T
- the type of objects that this object may be compared toArrayType
, ByteValue
, CharValue
, ChronoLocalDate
, ChronoLocalDateTime<D>
, Chronology
, ChronoZonedDateTime<D>
, ClassType
, Delayed
, DoubleValue
, Field
, FloatValue
, IntegerValue
, InterfaceType
, LocalVariable
, Location
, LongValue
, Method
, Name
, Path
, ProcessHandle
, ReferenceType
, RunnableScheduledFuture<V>
, ScheduledFuture<V>
, ShortValue
AbstractChronology
, AbstractRegionPainter.PaintContext.CacheMode
, AccessFlag
, AccessFlag.Location
, AccessMode
, AclEntryFlag
, AclEntryPermission
, AclEntryType
, AssociationChangeNotification.AssocChangeEvent
, AttributeTree.ValueKind
, Authenticator.RequestorType
, BigDecimal
, BigInteger
, Boolean
, Byte
, ByteBuffer
, Calendar
, CardTerminals.State
, CaseTree.CaseKind
, CatalogFeatures.Feature
, CertPathValidatorException.BasicReason
, Character
, Character.UnicodeScript
, CharBuffer
, Charset
, ChronoField
, ChronoUnit
, ClassFileFormatVersion
, ClientInfoStatus
, CollationKey
, Collector.Characteristics
, Component.BaselineResizeBehavior
, CompositeName
, CompoundName
, ConversionComparator.Comparison
, CRLReason
, CryptoPrimitive
, Date
, Date
, DayOfWeek
, Desktop.Action
, Diagnostic.Kind
, Dialog.ModalExclusionType
, Dialog.ModalityType
, DirectMethodHandleDesc.Kind
, Doclet.Option.Kind
, DocletEnvironment.ModuleMode
, DocTree.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
, Future.State
, GraphicsDevice.WindowTranslucency
, GregorianCalendar
, GroupLayout.Alignment
, HandlerResult
, HijrahChronology
, HijrahDate
, HijrahEra
, HotSpotDiagnosticMXBean.ThreadDumpFormat
, HttpClient.Redirect
, HttpClient.Version
, InquireType
, Instant
, IntBuffer
, Integer
, IsoChronology
, IsoEra
, JapaneseChronology
, JapaneseDate
, JavaFileObject.Kind
, JConsoleContext.ConnectionState
, JDBCType
, JTable.PrintMode
, KeyRep.Type
, LambdaExpressionTree.BodyKind
, LayoutStyle.ComponentPlacement
, LdapName
, LinkOption
, LocalDate
, LocalDateTime
, Locale.Category
, Locale.FilteringMode
, Locale.IsoCountryCode
, LocalTime
, Long
, LongBuffer
, MappedByteBuffer
, MemberReferenceTree.ReferenceMode
, MemoryType
, MethodHandles.Lookup.ClassOption
, 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
, ModuleTree.ModuleKind
, Month
, MonthDay
, MultipleGradientPaint.ColorSpaceType
, MultipleGradientPaint.CycleMethod
, NestingKind
, Normalizer.Form
, NumberFormat.Style
, NumericShaper.Range
, ObjectInputFilter.Status
, ObjectName
, ObjectStreamField
, OffsetDateTime
, OffsetTime
, PeerAddressChangeNotification.AddressChangeEvent
, PKIXReason
, PKIXRevocationChecker.Option
, PosixFilePermission
, ProcessBuilder.Redirect.Type
, Proxy.Type
, PseudoColumnUsage
, QuitStrategy
, Rdn
, RecordingState
, ResolverStyle
, RetentionPolicy
, RoundingMode
, RowFilter.ComparisonType
, RowIdLifetime
, RowSorterEvent.Type
, Runtime.Version
, Short
, ShortBuffer
, SignStyle
, SimpleFileServer.OutputLevel
, Snippet.Kind
, Snippet.Status
, Snippet.SubKind
, SortOrder
, SourceCodeAnalysis.Attribute
, SourceCodeAnalysis.Completeness
, SourceVersion
, SSLEngineResult.HandshakeStatus
, SSLEngineResult.Status
, StackWalker.Option
, StandardCopyOption
, StandardLocation
, StandardNamespace
, StandardOpenOption
, StandardOperation
, StandardProtocolFamily
, String
, StringBuffer
, StringBuilder
, StructuredTaskScope.Subtask.StatePREVIEW
, SwingWorker.StateValue
, System.Logger.Level
, Taglet.Location
, Taskbar.Feature
, Taskbar.State
, TaskEvent.Kind
, TextStyle
, ThaiBuddhistChronology
, ThaiBuddhistDate
, ThaiBuddhistEra
, Thread.State
, Time
, Timestamp
, TimeUnit
, TrayIcon.MessageType
, Tree.Kind
, TypeKind
, URI
, UserSessionEvent.Reason
, UUID
, VarHandle.AccessMode
, VectorShape
, VMOption.Origin
, Window.Type
, XPathEvaluationResult.XPathResultType
, Year
, YearMonth
, ZonedDateTime
, ZoneOffset
, ZoneOffsetTransition
, ZoneOffsetTransitionRule.TimeDefinition
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 BigDecimal
, whose natural ordering equates
BigDecimal
objects with equal numerical values and different representations (such as 4.0 and 4.00). For BigDecimal.equals()
to return true, the representation and numerical value of the two
BigDecimal
objects must be the same.
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)}.
In other words, when a class's natural ordering is consistent with equals, the equivalence classes defined by the equivalence relation of the equals
method and the equivalence classes defined by the quotient of the compareTo
method are the same.
This interface is a member of the Java Collections Framework.
int compareTo(T o)
The implementor must ensure signum
(x.compareTo(y)) == -signum(y.compareTo(x))
for all x
and y
. (This implies that
x.compareTo(y)
must throw an exception if and only if
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 signum(x.compareTo(z))
== signum(y.compareTo(z))
, for all z
.
(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."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.
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Comparable.html