()
(javadoc) must also be consistent (if the object is not modified in terms of equals()
, it must keep returning the same value).
The relation between the two methods is:
Whenever a.equals(b)
, then a.hashCode()
must be same as b.hashCode()
.
In practice:
If you override one, then you should override the other.
Use the same set of fields that you use to compute equals()
to compute hashCode()
.
Use the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Lang library. An example:
public class Person {
private String name;
private int age;
// ...
@Override
public int hashCode() {
return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers
// if deriving: appendSuper(super.hashCode()).
append(name).
append(age).
toHashCode();
}
@Override
public boolean equals(Object obj) {
if (!(obj instanceof Person))
return false;
if (obj == this)
return true;
Person rhs = (Person) obj;
return new EqualsBuilder().
// if deriving: appendSuper(super.equals(obj)).
append(name, rhs.name).
append(age, rhs.age).
isEquals();
}
}
Also remember:
When using a hash-based Collection or Map such as HashSet, LinkedHashSet, HashMap, Hashtable, or WeakHashMap, make sure that the hashCode() of the key objects that you put into the collection never changes while the object is in the collection. The bulletproof way to ensure this is to make your keys immutable, which has also other benefits.
Coding from the hip, it would be something like:
java.lang.reflect.Method method;
try {
method = obj.getClass().getMethod(methodName, param1.class, param2.class, ..);
} catch (SecurityException e) { ... }
catch (NoSuchMethodException e) { ... }
The parameters identify the very specific method you need (if there are several overloaded available, if the method has no arguments, only give methodName
).
Then you invoke that method by calling
try {
method.invoke(obj, arg1, arg2,...);
} catch (IllegalArgumentException e) { ... }
catch (IllegalAccessException e) { ... }
catch (InvocationTargetException e) { ... }
Again, leave out the arguments in .invoke
, if you don't have any. But yeah. Read about Java Reflection
Best Answer
Yes,
Locale.ENGLISH
is a safe choice for case operations for things like programming language identifiers and URL parts since it doesn't involve any special casing rules and all 7-bit ASCII characters in the ENGLISH case-convert to 7-bit ASCII characters.That is not true for all other locales. In Turkish, the 'I' and 'i' characters are not case-converted to one another.
"Dotted and dotless I" explains:
The list of special exceptions is maintained at http://unicode.org/Public/UNIDATA/SpecialCasing.txt