MustMatchers

Trait that provides a domain specific language (DSL) for expressing assertions in tests using the word must. (If you prefer the word should, you can alternatively mix in trait ShouldMatchers.) For example, if you mix MustMatchers into a suite class, you can write an equality assertion in that suite like this:

object must equal (3)

Here object is a variable, and can be of any type. If the object is anInt with the value 3, execution will continue (i.e., the expression will result in the unit value, ()). Otherwise, a TestFailedExceptionwill be thrown with a detail message that explains the problem, such as "7 did not equal 3". This TestFailedException will cause the test to fail.

The left must equal (right) syntax works by calling == on the leftvalue, passing in the right value, on every type except arrays. If left is an array, deepEqualswill be invoked on left, passing in right. Thus, even though this expression will yield false, because Array's equals method compares object identity:

Array(1, 2) == Array(1, 2) // yields false

The following expression will not result in a TestFailedException, because deepEquals compares the two arrays structurally, taking into consideration the equality of the array's contents:

Array(1, 2) must equal (Array(1, 2)) // succeeds (i.e., does not throw TestFailedException)

If you ever do want to verify that two arrays are actually the same object (have the same identity), you can use thebe theSameInstanceAs syntax, described below.

Checking size and length

You can check the size or length of just about any type of object for which it would make sense. Here's how checking for length looks:

object must have length (3)

Size is similar:

object must have size (10)

The length syntax can be used with any object that has a field or method named lengthor a method named getLength. Similarly, the size syntax can be used with any object that has a field or method named size or a method named getSize. The type of a length or size field, or return type of a method, must be either Intor Long. Any such method must take no parameters. (The Scala compiler will ensure at compile time that the object on which must is being invoked has the appropriate structure.)

Checking strings

You can check for whether a string starts with, ends with, or includes a substring like this:

string must startWith ("Hello")
string must endWith ("world")
string must include ("seven")

You can check for whether a string starts with, ends with, or includes a regular expression, like this:

string must startWith regex ("Hel*o")
string must endWith regex ("wo.ld")
string must include regex ("wo.ld")

And you can check whether a string fully matches a regular expression, like this:

string must fullyMatch regex ("""(-)?(\d+)(\.\d*)?""")

The regular expression passed following the regex token can be either a Stringor a scala.util.matching.Regex.

Greater and less than

You can check whether any type that is, or can be implicitly converted to, an Ordered[T] is greater than, less than, greater than or equal, or less than or equal to a value of type T. The syntax is:

one must be < (7)
one must be > (0)
one must be <= (7)
one must be >= (0)

Checking equality with be ===

An alternate way to check for equality of two objects is to use be with===. Here's an example:

object must be === (3)

Here object is a variable, and can be of any type. If the object is anInt with the value 3, execution will continue (i.e., the expression will result in the unit value, ()). Otherwise, a TestFailedExceptionwill be thrown with a detail message that explains the problem, such as "7 was not equal to 3". This TestFailedException will cause the test to fail.

The left must be === (right) syntax works by calling == on the leftvalue, passing in the right value, on every type except arrays. If left is an array, deepEqualswill be invoked on left, passing in right. Thus, even though this expression will yield false, because Array's equals method compares object identity:

Array(1, 2) == Array(1, 2) // yields false

The following expression will not result in a TestFailedException, because deepEquals compares the two arrays structurally, taking into consideration the equality of the array's contents:

Array(1, 2) must be === (Array(1, 2)) // succeeds (i.e., does not throw TestFailedException)

If you ever do want to verify that two arrays are actually the same object (have the same identity), you can use thebe theSameInstanceAs syntax, described below.

Checking Boolean properties with be

If an object has a method that takes no parameters and returns boolean, you can check it by placing a Symbol (after be) that specifies the name of the method (excluding an optional prefix of "is"). A symbol literal in Scala begins with a tick mark and ends at the first non-identifier character. Thus,'empty results in a Symbol object at runtime, as does'defined and 'file. Here's an example:

emptySet must be ('empty)

Given this code, ScalaTest will use reflection to look on the object referenced fromemptySet for a method that takes no parameters and results in Boolean, with either the name empty or isEmpty. If found, it will invoke that method. If the method returns true, execution will continue. But if it returnsfalse, a TestFailedException will be thrown that will contain a detail message, such as:

Set(1, 2, 3) was not empty

This be syntax can be used with any type. If the object does not have an appropriately named predicate method, you'll get a TestFailedExceptionat runtime with a detail message that explains the problem. (For the details on how a field or method is selected during this process, see the documentation for BeWord.)

