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README.md

HamKrest - Hamcrest for Kotlin

A reimplementation of Hamcrest to take advantage of Kotlin language features.

Kotlin Build Status Maven Central

Note: as of version 1.4.0.0, you must add kotlin-reflect to the classpath to use HamKrest's reflective features.

When working in Kotlin, Hamkrest provides these benefits over using the Java Hamcrest library:

  • Kotlin's type system means that developers don't have to worry about getting the variance of generic signatures right. Variance is defined on the abstract Matcher type and Kotlin makes sure composition and subtyping work together the way you expect.

  • Syntactic sugar. You can negate a matcher with the ! operator and compose matchers with infix and and or functions:

    import com.natpryce.hamkrest.assertion.assert
          
    ...
          
    assertThat("xyzzy", startsWith("x") and endsWith("y") and !containsSubstring("a"))
  • Easier to extend. You can convert named unary predicates into matchers.

    val isBlank = Matcher(String::isBlank)
    
    assertThat(input, isBlank)

    As a shortcut, you can pass named functions to the assertThat, and, or and many other functions that take a matcher.

    assertThat(input, String::isBlank)

    You can also convert a named binary predicate and the second argument to a matcher for first argument, which works well for extension methods.

    fun String.hasLength(n: Int): Boolean = this.length == n
    
    val isTheRightLength = Matcher(String::hasLength, 8)
    
    assertThat(secretCode, isTheRightLength)

    You can use function and property references to match features of a value:

    val isLongEnough = has(String::length, greaterThan(8))
    
    assertThat(password, isLongEnough)

    All of these shortcuts produce good, human-readable diagnostics.

    You can customise how diagnostics are generated by creating a project-specific assert object.

More documentation

More detailed documentation of specific library features is in the docs/ directory.

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