T
 the type of objects that this object may be compared toProcessHandle
Boolean
, Byte
, Character
, Character.UnicodeScript
, DirectMethodHandleDesc.Kind
, Double
, Enum
, Float
, Integer
, Long
, ProcessBuilder.Redirect.Type
, Runtime.Version
, Short
, StackWalker.Option
, String
, StringBuffer
, StringBuilder
, System.Logger.Level
, Thread.State
, VarHandle.AccessMode
public interface Comparable<T>
compareTo
method is referred to as
its natural comparison method.
Lists (and arrays) of objects that implement this interface can be sorted
automatically by Collections.sort
(and
Arrays.sort
). Objects that implement this
interface can be used as keys in a sorted map or as
elements in a sorted set, without the need to
specify a comparator.
The natural ordering for a class C
is said to be consistent
with equals if and only if e1.compareTo(e2) == 0
has
the same boolean value as e1.equals(e2)
for every
e1
and e2
of class C
. Note that null
is not an instance of any class, and e.compareTo(null)
should
throw a NullPointerException
even though e.equals(null)
returns false
.
It is strongly recommended (though not required) that natural orderings be
consistent with equals. This is so because sorted sets (and sorted maps)
without explicit comparators behave "strangely" when they are used with
elements (or keys) whose natural ordering is inconsistent with equals. In
particular, such a sorted set (or sorted map) violates the general contract
for set (or map), which is defined in terms of the equals
method.
For example, if one adds two keys a
and b
such that
(!a.equals(b) && a.compareTo(b) == 0)
to a sorted
set that does not use an explicit comparator, the second add
operation returns false (and the size of the sorted set does not increase)
because a
and b
are equivalent from the sorted set's
perspective.
Virtually all Java core classes that implement Comparable
have natural
orderings that are consistent with equals. One exception is
java.math.BigDecimal
, whose natural ordering equates
BigDecimal
objects with equal values and different precisions
(such as 4.0 and 4.00).
For the mathematically inclined, the relation that defines the natural ordering on a given class C is:
{(x, y) such that x.compareTo(y) <= 0}.
The quotient for this total order is:
{(x, y) such that x.compareTo(y) == 0}.
It follows immediately from the contract for compareTo
that the
quotient is an equivalence relation on C
, and that the
natural ordering is a total order on C
. When we say that a
class's natural ordering is consistent with equals, we mean that the
quotient for the natural ordering is the equivalence relation defined by
the class's equals(Object)
method:{(x, y) such that x.equals(y)}.
This interface is a member of the Java Collections Framework.
Comparator
Modifier and Type  Method  Description 

int 
compareTo(T o) 
Compares this object with the specified object for order.

int compareTo(T o)
The implementor must ensure
sgn(x.compareTo(y)) == sgn(y.compareTo(x))
for all x
and y
. (This
implies that x.compareTo(y)
must throw an exception iff
y.compareTo(x)
throws an exception.)
The implementor must also ensure that the relation is transitive:
(x.compareTo(y) > 0 && y.compareTo(z) > 0)
implies
x.compareTo(z) > 0
.
Finally, the implementor must ensure that x.compareTo(y)==0
implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z))
, for
all z
.
It is strongly recommended, but not strictly required that
(x.compareTo(y)==0) == (x.equals(y))
. Generally speaking, any
class that implements the Comparable
interface and violates
this condition should clearly indicate this fact. The recommended
language is "Note: this class has a natural ordering that is
inconsistent with equals."
In the foregoing description, the notation
sgn(
expression)
designates the mathematical
signum function, which is defined to return one of 1
,
0
, or 1
according to whether the value of
expression is negative, zero, or positive, respectively.
o
 the object to be compared.NullPointerException
 if the specified object is nullClassCastException
 if the specified object's type prevents it
from being compared to this object.