An instance of A <:< B witnesses that A is a subtype of B. Requiring an implicit argument of the type A <:< B encodes the generalized constraint A <: B.
we need a new type constructor <:< and evidence conforms, as reusing Function1 and identity leads to ambiguities in case of type errors (any2stringadd is inferred) To constrain any abstract type T that's in scope in a method's argument list (not just the method's own type parameters) simply add an implicit argument of type T <:< U, where U is the required upper bound; or for lower-bounds, use: L <:< T, where L is the required lower bound. In part contributed by Jason Zaugg.
An instance of A =:= B witnesses that the types A and B are equal.
<:< for expressing subtyping constraints
A type for which there is always an implicit value.
scala.Array$, method fallbackCanBuildFrom
(Since version 2.10.0) use scala.reflect.ClassTag instead
(Since version 2.11.0) use built-in tuple syntax or Tuple2 instead
(Since version 2.11.0) use built-in tuple syntax or Tuple3 instead
Test two objects for inequality.
true if !(this == that), false otherwise.
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.
a hash value consistent with ==
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
true if the receiver object is equivalent to the argument; false otherwise.
??? can be used for marking methods that remain to be implemented.
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.
the receiver object.
ClassCastException if the receiver object is not an instance of the erasure of type T0.
Tests an expression, throwing an AssertionError if false. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.
the expression to test
a String to include in the failure message
Tests an expression, throwing an AssertionError if false. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.
the expression to test
Tests an expression, throwing an AssertionError if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.
the expression to test
a String to include in the failure message
Tests an expression, throwing an AssertionError if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.
the expression to test
We prefer the java.lang.* boxed types to these wrappers in any potential conflicts. Conflicts do exist because the wrappers need to implement ScalaNumber in order to have a symmetric equals method, but that implies implementing java.lang.Number as well.
Note - these are inlined because they are value classes, but the call to xxxWrapper is not eliminated even though it does nothing. Even inlined, every call site does a no-op retrieval of Predef's MODULE$ because maybe loading Predef has side effects!
Retrieve the runtime representation of a class type. classOf[T] is equivalent to the class literal T.class in Java.
val listClass = classOf[List[_]] // listClass is java.lang.Class[List[_]] = class scala.collection.immutable.List val mapIntString = classOf[Map[Int,String]] // mapIntString is java.lang.Class[Map[Int,String]] = interface scala.collection.immutable.Map
Create a copy of the receiver object.
The default implementation of the clone method is platform dependent.
a copy of the receiver object.
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:
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).
true if the argument is a reference to the receiver object; false otherwise.
The equality method for reference types. Default implementation delegates to eq.
See also equals in scala.Any.
true if the receiver object is equivalent to the argument; false otherwise.
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.
not specified by SLS as a member of AnyRef
Returns the runtime class representation of the object.
a class object corresponding to the runtime type of the receiver.
The hashCode method for reference types. See hashCode in scala.Any.
the hash code value for this object.
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.
true if the receiver object is an instance of erasure of type T0; false otherwise.
Equivalent to !(this eq that).
true if the argument is not a reference to the receiver object; false otherwise.
Wakes up a single thread that is waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Wakes up all threads that are waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Prints an object to out using its toString method.
the object to print; may be null.
Prints its arguments as a formatted string to the default output, based on a string pattern (in a fashion similar to printf in C).
The interpretation of the formatting patterns is described in java.util.Formatter.
Consider using the f interpolator as more type safe and idiomatic.
the pattern for formatting the arguments.
java.lang.IllegalArgumentException if there was a problem with the format string or arguments
Prints out an object to the default output, followed by a newline character.
the object to print.
Tests an expression, throwing an IllegalArgumentException if false. This method is similar to assert, but blames the caller of the method for violating the condition.
the expression to test
a String to include in the failure message
Tests an expression, throwing an IllegalArgumentException if false. This method is similar to assert, but blames the caller of the method for violating the condition.
the expression to test
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.
a String representation of the object.
© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.12.9/scala/Predef$.html
The
Predefobject provides definitions that are accessible in all Scala compilation units without explicit qualification.Commonly Used Types
Predef provides type aliases for types which are commonly used, such as the immutable collection types scala.collection.immutable.Map, scala.collection.immutable.Set, and the scala.collection.immutable.List constructors (scala.collection.immutable.:: and scala.collection.immutable.Nil).
Console Output
For basic console output,
Predefprovides convenience methods print and println, which are aliases of the methods in the object scala.Console.Assertions
A set of
assertfunctions are provided for use as a way to document and dynamically check invariants in code. Invocations ofassertcan be elided at compile time by providing the command line option-Xdisable-assertions, which raises-Xelide-belowaboveelidable.ASSERTION, to thescalaccommand.Variants of
assertintended for use with static analysis tools are also provided:assume,requireandensuring.requireandensuringare intended for use as a means of design-by-contract style specification of pre- and post-conditions on functions, with the intention that these specifications could be consumed by a static analysis tool. For instance,def addNaturals(nats: List[Int]): Int = { require(nats forall (_ >= 0), "List contains negative numbers") nats.foldLeft(0)(_ + _) } ensuring(_ >= 0)The declaration of
addNaturalsstates that the list of integers passed should only contain natural numbers (i.e. non-negative), and that the result returned will also be natural.requireis distinct fromassertin that if the condition fails, then the caller of the function is to blame rather than a logical error having been made withinaddNaturalsitself.ensuringis a form ofassertthat declares the guarantee the function is providing with regards to its return value.Implicit Conversions
A number of commonly applied implicit conversions are also defined here, and in the parent type scala.LowPriorityImplicits. Implicit conversions are provided for the "widening" of numeric values, for instance, converting a Short value to a Long value as required, and to add additional higher-order functions to Array values. These are described in more detail in the documentation of scala.Array.