Source & Full Article
When you really need to know if two references are identical, use ==. But when you need to know if the objects themselves (not the references) are equal, use the equals() method.
If you don't override a class's equals() method, you won't be able to use those objects as a key in a hashtable and you probably won't get accurate Sets, such that there are no conceptual duplicates.
Equality comparison: One way for primitives, Four ways for objects
When you really need to know if two references are identical, use ==. But when you need to know if the objects themselves (not the references) are equal, use the equals() method.
If you don't override a class's equals() method, you won't be able to use those objects as a key in a hashtable and you probably won't get accurate Sets, such that there are no conceptual duplicates.
Equality comparison: One way for primitives, Four ways for objects
| Comparison | Objects |
|---|---|
a == b, a != b | Compares references, not values. The use of == with object references is generally limited to the following:
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a.equals(b) | Compares values for equality. Because this method is defined in the Object class, from which all other classes are derived, it's automatically defined for every class. However, it doesn't perform an intelligent comparison for most classes unless the class overrides it. It has been defined in a meaningful way for most Java core classes. If it's not defined for a (user) class, it behaves the same as ==. It turns out that defining equals() isn't trivial; in fact it's moderately hard to get it right, especially in the case of subclasses. The best treatment of the issues is in Horstmann's Core Java Vol 1. [TODO: Add explanation and example] |
a.compareTo(b) | Comparable interface. Compares values and returns an int which tells if the values compare less than, equal, or greater than. If your class objects have a natural order, implement the Comparable<T> interface and define this method. All Java classes that have a natural ordering implement this (String, Double, BigInteger, ...). |
compare(a, b) | Comparator interface. Compares values of two objects. This is implemented as part of the Comparator<T> sort() or for use by sorting data structures such as TreeMap and TreeSet. You might want to create a Comparator object for the following. interface, and the typical use is to define one or more small utility classes that implement this, to pass to methods such as
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