# Ordering

### Related Docs: object Ordering | package math

#### trait Ordering[T] extends Comparator[T] with PartialOrdering[T] with Serializable

Ordering is a trait whose instances each represent a strategy for sorting instances of a type.

Ordering's companion object defines many implicit objects to deal with subtypes of AnyVal (e.g. Int, Double), String, and others.

To sort instances by one or more member variables, you can take advantage of these built-in orderings using Ordering.by and Ordering.on:

```import scala.util.Sorting
val pairs = Array(("a", 5, 2), ("c", 3, 1), ("b", 1, 3))

// sort by 2nd element
Sorting.quickSort(pairs)(Ordering.by[(String, Int, Int), Int](_._2))

// sort by the 3rd element, then 1st
Sorting.quickSort(pairs)(Ordering[(Int, String)].on(x => (x._3, x._1)))```

An Ordering[T] is implemented by specifying compare(a:T, b:T), which decides how to order two instances a and b. Instances of Ordering[T] can be used by things like scala.util.Sorting to sort collections like Array[T].

For example:

```import scala.util.Sorting

case class Person(name:String, age:Int)
val people = Array(Person("bob", 30), Person("ann", 32), Person("carl", 19))

// sort by age
object AgeOrdering extends Ordering[Person] {
def compare(a:Person, b:Person) = a.age compare b.age
}
Sorting.quickSort(people)(AgeOrdering)```

This trait and scala.math.Ordered both provide this same functionality, but in different ways. A type T can be given a single way to order itself by extending Ordered. Using Ordering, this same type may be sorted in many other ways. Ordered and Ordering both provide implicits allowing them to be used interchangeably.

Self Type
Ordering[T]
Annotations
( msg = ... )
Source
Ordering.scala
Version

0.9.5, 2008-04-15

Since

2.7

Linear Supertypes
PartialOrdering[T], Equiv[T], Serializable, java.io.Serializable, Comparator[T], AnyRef, Any
Ordering
1. Alphabetic
2. By inheritance
Inherited
1. Ordering
2. PartialOrdering
3. Equiv
4. Serializable
5. Serializable
6. Comparator
7. AnyRef
8. Any
Implicitly
2. by StringFormat
3. by Ensuring
4. by ArrowAssoc
1. Hide All
2. Show all
Visibility
1. Public
2. All

### Type Members

1. #### class Ops extends AnyRef

This inner class defines comparison operators available for `T`.

### Abstract Value Members

1. #### abstract def compare(x: T, y: T): Int

Returns an integer whose sign communicates how x compares to y.

Returns an integer whose sign communicates how x compares to y.

The result sign has the following meaning:

• negative if x < y
• positive if x > y
• zero otherwise (if x == y)
Definition Classes
Ordering → Comparator

### Concrete Value Members

1. #### final def !=(arg0: Any): Boolean

Test two objects for inequality.

Test two objects for inequality.

returns

`true` if !(this == that), false otherwise.

Definition Classes
AnyRef → Any
2. #### final def ##(): Int

Equivalent to `x.hashCode` except for boxed numeric types and `null`.

Equivalent to `x.hashCode` except for boxed numeric types and `null`. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For `null` returns a hashcode where `null.hashCode` throws a `NullPointerException`.

returns

a hash value consistent with ==

Definition Classes
AnyRef → Any
3. #### def +(other: String): String

Implicit information
This member is added by an implicit conversion from Ordering[T] to any2stringadd[Ordering[T]] performed by method any2stringadd in scala.Predef.
Definition Classes
4. #### def ->[B](y: B): (Ordering[T], B)

Implicit information
This member is added by an implicit conversion from Ordering[T] to ArrowAssoc[Ordering[T]] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc
Annotations
()
5. #### final def ==(arg0: Any): Boolean

The expression `x == that` is equivalent to `if (x eq null) that eq null else x.equals(that)`.

The expression `x == that` is equivalent to `if (x eq null) that eq null else x.equals(that)`.

returns

`true` if the receiver object is equivalent to the argument; `false` otherwise.

