org.scalatest.Assertions

Trait that contains ScalaTest's basic assertion methods.

You can use the assertions provided by this trait in any ScalaTest Suite, because Suite mixes in this trait. This trait is designed to be used independently of anything else in ScalaTest, though, so you can mix it into anything. (You can alternatively import the methods defined in this trait. For details, see the documentation for the Assertions companion object.

In any Scala program, you can write assertions by invoking assert and passing in a Boolean expression, such as:

val left = 2
val right = 1
assert(left == right)

If the passed expression is true, assert will return normally. If false, assert will complete abruptly with an AssertionError. This behavior is provided by the assert method defined in object Predef, whose members are implicitly imported into every Scala source file. This Assertions traits defines another assert method that hides the one in Predef. It behaves the same, except that if false is passed it throws TestFailedException instead of AssertionError. The reason it throws TestFailedException is because TestFailedException carries information about exactly which item in the stack trace represents the line of test code that failed, which can help users more quickly find an offending line of code in a failing test.

If you pass the previous Boolean expression, left == right to assert in a ScalaTest test, a failure will be reported, but without reporting the left and right values. You can alternatively encode these values in a String passed as a second argument to assert, like this:

val left = 2
val right = 1
assert(left == right, left + " did not equal " + right)

Using this form of assert, the failure report will include the left and right values, thereby helping you debug the problem. However, ScalaTest provides the === operator to make this easier. You use it like this:

val left = 2
val right = 1
assert(left === right)

Because you use === here instead of ==, the failure report will include the left and right values. For example, the detail message in the thrown TestFailedException from the assert shown previously will include, "2 did not equal 1". From this message you will know that the operand on the left had the value 2, and the operand on the right had the value 1.

If you're familiar with JUnit, you would use === in a ScalaTest Suite where you'd use assertEquals in a JUnit TestCase. The === operator is made possible by an implicit conversion from Any to Equalizer. If you're curious to understand the mechanics, see the documentation for Equalizer and the convertToEqualizer method.

Expected results

Although === provides a natural, readable extension to Scala's assert mechanism, as the operands become lengthy, the code becomes less readable. In addition, the === comparison doesn't distinguish between actual and expected values. The operands are just called left and right, because if one were named expected and the other actual, it would be difficult for people to remember which was which. To help with these limitations of assertions, Suite includes a method called expect that can be used as an alternative to assert with ===. To use expect, you place the expected value in parentheses after expect, followed by curly braces containing code that should result in the expected value. For example:

val a = 5
val b = 2
expect(2) {
  a - b
}

In this case, the expected value is 2, and the code being tested is a - b. This expectation will fail, and the detail message in the TestFailedException will read, "Expected 2, but got 3."

Intercepted exceptions

Sometimes you need to test whether a method throws an expected exception under certain circumstances, such as when invalid arguments are passed to the method. You can do this in the JUnit 3 style, like this:

val s = "hi"
try {
  s.charAt(-1)
  fail()
}
catch {
  case _: IndexOutOfBoundsException => // Expected, so continue
}

If charAt throws IndexOutOfBoundsException as expected, control will transfer to the catch case, which does nothing. If, however, charAt fails to throw an exception, the next statement, fail(), will be run. The fail method always completes abruptly with a TestFailedException, thereby signaling a failed test.

To make this common use case easier to express and read, ScalaTest provides an intercept method. You use it like this:

val s = "hi"
intercept[IndexOutOfBoundsException] {
  s.charAt(-1)
}

This code behaves much like the previous example. If charAt throws an instance of IndexOutOfBoundsException, intercept will return that exception. But if charAt completes normally, or throws a different exception, intercept will complete abruptly with a TestFailedException. intercept returns the caught exception so that you can inspect it further if you wish, for example, to ensure that data contained inside the exception has the expected values.

