Java Notes

==, .equals(), and compareTo()

Equality comparison: One way for primitives, Four ways for objects

Comparison	Primitives	Objects
a == b, a != b	Equal values	Refer to the same object.
a.equals(b)	N/A	Compares values, if it's defined for this class, as it is for most Java core classes. If it's not defined for a (user) class, it behaves the same as ==.
a.compareTo(b)	N/A	Compares values. Class must implement the Comparable<T> interface. All Java classes that have a natural ordering implement this (String, Double, BigInteger, ...), but BigDecimal may not do what you expect. Comparable objects can be used by the Collections sort() method and data structures that implicitly sort (eg, TreeSet, TreeMap).
compareTo(a, b)	N/A	Compares values. Available only if the Comparator<T> interface has been implemented, which is not the case for the Java classes. Typically used to define a comparator object that can be passed to Collections sort() method or ordered data structures.

Comparing Object references with the == and != Operators

The two operators that can be used with object references are comparing for equality (==) and inequality (!=). These operators compare two values to see if they refer to the same object. Although this comparison is very fast, it is often not what you want.

Usually you want to know if the objects have the same value, and not whether two objects are a reference to the same object. For example,

if (name == "Mickey Mouse")   // ALMOST SURELY WRONG

This be true if name is a reference to the same object that "Mickey Mouse" refers to. This will probably be false if the String in name was read from input or computed (by putting strings together or taking the substring), even though name really does have exactly those characters in it.

Many classes (eg, String) define the equals() method to compare the values of objects.

Comparing Object values with the equals() Method

Use the equals() method to compare object values. The equals() method returns a boolean value. The previous example can be fixed by writing:

if (name.equals("Mickey Mouse"))  // Compares values, not referernces.

Other comparisons - Comparable<T> interface

The equals method and == and != operators test for equality/inequality, but do not provide a way to test for the relative values. Some classes (eg, String and other classes with a natural ordering) implement the Comparable<T> interface, which defines a compareTo method. You will want to implement Comparable<T> in your class if you want to use it with Collections.sort() or Arrays.sort() methods. The String class also provides case insensitive comparisons.

The === operator (Doesn't exist - yet?)

Comparing objects is somewhat awkward, so a === operator has been proposed.
a === b would be the same as ((a == b) || ((a != null) && a.equals(b)))

Common Errors

Using == instead of equals() with Objects

When you want to compare objects, you need to know whether you should use == to see if they are the same object, or equals() to see if they may be a different object, but have the same value. This kind of error can be very hard to find.