Some groovy gotchas

• groovy -e “println ‘Hello World’” - execute groovy code from comand line
• groovy Example.groovy - run groovy code from the file Example.groovy
• groovyc Example.groovy - compile groovy code from the file to the JVM class. Then you can run this code with JVM adding groovy libraries: java -classpath groovy-all.jar:. Example
• groovysh - interactive command line shell
• groovyConsole - interactive GUI shell
• Arithmetic:
• BigDecimal is used by default for number literal with fraction
• Number literals without fraction are the smallest type that fits Integer, Long, BigInteger
• Strings:
• Single quoted is standard java String
• Double quoted is GString (may contain groovy expressions, can be substring using [])
• Tripple quoted are multiline strings
• Slashy - no escaping needed - useful for regex
• Operator overloading - operators map to specific methods f.e. + is same ad method add()
• Method can be referenced as closure with .& operator
• Closures has properties:
• maximumNumberOfParameters - number of parameters provided to closure
• parameterTypes - array of types of parameters
• Collection methods: collect - as map in Scala and inject as foldLeft
• Objects evaluate to false: null, empty collections, iterators, empty GStrings, numbers equals 0, non-matching regexp pattern matchers, everything else evaluete to true
• == is treated as equals() in java but null safe. If objects implement Comparable then compareTo method is used
• a.is(b) - for java ==
• string.contains(gstring) is false due to internal equals method usage
• Null-safe operator returns null without futher evaluation in case od null ?.
• Elvis operator shorthand for returning default value ?:
• Spaceship operator <=> which replace compareTo method
• regexp operator ~ what creates a Pattern, =~ what creates Matcher, ==~ matching String against regexp
• spread operator *. is a shorthand for Collection.collect(closure)
• as operator transforms a variable to another type
• xml support with classes MarkupBuilder and XmlParser
• json suppert with JsonBuilder, JsonSlurper
• Integrations with java:
• Groovy shell - evaluates expressions in Java with passing in and out variables
• GroovyClassLoader - parse and compile classes
• Groovy Joint Compiler - compile all groovy and java sources. Allows circular dependencies between them, but on other layers
• Spring Groovy Integration - beans can be defined by POGO with lang namespace
• Changes between groovy and java:
• this inside static method refers to class
• do-while loops are not supperted
• all exceptions are unchecked
• inner classes are not allowed
• Built-in AST Transformations: @Singleton, @Delegate, @TupleConstructor, @TimedInterrupt
• @TypedChecked - analyze at compile-time
• @CompileStatic - compile like javac compiler, no groovy dynamic features