Getting a clue

If you want more information that is provided by default by the methods if this trait, you can supply a "clue" string in one of several ways. The extra information (or "clues") you provide will be included in the detail message of the thrown exception. Both assert and expect provide a way for a clue to be included directly, intercept does not. Here's an example of clues provided directly in assert:

assert(1 + 1 === 3, "this is a clue")

and in expect:

expect(3, "this is a clue") { 1 + 1 }

The exceptions thrown by the previous two statements will include the clue string, "this is a clue", in the exception's detail message. To get the same clue in the detail message of an exception thrown by a failed intercept call requires using withClue:

withClue("this is a clue") {
  intercept[IndexOutOfBoundsException] {
    "hi".charAt(-1)
  }
}

The withClue method will only prepend the clue string to the detail message of exception types that mix in the ModifiableMessage trait. See the documentation for ModifiableMessage for more information.

Linear Supertypes

AnyRef, Any

Known Subclasses

Assertions, AssertionsForJUnit, AssertionsForJUnit, ConfigMapWrapperSuite, FeatureSpec, FeatureSpec, FixtureFeatureSpec, FixtureFlatSpec, FixtureFreeSpec, FixtureFunSuite, FixturePropSpec, FixtureSpec, FixtureSuite, FixtureWordSpec, FlatSpec, FlatSpec, FreeSpec, FreeSpec, FreeSpec, FunSpec, FunSpec, FunSpec, FunSuite, FunSuite, JUnit3Suite, JUnitSuite, JUnitWrapperSuite, Matchers, MultipleFixtureFeatureSpec, MultipleFixtureFlatSpec, MultipleFixtureFreeSpec, MultipleFixtureFunSuite, MultipleFixturePropSpec, MultipleFixtureSpec, MultipleFixtureWordSpec, MustMatchers, MustMatchers, MustMatchersForJUnit, MustMatchersForJUnit, NonImplicitAssertions, NonImplicitAssertions, PropSpec, PropSpec, ShouldMatchers, ShouldMatchers, ShouldMatchersForJUnit, ShouldMatchersForJUnit, Spec, Specs, Suite, Suite, Suites, SuperSuite, TestNGSuite, TestNGWrapperSuite, WordSpec, WordSpec

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Inherited

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Assertions
AnyRef
Any

Visibility

Public
All

Type Members

class Equalizer extends AnyRef

Class used via an implicit conversion to enable any two objects to be compared with === in assertions in tests.

Value Members

def != (arg0: AnyRef): Boolean

Attributes
final
Definition Classes
AnyRef
def != (arg0: Any): Boolean

Attributes
final
Definition Classes
Any
def ## (): Int

Attributes
final
Definition Classes
AnyRef → Any
def == (arg0: AnyRef): Boolean

Attributes
final
Definition Classes
AnyRef
def == (arg0: Any): Boolean

Attributes
final
Definition Classes
Any
def asInstanceOf [T0] : T0

Attributes
final
Definition Classes
Any
def assert (o: Option[String]): Unit

Assert that an Option[String] is None.
Assert that an Option[String] is None. If the condition is None, this method returns normally. Else, it throws TestFailedException with the String value of the Some included in the TestFailedException's detail message.
This form of assert is usually called in conjunction with an implicit conversion to Equalizer, using a === comparison, as in:
```
assert(a === b)
```
For more information on how this mechanism works, see the documentation for Equalizer.
o
the Option[String] to assert
def assert (o: Option[String], clue: Any): Unit

Assert that an Option[String] is None.
Assert that an Option[String] is None. If the condition is None, this method returns normally. Else, it throws TestFailedException with the String value of the Some, as well as the String obtained by invoking toString on the specified message, included in the TestFailedException's detail message.
This form of assert is usually called in conjunction with an implicit conversion to Equalizer, using a === comparison, as in:
```
assert(a === b, "extra info reported if assertion fails")
```
For more information on how this mechanism works, see the documentation for Equalizer.
o
the Option[String] to assert
clue
An objects whose toString method returns a message to include in a failure report.
def assert (condition: Boolean, clue: Any): Unit

Assert that a boolean condition, described in String message, is true.
Assert that a boolean condition, described in String message, is true. If the condition is true, this method returns normally. Else, it throws TestFailedException with the String obtained by invoking toString on the specified message as the exception's detail message.
condition
the boolean condition to assert
clue
An objects whose toString method returns a message to include in a failure report.
def assert (condition: Boolean): Unit