If you think it reads better, you can optionally put a or an afterbe. For example, java.io.File has two predicate methods,isFile and isDirectory. Thus with a File object named temp, you could write:

temp must be a ('file)

Or, given java.awt.event.KeyEvent has a method isActionKey that takes no arguments and returns Boolean, you could assert that a KeyEvent is an action key with:

keyEvent must be an ('actionKey)

If you prefer to check Boolean properties in a type-safe manner, you can use a BePropertyMatcher. This would allow you to write expressions such as:

emptySet must be (empty)
temp must be a (file)
keyEvent must be an (actionKey)

These expressions would fail to compile if must is used on an inappropriate type, as determined by the type parameter of the BePropertyMatcher being used. (For example, file in this example would likely be of type BePropertyMatcher[java.io.File]. If used with an appropriate type, such an expression will compile and at run time the Boolean property method or field will be accessed directly; i.e., no reflection will be used. See the documentation for BePropertyMatcher for more information.

Using custom BeMatchers

If you want to create a new way of using be, which doesn't map to an actual property on the type you care about, you can create a BeMatcher. You could use this, for example, to create BeMatcher[Int]called odd, which would match any odd Int, and even, which would match any even Int. Given this pair of BeMatchers, you could check whether an Int was odd or even with expressions like:

num must be (odd)
num must not be (even)

For more information, see the documentation for BeMatcher.

Checking object identity

If you need to check that two references refer to the exact same object, you can write:

ref1 must be theSameInstanceAs (ref2)

Checking numbers against a range

To check whether a floating point number has a value that exactly matches another, you can use must equal:

sevenDotOh must equal (7.0)

Often, however, you may want to check whether a floating point number is within a range. You can do that using be and plusOrMinus, like this:

sevenDotOh must be (6.9 plusOrMinus 0.2)

This expression will cause a TestFailedException to be thrown if the floating point value, sevenDotOh is outside the range 6.7 to 7.1. You can also use plusOrMinus with integral types, for example:

seven must be (6 plusOrMinus 2)

Iterables, collections, sequences, and maps

You can use some of the syntax shown previously with Iterable and its subtypes. For example, you can check whether an Iterable is empty, like this:

iterable must be ('empty)

You can check the length of an Seq (Array, List, etc.), like this:

array must have length (3)
list must have length (9)

You can check the size of any Collection, like this:

map must have size (20)
set must have size (90)

In addition, you can check whether an Iterable contains a particular element, like this:

iterable must contain ("five")

You can also check whether a Map contains a particular key, or value, like this:

map must contain key (1)
map must contain value ("Howdy")

Java collections and maps

You can use similar syntax on Java collections (java.util.Collection) and maps (java.util.Map). For example, you can check whether a Java Collection or Map is empty, like this:

javaCollection must be ('empty)
javaMap must be ('empty)

Even though Java's List type doesn't actually have a length or getLength method, you can nevertheless check the length of a Java List (java.util.List) like this:

javaList must have length (9)

You can check the size of any Java Collection or Map, like this:

javaMap must have size (20)
javaSet must have size (90)

In addition, you can check whether a Java Collection contains a particular element, like this:

javaCollection must contain ("five")

One difference to note between the syntax supported on Java collections and that of Scala iterables is that you can't use contain (...) syntax with a Java Map. Java differs from Scala in that its Map is not a subtype of its Collection type. If you want to check that a Java Map contains a specific key/value pair, the best approach is to invoke entrySet on the Java Map and check that entry set for the appropriate element (a java.util.Map.Entry) using contain (...).

Despite this difference, the other (more commonly used) map matcher syntax works just fine on Java Maps. You can, for example, check whether a Java Map contains a particular key, or value, like this:

javaMap must contain key (1)
javaMap must contain value ("Howdy")

Be as an equality comparison

All uses of be other than those shown previously perform an equality comparison. In other words, they work the same as equals. This redundance between be and equals exists because it enables syntax that sometimes sounds more natural. For example, instead of writing:

result must equal (null)

You can write:

result must be (null)

(Hopefully you won't write that too much given null is error prone, and Optionis usually a better, well, option.) Here are some other examples of be used for equality comparison:

sum must be (7.0)
boring must be (false)
fun must be (true)
list must be (Nil)
option must be (None)
option must be (Some(1))

As with equal, using be on arrays results in deepEquals being called, not equals. As a result, the following expression would not throw a TestFailedException:

Array(1, 2) must be (Array(1, 2)) // succeeds (i.e., does not throw TestFailedException)

