
 Type Parameters:
T
 the type of objects that this object may be compared to
 All Known Subinterfaces:
ChronoLocalDate
,ChronoLocalDateTime<D>
,Chronology
,ChronoZonedDateTime<D>
,Delayed
,Name
,Path
,ProcessHandle
,RunnableScheduledFuture<V>
,ScheduledFuture<V>
 All Known Implementing Classes:
AbstractChronology
,AbstractRegionPainter.PaintContext.CacheMode
,AccessMode
,AclEntryFlag
,AclEntryPermission
,AclEntryType
,Authenticator.RequestorType
,BigDecimal
,BigInteger
,Boolean
,Byte
,ByteBuffer
,Calendar
,CatalogFeatures.Feature
,CertPathValidatorException.BasicReason
,Character
,Character.UnicodeScript
,CharBuffer
,Charset
,ChronoField
,ChronoUnit
,ClientInfoStatus
,CollationKey
,Collector.Characteristics
,Component.BaselineResizeBehavior
,CompositeName
,CompoundName
,CRLReason
,CryptoPrimitive
,Date
,Date
,DayOfWeek
,Desktop.Action
,Diagnostic.Kind
,Dialog.ModalExclusionType
,Dialog.ModalityType
,DirectMethodHandleDesc.Kind
,DocumentationTool.Location
,Double
,DoubleBuffer
,DrbgParameters.Capability
,DropMode
,Duration
,ElementKind
,Elements.Origin
,ElementType
,Enum
,File
,FileTime
,FileVisitOption
,FileVisitResult
,Float
,FloatBuffer
,FocusEvent.Cause
,FormatStyle
,Formatter.BigDecimalLayoutForm
,FormSubmitEvent.MethodType
,GraphicsDevice.WindowTranslucency
,GregorianCalendar
,GroupLayout.Alignment
,HijrahChronology
,HijrahDate
,HijrahEra
,HttpClient.Redirect
,HttpClient.Version
,Instant
,IntBuffer
,Integer
,IsoChronology
,IsoEra
,JapaneseChronology
,JapaneseDate
,JavaFileObject.Kind
,JDBCType
,JTable.PrintMode
,KeyRep.Type
,LayoutStyle.ComponentPlacement
,LdapName
,LinkOption
,LocalDate
,LocalDateTime
,Locale.Category
,Locale.FilteringMode
,Locale.IsoCountryCode
,LocalTime
,Long
,LongBuffer
,MappedByteBuffer
,MemoryType
,MinguoChronology
,MinguoDate
,MinguoEra
,Modifier
,ModuleDescriptor
,ModuleDescriptor.Exports
,ModuleDescriptor.Exports.Modifier
,ModuleDescriptor.Modifier
,ModuleDescriptor.Opens
,ModuleDescriptor.Opens.Modifier
,ModuleDescriptor.Provides
,ModuleDescriptor.Requires
,ModuleDescriptor.Requires.Modifier
,ModuleDescriptor.Version
,ModuleElement.DirectiveKind
,Month
,MonthDay
,MultipleGradientPaint.ColorSpaceType
,MultipleGradientPaint.CycleMethod
,NestingKind
,Normalizer.Form
,NumberFormat.Style
,NumericShaper.Range
,ObjectInputFilter.Status
,ObjectName
,ObjectStreamField
,OffsetDateTime
,OffsetTime
,PKIXReason
,PKIXRevocationChecker.Option
,PosixFilePermission
,ProcessBuilder.Redirect.Type
,Proxy.Type
,PseudoColumnUsage
,QuitStrategy
,Rdn
,ResolverStyle
,RetentionPolicy
,RoundingMode
,RowFilter.ComparisonType
,RowIdLifetime
,RowSorterEvent.Type
,Runtime.Version
,Short
,ShortBuffer
,SignStyle
,SortOrder
,SourceVersion
,SSLEngineResult.HandshakeStatus
,SSLEngineResult.Status
,StackWalker.Option
,StandardCopyOption
,StandardLocation
,StandardOpenOption
,StandardProtocolFamily
,String
,StringBuffer
,StringBuilder
,SwingWorker.StateValue
,System.Logger.Level
,Taskbar.Feature
,Taskbar.State
,TextStyle
,ThaiBuddhistChronology
,ThaiBuddhistDate
,ThaiBuddhistEra
,Thread.State
,Time
,Timestamp
,TimeUnit
,TrayIcon.MessageType
,TypeKind
,URI
,UserSessionEvent.Reason
,UUID
,VarHandle.AccessMode
,Window.Type
,XPathEvaluationResult.XPathResultType
,Year
,YearMonth
,ZonedDateTime
,ZoneOffset
,ZoneOffsetTransition
,ZoneOffsetTransitionRule.TimeDefinition
public interface Comparable<T>
This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class'scompareTo
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
(andArrays.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 ife1.compareTo(e2) == 0
has the same boolean value ase1.equals(e2)
for everye1
ande2
of classC
. Note thatnull
is not an instance of any class, ande.compareTo(null)
should throw aNullPointerException
even thoughe.equals(null)
returnsfalse
.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
andb
such that(!a.equals(b) && a.compareTo(b) == 0)
to a sorted set that does not use an explicit comparator, the secondadd
operation returns false (and the size of the sorted set does not increase) becausea
andb
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 isjava.math.BigDecimal
, whose natural ordering equatesBigDecimal
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:
The quotient for this total order is:{(x, y) such that x.compareTo(y) <= 0}.
It follows immediately from the contract for{(x, y) such that x.compareTo(y) == 0}.
compareTo
that the quotient is an equivalence relation onC
, and that the natural ordering is a total order onC
. 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'sequals(Object)
method:{(x, y) such that x.equals(y)}.
This interface is a member of the Java Collections Framework.
 Since:
 1.2
 See Also:
Comparator


Method Detail

compareTo
int compareTo(T o)
Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.The implementor must ensure
sgn(x.compareTo(y)) == sgn(y.compareTo(x))
for allx
andy
. (This implies thatx.compareTo(y)
must throw an exception iffy.compareTo(x)
throws an exception.)The implementor must also ensure that the relation is transitive:
(x.compareTo(y) > 0 && y.compareTo(z) > 0)
impliesx.compareTo(z) > 0
.Finally, the implementor must ensure that
x.compareTo(y)==0
implies thatsgn(x.compareTo(z)) == sgn(y.compareTo(z))
, for allz
.It is strongly recommended, but not strictly required that
(x.compareTo(y)==0) == (x.equals(y))
. Generally speaking, any class that implements theComparable
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 of1
,0
, or1
according to whether the value of expression is negative, zero, or positive, respectively. Parameters:
o
 the object to be compared. Returns:
 a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.
 Throws:
NullPointerException
 if the specified object is nullClassCastException
 if the specified object's type prevents it from being compared to this object.