Assert that a boolean condition is true.
Assert that a boolean condition is true. If the condition is true, this method returns normally. Else, it throws TestFailedException.
condition
the boolean condition to assert
def clone (): AnyRef

Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
@throws()
implicit def convertToEqualizer (left: Any): Equalizer

Implicit conversion from Any to Equalizer, used to enable assertions with === comparisons.
Implicit conversion from Any to Equalizer, used to enable assertions with === comparisons.
For more information on this mechanism, see the documentation for Equalizer.
Because trait Suite mixes in Assertions, this implicit conversion will always be available by default in ScalaTest Suites. This is the only implicit conversion that is in scope by default in every ScalaTest Suite. Other implicit conversions offered by ScalaTest, such as those that support the matchers DSL or invokePrivate, must be explicitly invited into your test code, either by mixing in a trait or importing the members of its companion object. The reason ScalaTest requires you to invite in implicit conversions (with the exception of the implicit conversion for === operator) is because if one of ScalaTest's implicit conversions clashes with an implicit conversion used in the code you are trying to test, your program won't compile. Thus there is a chance that if you are ever trying to use a library or test some code that also offers an implicit conversion involving a === operator, you could run into the problem of a compiler error due to an ambiguous implicit conversion. If that happens, you can turn off the implicit conversion offered by this convertToEqualizer method simply by overriding the method in your Suite subclass, but not marking it as implicit:
```
// In your Suite subclass
override def convertToEqualizer(left: Any) = new Equalizer(left)
```
left
the object whose type to convert to Equalizer.

Attributes
implicit
def eq (arg0: AnyRef): Boolean

Attributes
final
Definition Classes
AnyRef
def equals (arg0: Any): Boolean

Definition Classes
AnyRef → Any
def expect (expected: Any)(actual: Any): Unit

Expect that the value passed as expected equals the value passed as actual.
Expect that the value passed as expected equals the value passed as actual. If the actual value equals the expected value (as determined by ==), expect returns normally. Else, expect throws an TestFailedException whose detail message includes the expected and actual values.
expected
the expected value
actual
the actual value, which should equal the passed expected value
def expect (expected: Any, clue: Any)(actual: Any): Unit

Expect that the value passed as expected equals the value passed as actual.
Expect that the value passed as expected equals the value passed as actual. If the actual equals the expected (as determined by ==), expect returns normally. Else, if actual is not equal to expected, expect throws an TestFailedException whose detail message includes the expected and actual values, as well as the String obtained by invoking toString on the passed message.
expected
the expected value
clue
An object whose toString method returns a message to include in a failure report.
actual
the actual value, which should equal the passed expected value
def fail (cause: Throwable): Nothing

Throws TestFailedException, with the passed Throwable cause, to indicate a test failed.
Throws TestFailedException, with the passed Throwable cause, to indicate a test failed. The getMessage method of the thrown TestFailedException will return cause.toString().
cause
a Throwable that indicates the cause of the failure.
def fail (message: String, cause: Throwable): Nothing

Throws TestFailedException, with the passed String message as the exception's detail message and Throwable cause, to indicate a test failed.
Throws TestFailedException, with the passed String message as the exception's detail message and Throwable cause, to indicate a test failed.
message
A message describing the failure.
cause
A Throwable that indicates the cause of the failure.
def fail (message: String): Nothing

Throws TestFailedException, with the passed String message as the exception's detail message, to indicate a test failed.
Throws TestFailedException, with the passed String message as the exception's detail message, to indicate a test failed.
message
A message describing the failure.
def fail (): Nothing

Throws TestFailedException to indicate a test failed.
def finalize (): Unit

Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
@throws()
def getClass (): java.lang.Class[_]

Attributes
final
Definition Classes
AnyRef
def hashCode (): Int

Definition Classes
AnyRef → Any
def intercept [T <: AnyRef] (f: ⇒ Any)(implicit manifest: Manifest[T]): T