Because be is used in several ways in ScalaTest matcher syntax, just as it is used in many ways in English, one potential point of confusion in the event of a failure is determining whether be was being used as an equality comparison or in some other way, such as a property assertion. To make it more obvious when be is being used for equality, the failure messages generated for those equality checks will include the word equal in them. For example, if this expression fails with aTestFailedException:

option must be (Some(1))

The detail message in that TestFailedException will include the words "equal to" to signify bewas in this case being used for equality comparison:

Some(2) was not equal to Some(1)

Being negative

If you wish to check the opposite of some condition, you can simply insert not in the expression. Here are a few examples:

object must not be (null)
sum must not be <= (10)
mylist must not equal (yourList)
string must not startWith ("Hello")

Logical expressions with and and or

You can also combine matcher expressions with and and/or or, however, you must place parentheses or curly braces around the and or or expression. For example, this and-expression would not compile, because the parentheses are missing:

map must contain key ("two") and not contain value (7) // ERROR, parentheses missing!

Instead, you need to write:

map must (contain key ("two") and not contain value (7))

Here are some more examples:

number must (be > (0) and be <= (10))
option must (equal (Some(List(1, 2, 3))) or be (None))
string must (
  equal ("fee") or
  equal ("fie") or
  equal ("foe") or
  equal ("fum")
)

Two differences exist between expressions composed of these and and or operators and the expressions you can write on regular Booleans using its && and || operators. First, expressions with andand or do not short-circuit. The following contrived expression, for example, would print "hello, world!":

"yellow" must (equal ("blue") and equal { println("hello, world!"); "green" })

In other words, the entire and or or expression is always evaluated, so you'll see any side effects of the right-hand side even if evaluating only the left-hand side is enough to determine the ultimate result of the larger expression. Failure messages produced by these expressions will "short-circuit," however, mentioning only the left-hand side if that's enough to determine the result of the entire expression. This "short-circuiting" behavior of failure messages is intended to make it easier and quicker for you to ascertain which part of the expression caused the failure. The failure message for the previous expression, for example, would be:

"yellow" did not equal "blue"

Most likely this lack of short-circuiting would rarely be noticeable, because evaluating the right hand side will usually not involve a side effect. One situation where it might show up, however, is if you attempt to and a null check on a variable with an expression that uses the variable, like this:

map must (not be (null) and contain key ("ouch"))

If map is null, the test will indeed fail, but with a NullPointerException, not aTestFailedException. Here, the NullPointerException is the visible right-hand side effect. To get aTestFailedException, you would need to check each assertion separately:

map must not be (null)
map must contain key ("ouch")

If map is null in this case, the null check in the first expression will fail with a TestFailedException, and the second expression will never be executed.

The other difference with Boolean operators is that although && has a higher precedence than ||,and and orhave the same precedence. Thus although the Boolean expression (a || b && c) will evaluate the && expression before the || expression, like (a || (b && c)), the following expression:

collection must (contain (7) or contain (8) and have size (9))

Will evaluate left to right, as:

collection must ((contain (7) or contain (8)) and have size (9))

If you really want the and part to be evaluated first, you'll need to put in parentheses, like this:

collection must (contain (7) or (contain (8) and have size (9)))

Working with Options

ScalaTest matchers has no special support for Options, but you can work with them quite easily using syntax shown previously. For example, if you wish to check whether an option is None, you can write any of:

option must equal (None)
option must be (None)
option must not be ('defined)
option must be ('empty)

If you wish to check an option is defined, and holds a specific value, you can write either of:

option must equal (Some("hi"))
option must be (Some("hi"))

If you only wish to check that an option is defined, but don't care what it's value is, you can write:

option must be ('defined)

Checking arbitrary properties with have

Using have, you can check properties of any type, where a property is an attribute of any object that can be retrieved either by a public field, method, or JavaBean-style getor is method, like this:

book must have (
  'title ("Programming in Scala"),
  'author (List("Odersky", "Spoon", "Venners")),
  'pubYear (2008)
)

This expression will use reflection to ensure the title, author, and pubYear properties of object bookare equal to the specified values. For example, it will ensure that book has either a public Java field or method named title, or a public method named getTitle, that when invoked (or accessed in the field case) results in a the string "Programming in Scala". If all specified properties exist and have their expected values, respectively, execution will continue. If one or more of the properties either does not exist, or exists but results in an unexpected value, a TestFailedException will be thrown that explains the problem. (For the details on how a field or method is selected during this process, see the documentation for HavePropertyMatcherGenerator.)