Definition Classes
AnyRef → Any
6. #### final def asInstanceOf[T0]: T0

Cast the receiver object to be of type `T0`.

Cast the receiver object to be of type `T0`.

Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression `1.asInstanceOf[String]` will throw a `ClassCastException` at runtime, while the expression `List(1).asInstanceOf[List[String]]` will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

returns

Definition Classes
Any
Exceptions thrown

`ClassCastException` if the receiver object is not an instance of the erasure of type `T0`.

7. #### def clone(): AnyRef

Create a copy of the receiver object.

Create a copy of the receiver object.

The default implementation of the `clone` method is platform dependent.

returns

a copy of the receiver object.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
( ... )
Note

not specified by SLS as a member of AnyRef

8. #### def ensuring(cond: (Ordering[T]) ⇒ Boolean, msg: ⇒ Any): Ordering[T]

Implicit information
This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
9. #### def ensuring(cond: (Ordering[T]) ⇒ Boolean): Ordering[T]

Implicit information
This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
10. #### def ensuring(cond: Boolean, msg: ⇒ Any): Ordering[T]

Implicit information
This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
11. #### def ensuring(cond: Boolean): Ordering[T]

Implicit information
This member is added by an implicit conversion from Ordering[T] to Ensuring[Ordering[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
12. #### final def eq(arg0: AnyRef): Boolean

Tests whether the argument (`that`) is a reference to the receiver object (`this`).

Tests whether the argument (`that`) is a reference to the receiver object (`this`).

The `eq` method implements an equivalence relation on non-null instances of `AnyRef`, and has three additional properties:

• It is consistent: for any non-null instances `x` and `y` of type `AnyRef`, multiple invocations of `x.eq(y)` consistently returns `true` or consistently returns `false`.
• For any non-null instance `x` of type `AnyRef`, `x.eq(null)` and `null.eq(x)` returns `false`.
• `null.eq(null)` returns `true`.

When overriding the `equals` or `hashCode` methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (`o1 eq o2`), they should be equal to each other (`o1 == o2`) and they should hash to the same value (`o1.hashCode == o2.hashCode`).

returns

`true` if the argument is a reference to the receiver object; `false` otherwise.

Definition Classes
AnyRef
13. #### def equals(arg0: Any): Boolean

The equality method for reference types.

The equality method for reference types. Default implementation delegates to `eq`.

See also `equals` in scala.Any.

returns

`true` if the receiver object is equivalent to the argument; `false` otherwise.

Definition Classes
AnyRef → Any
14. #### def equiv(x: T, y: T): Boolean

Return true if `x` == `y` in the ordering.

Return true if `x` == `y` in the ordering.

Definition Classes
OrderingPartialOrderingEquiv
15. #### def finalize(): Unit

Called by the garbage collector on the receiver object when there are no more references to the object.

Called by the garbage collector on the receiver object when there are no more references to the object.

The details of when and if the `finalize` method is invoked, as well as the interaction between `finalize` and non-local returns and exceptions, are all platform dependent.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
( classOf[java.lang.Throwable] )
Note

not specified by SLS as a member of AnyRef

16. #### def formatted(fmtstr: String): String

Returns string formatted according to given `format` string.

Returns string formatted according to given `format` string. Format strings are as for `String.format` (@see java.lang.String.format).

Implicit information
This member is added by an implicit conversion from Ordering[T] to StringFormat[Ordering[T]] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
()
17. #### final def getClass(): Class[_]

Returns the runtime class representation of the object.

Returns the runtime class representation of the object.

returns

a class object corresponding to the runtime type of the receiver.

Definition Classes
AnyRef → Any
18. #### def gt(x: T, y: T): Boolean

Return true if `x` > `y` in the ordering.

Return true if `x` > `y` in the ordering.

Definition Classes
OrderingPartialOrdering
19. #### def gteq(x: T, y: T): Boolean

Return true if `x` >= `y` in the ordering.

Return true if `x` >= `y` in the ordering.

Definition Classes
OrderingPartialOrdering
20. #### def hashCode(): Int

The hashCode method for reference types.

The hashCode method for reference types. See hashCode in scala.Any.

returns

the hash code value for this object.