Intercept and return an exception that's expected to be thrown by the passed function value.
Intercept and return an exception that's expected to be thrown by the passed function value. The thrown exception must be an instance of the type specified by the type parameter of this method. This method invokes the passed function. If the function throws an exception that's an instance of the specified type, this method returns that exception. Else, whether the passed function returns normally or completes abruptly with a different exception, this method throws TestFailedException.
Note that the type specified as this method's type parameter may represent any subtype of AnyRef, not just Throwable or one of its subclasses. In Scala, exceptions can be caught based on traits they implement, so it may at times make sense to specify a trait that the intercepted exception's class must mix in. If a class instance is passed for a type that could not possibly be used to catch an exception (such as String, for example), this method will complete abruptly with a TestFailedException.
f
the function value that should throw the expected exception
manifest
an implicit Manifest representing the type of the specified type parameter.
returns
the intercepted exception, if it is of the expected type
def isInstanceOf [T0] : Boolean

Attributes
final
Definition Classes
Any
def ne (arg0: AnyRef): Boolean

Attributes
final
Definition Classes
AnyRef
def notify (): Unit

Attributes
final
Definition Classes
AnyRef
def notifyAll (): Unit

Attributes
final
Definition Classes
AnyRef
def synchronized [T0] (arg0: ⇒ T0): T0

Attributes
final
Definition Classes
AnyRef
def toString (): String

Definition Classes
AnyRef → Any
def wait (): Unit

Attributes
final
Definition Classes
AnyRef
Annotations
@throws()
def wait (arg0: Long, arg1: Int): Unit

Attributes
final
Definition Classes
AnyRef
Annotations
@throws()
def wait (arg0: Long): Unit

Attributes
final
Definition Classes
AnyRef
Annotations
@throws()
def withClue [T] (clue: Any)(fun: ⇒ T): T

Executes the block of code passed as the second parameter, and, if it completes abruptly with a ModifiableMessage exception, prepends the "clue" string passed as the first parameter to the beginning of the detail message of that thrown exception, then rethrows it.
Executes the block of code passed as the second parameter, and, if it completes abruptly with a ModifiableMessage exception, prepends the "clue" string passed as the first parameter to the beginning of the detail message of that thrown exception, then rethrows it. If clue does not end in a white space character, one space will be added between it and the existing detail message (unless the detail message is not defined).
This method allows you to add more information about what went wrong that will be reported when a test fails. Here's an example:
```
withClue("(Employee's name was: " + employee.name + ")") {
  intercept[IllegalArgumentException] {
    employee.getTask(-1)
  }
}
```
If an invocation of intercept completed abruptly with an exception, the resulting message would be something like:
```
(Employee's name was Bob Jones) Expected IllegalArgumentException to be thrown, but no exception was thrown
```

Assertions

trait Assertions extends AnyRef

Expected results

Intercepted exceptions

Getting a clue

Type Members

class Equalizer extends AnyRef

Value Members

def != (arg0: AnyRef): Boolean

def != (arg0: Any): Boolean

def ## (): Int

def == (arg0: AnyRef): Boolean

def == (arg0: Any): Boolean

def asInstanceOf [T0] : T0

def assert (o: Option[String]): Unit

def assert (o: Option[String], clue: Any): Unit

def assert (condition: Boolean, clue: Any): Unit

def assert (condition: Boolean): Unit

def clone (): AnyRef

implicit def convertToEqualizer (left: Any): Equalizer

def eq (arg0: AnyRef): Boolean

def equals (arg0: Any): Boolean

def expect (expected: Any)(actual: Any): Unit

def expect (expected: Any, clue: Any)(actual: Any): Unit

def fail (cause: Throwable): Nothing

def fail (message: String, cause: Throwable): Nothing

def fail (message: String): Nothing

def fail (): Nothing

def finalize (): Unit

def getClass (): java.lang.Class[_]

def hashCode (): Int

def intercept [T <: AnyRef] (f: ⇒ Any)(implicit manifest: Manifest[T]): T

def isInstanceOf [T0] : Boolean

def ne (arg0: AnyRef): Boolean

def notify (): Unit

def notifyAll (): Unit

def synchronized [T0] (arg0: ⇒ T0): T0

def toString (): String

def wait (): Unit

def wait (arg0: Long, arg1: Int): Unit

def wait (arg0: Long): Unit

def withClue [T] (clue: Any)(fun: ⇒ T): T

Inherited from AnyRef

Inherited from Any