When you use this syntax, you must place one or more property values in parentheses after have, seperated by commas, where a property value is a symbol indicating the name of the property followed by the expected value in parentheses. The only exceptions to this rule is the syntax for checking size and length shown previously, which does not require parentheses. If you forget and put parentheses in, however, everything will still work as you'd expect. Thus instead of writing:

array must have length (3)
set must have size (90)

You can alternatively, write:

array must have (length (3))
set must have (size (90))

If a property has a value different from the specified expected value, a TestFailedError will be thrown with a detail message that explains the problem. For example, if you assert the following on a book whose title is Moby Dick:

book must have ('title ("A Tale of Two Cities"))

You'll get a TestFailedException with this detail message:

The title property had value "Moby Dick", instead of its expected value "A Tale of Two Cities",
on object Book("Moby Dick", "Melville", 1851)

If you prefer to check properties in a type-safe manner, you can use a HavePropertyMatcher. This would allow you to write expressions such as:

book must have (
  title ("Programming in Scala"),
  author (List("Odersky", "Spoon", "Venners")),
  pubYear (2008)
)

These expressions would fail to compile if must is used on an inappropriate type, as determined by the type parameter of the HavePropertyMatcher being used. (For example, title in this example might be of type HavePropertyMatcher[org.publiclibrary.Book]. If used with an appropriate type, such an expression will compile and at run time the property method or field will be accessed directly; i.e., no reflection will be used. See the documentation for HavePropertyMatcher for more information.

Using custom matchers

If none of the built-in matcher syntax (or options shown so far for extending the syntax) satisfy a particular need you have, you can create custom Matchers that allow you to place your own syntax directly after must. For example, class java.io.File has a method exists, which indicates whether a file of a certain path and name exists. Because the exists method takes no parameters and returns Boolean, you can call it using be with a symbol or BePropertyMatcher, yielding assertions like:

file must be ('exists)  // using a symbol
file must be (inExistance)   // using a BePropertyMatcher

Although these expressions will achieve your goal of throwing a TestFailedException if the file does not exist, they don't produce the most readable code because the English is either incorrect or awkward. In this case, you might want to create a custom Matcher[java.io.File]named exist, which you could then use to write expressions like:

// using a plain-old Matcher
file must exist
file must not (exist)
file must (exist and have ('name ("temp.txt")))

Note that when you use custom Matchers, you will need to put parentheses around the custom matcher in more cases than with the built-in syntax. For example you will often need the parentheses after not, as shown above. (There's no penalty for always surrounding custom matchers with parentheses, and if you ever leave them off when they are needed, you'll get a compiler error.) For more information about how to create custom Matchers, please see the documentation for the Matcher trait.

Checking for expected 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. With MustMatchers mixed in, you can check for an expected exception like this:

evaluating { s.charAt(-1) } must produce [IndexOutOfBoundsException]

If charAt throws an instance of StringIndexOutOfBoundsException, this expression will result in that exception. But if charAt completes normally, or throws a different exception, this expression will complete abruptly with a TestFailedException. This expression 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. Here's an example:

val thrown = evaluating { s.charAt(-1) } must produce [IndexOutOfBoundsException]
thrown.getMessage must equal ("String index out of range: -1")

Those pesky parens

Perhaps the most tricky part of writing assertions using ScalaTest matchers is remembering when you need or don't need parentheses, but bearing in mind a few simple rules should help. It is also reassuring to know that if you ever leave off a set of parentheses when they are required, your code will not compile. Thus the compiler will help you remember when you need the parens. That said, the rules are:

1. Although you don't always need them, it is recommended style to always put parentheses around right-hand values, such as the 7 in num must equal (7):

result must equal (4)array must have length (3)book must have (
  'title ("Programming in Scala"),
  'author (List("Odersky", "Spoon", "Venners")),
  'pubYear (2008))
option must be ('defined)catMap must (contain key (9) and contain value ("lives"))keyEvent must be an ('actionKey)javaSet must have size (90)

2. Except for length and size, you must always put parentheses around the list of one or more property values following a have:

file must (exist and have ('name ("temp.txt")))
book must have (  title ("Programming in Scala"),
  author (List("Odersky", "Spoon", "Venners")),
  pubYear (2008))javaList must have length (9) // parens optional for length and size

3. You must always put parentheses around and and or expressions, as in:

catMap must (contain key (9) and contain value ("lives"))number must (equal (2) or equal (4) or equal (8))

4. Although you don't always need them, it is recommended style to always put parentheses around custom Matchers when they appear directly after not:

file must exist
file must not (exist)file must (exist and have ('name ("temp.txt")))
file must (not (exist) and have ('name ("temp.txt"))
file must (have ('name ("temp.txt") or exist)
file must (have ('name ("temp.txt") or not (exist))

That's it. With a bit of practice it should become natural to you, and the compiler will always be there to tell you if you forget a set of needed parentheses.