The Road from Java to Groovy

Greach Lightning Talk

About Me

1st Career

About Me

2nd Career

OCI Grails Team

Legend

Cruisin' to easy migration and interop

Caution for potholes & speed bumps

Stopping for rework or collisions

Legend

Anywhere you see a parrot means this inconsistency with Java will be changing when the new "parrot" grammar changes are available in Groovy 3.0 and maybe as options in 2.5.x  

  • Generally, migration is very easy
  • Often, just renaming the file works
  • There are a few roadblocks and detours

Java to Groovy Migration

  • Kind of like Java 8 interfaces
  • Kind of like classes
  • Can have state and behavior

In Groovy we can't create default methods in interfaces, but we have traits! 

trait Singer {
    int songLength

    void sing() {
        println "Oh say can you see..."
    }
}

class Teacher implements Singer { }

class Student implements Singer {
    void sing() {
        println "Twinkle twinkle little star..."
    }
}

Java 8 Streams, Method Refs & Lambdas

Java 8 introduced streams, method references and lambdas

Groovy doesn't support lambdas and Java method references, but has its own method reference mechanism)

GroovY "Parrot" Grammars to Support Lambda expressions

Detouring Around The Java 8 Gaps

  • Java 8 method references are a kind of shorthand for lambdas
  • Lambdas implement functional interfaces
  • Groovy closures can be coerced into meeting contract of the functional interface
  • Wherever you find a Java 8 method reference or lambda, you can replace it with a closure

Mostly Smooth running in Regards

to Migration and Interoperability

  • Often can simply rename file to .groovy
  • Use closures to work around Java 8 gaps
  • AST transformations are at compile time, so support interop with Java

Method Dispatching Differences (JAVA)

Method Dispatching Differences (Groovy)

Groovy Equality

== means compareTo(obj) or equals(obj)

"spacheship" operator means compareTo

In Java, x == y means: Is x same obj ref as y?

Groovy Equality

a.is(b)

a === b

Groovy Dynamic Method Dispatch

Cruising Performance (Groovy)

@CompileStatic bypasses Groovy MOP

Note Groovy method pointers

Missing Pavement

Do / While (Java)

Do / While (Groovy)

Groovy has never supported do/while

To Groovy, do { } looks like do(Closure)

Types and promotion - Java vs Groovy

In Groovy, all types are objects so the variable five is not an int, but an Integer. This means the method(Integer) will be called.

Output in Java:

In Java, promotion has higher precedence than autoboxing

Keyword Collisons

Summary

Migration from Java and interop with Java is almost trivial

There are a few issues arising from the differences between Groovy and Java, but these are easy to remedy

Very few issues are irreconcilable, such as keyword conflicts.

Resources

Thank you!