Definition Classes
AnyRef → Any
21. #### final def isInstanceOf[T0]: Boolean

Test whether the dynamic type of the receiver object is `T0`.

Test whether the dynamic type of the receiver object is `T0`.

Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression `1.isInstanceOf[String]` will return `false`, while the expression `List(1).isInstanceOf[List[String]]` will return `true`. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

returns

`true` if the receiver object is an instance of erasure of type `T0`; `false` otherwise.

Definition Classes
Any
22. #### def lt(x: T, y: T): Boolean

Return true if `x` < `y` in the ordering.

Return true if `x` < `y` in the ordering.

Definition Classes
OrderingPartialOrdering
23. #### def lteq(x: T, y: T): Boolean

Return true if `x` <= `y` in the ordering.

Return true if `x` <= `y` in the ordering.

Definition Classes
OrderingPartialOrdering
24. #### def max(x: T, y: T): T

Return `x` if `x` >= `y`, otherwise `y`.

25. #### def min(x: T, y: T): T

Return `x` if `x` <= `y`, otherwise `y`.

26. #### implicit def mkOrderingOps(lhs: T): Ops

This implicit method augments `T` with the comparison operators defined in `scala.math.Ordering.Ops`.

27. #### final def ne(arg0: AnyRef): Boolean

Equivalent to `!(this eq that)`.

Equivalent to `!(this eq that)`.

returns

`true` if the argument is not a reference to the receiver object; `false` otherwise.

Definition Classes
AnyRef
28. #### final def notify(): Unit

Wakes up a single thread that is waiting on the receiver object's monitor.

Wakes up a single thread that is waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note

not specified by SLS as a member of AnyRef

29. #### final def notifyAll(): Unit

Wakes up all threads that are waiting on the receiver object's monitor.

Wakes up all threads that are waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note

not specified by SLS as a member of AnyRef

30. #### def on[U](f: (U) ⇒ T): Ordering[U]

Given f, a function from U into T, creates an Ordering[U] whose compare function is equivalent to:

Given f, a function from U into T, creates an Ordering[U] whose compare function is equivalent to:

`def compare(x:U, y:U) = Ordering[T].compare(f(x), f(y))`
31. #### def reverse: Ordering[T]

Return the opposite ordering of this one.

Return the opposite ordering of this one.

Definition Classes
OrderingPartialOrdering
32. #### def reversed(): Comparator[T]

Definition Classes
Comparator
33. #### final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
34. #### def thenComparing[U <: Comparable[_ >: U]](arg0: java.util.function.Function[_ >: T, _ <: U]): Comparator[T]

Definition Classes
Comparator
35. #### def thenComparing[U](arg0: java.util.function.Function[_ >: T, _ <: U], arg1: Comparator[_ >: U]): Comparator[T]

Definition Classes
Comparator
36. #### def thenComparing(arg0: Comparator[_ >: T]): Comparator[T]

Definition Classes
Comparator
37. #### def thenComparingDouble(arg0: ToDoubleFunction[_ >: T]): Comparator[T]

Definition Classes
Comparator
38. #### def thenComparingInt(arg0: ToIntFunction[_ >: T]): Comparator[T]

Definition Classes
Comparator
39. #### def thenComparingLong(arg0: ToLongFunction[_ >: T]): Comparator[T]

Definition Classes
Comparator
40. #### def toString(): String

Creates a String representation of this object.

Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

returns

a String representation of the object.

Definition Classes
AnyRef → Any
41. #### def tryCompare(x: T, y: T): Some[Int]

Returns whether a comparison between `x` and `y` is defined, and if so the result of `compare(x, y)`.

Returns whether a comparison between `x` and `y` is defined, and if so the result of `compare(x, y)`.

Definition Classes
OrderingPartialOrdering
42. #### final def wait(): Unit

Definition Classes
AnyRef
Annotations
( ... )
43. #### final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
( ... )
44. #### final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
( ... )
45. #### def →[B](y: B): (Ordering[T], B)

Implicit information
This member is added by an implicit conversion from Ordering[T] to ArrowAssoc[Ordering[T]] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc