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= Differences with Java

Groovy tries to be as natural as possible for Java developers. We’ve
tried to follow the principle of least surprise when designing Groovy,
particularly for developers learning Groovy who’ve come from a Java
background.

Here we list all the major differences between Java and Groovy.

== Default imports

All these packages and classes are imported by default, i.e. you do not
have to use an explicit `import` statement to use them:

* java.io.*
* java.lang.*
* java.math.BigDecimal
* java.math.BigInteger
* java.net.*
* java.util.*
* groovy.lang.*
* groovy.util.*

== Multi-methods

In Groovy, the methods which will be invoked are chosen at runtime. This is called runtime dispatch or multi-methods. It
means that the method will be chosen based on the types of the arguments at runtime. In Java, this is the opposite: methods
are chosen at compile time, based on the declared types.

The following code, written as Java code, can be compiled in both Java and Groovy, but it will behave differently:

[source,java]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=multimethods,indent=0]
----

In Java, you would have:

[source,java]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=multimethods_java,indent=0]
----

Whereas in Groovy:

[source,java]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=multimethods_groovy,indent=0]
----

That is because Java will use the static information type, which is that `o` is declared as an `Object`, whereas
Groovy will choose at runtime, when the method is actually called. Since it is called with a `String`, then the
`String` version is called.

== Array initializers

In Groovy, the `{ ... }` block is reserved for closures. That means that you cannot create array literals with this
syntax:

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=arraycreate_fail,indent=0]
----

You actually have to use:

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=arraycreate_success,indent=0]
----

== Package scope visibility

In Groovy, omitting a modifier on a field doesn't result in a package-private field like in Java:

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=packageprivate_property,indent=0]
----

Instead, it is used to create a _property_, that is to say a _private field_, an associated _getter_ and an associated
_setter_.

It is possible to create a package-private field by annotating it with `@PackageScope`:

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=packageprivate_field,indent=0]
----

== ARM blocks

ARM (Automatic Resource Management) block from Java 7 are not supported in Groovy. Instead, Groovy provides various
methods relying on closures, which have the same effect while being more idiomatic. For example:

[source,java]
----
Path file = Paths.get("/path/to/file");
Charset charset = Charset.forName("UTF-8");
try (BufferedReader reader = Files.newBufferedReader(file, charset)) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }

} catch (IOException e) {
    e.printStackTrace();
}
----

can be written like this:

[source,groovy]
----
new File('/path/to/file').eachLine('UTF-8') {
   println it
}
----

or, if you want a version closer to Java:

[source,groovy]
----
new File('/path/to/file').withReader('UTF-8') { reader ->
   reader.eachLine {
       println it
   }
}
----

== Inner classes

WARNING: The implementation of anonymous inner classes and nested classes follows the Java lead, but
you should not take out the Java Language Spec and keep shaking the head
about things that are different. The implementation done looks much like
what we do for `groovy.lang.Closure`, with some benefits and some
differences. Accessing private fields and methods for example can become
a problem, but on the other hand local variables don’t have to be final.

=== Static inner classes

Here’s an example of static inner class:

[source,groovy]
---------------------
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=innerclass_1,indent=0]
---------------------

The usage of static inner classes is the best supported one. If you
absolutely need an inner class, you should make it a static one.

=== Anonymous Inner Classes

[source,groovy]
---------------------
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=innerclass_2,indent=0]
---------------------

=== Creating Instances of Non-Static Inner Classes

In Java you can do this:

[source,java]
----------------------------------
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=innerclass_3_java,indent=0]
----------------------------------

Groovy doesn't support the `y.new X()` syntax. Instead, you have to write `new X(y)`, like in the code below:

[source,groovy]
----------------------------------
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=innerclass_3,indent=0]
----------------------------------

[WARNING]
Caution though, Groovy supports calling methods with one
parameter without giving an argument. The parameter will then have the
value null. Basically the same rules apply to calling a constructor.
There is a danger that you will write new X() instead of new X(this) for
example. Since this might also be the regular way we have not yet found
a good way to prevent this problem.

== Lambda expressions and the method reference operator

Java 8+ supports lambda expressions and the method reference operator (`::`):

[source,java]
----
Runnable run = () -> System.out.println("Run");  // Java
list.forEach(System.out::println);
----

Groovy 3 and above also support these within the Parrot parser.
In earlier versions of Groovy you should use closures instead:

[source,groovy]
----
Runnable run = { println 'run' }
list.each { println it } // or list.each(this.&println)
----


== GStrings

As double-quoted string literals are interpreted as `GString` values, Groovy may fail
with compile error or produce subtly different code if a class with `String` literal
containing a dollar character is compiled with Groovy and Java compiler.

While typically, Groovy will auto-cast between `GString` and `String` if an API declares
the type of a parameter, beware of Java APIs that accept an `Object` parameter and then
check the actual type.

== String and Character literals

Singly-quoted literals in Groovy are used for `String`, and double-quoted result in
`String` or `GString`, depending whether there is interpolation in the literal.

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=type_depends_on_quoting_AND_whether_we_actually_interpolate,indent=0]
----

Groovy will automatically cast a single-character `String` to `char` only when assigning to
a variable of type `char`. When calling methods with arguments of type `char` we need
to either cast explicitly or make sure the value has been cast in advance.

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=single_char_strings_are_autocasted,indent=0]
----

Groovy supports two styles of casting and in the case of casting to `char` there
are subtle differences when casting a multi-char strings. The Groovy style cast is
more lenient and will take the first character, while the C-style cast will fail
with exception.

[source,groovy]
----
include::{projectdir}/src/spec/test/DifferencesFromJavaTest.groovy[tags=chars_c_vs_groovy_cast,indent=0]
----

== Primitives and wrappers

Because Groovy uses Objects for everything, it link:core-object-orientation.html#_primitive_types[autowraps] references
to primitives. Because of this, it does not follow Java's behavior of widening taking priority over boxing.
Here's an example using `int`

[source,groovy]
----
include::{projectdir}/src/spec/test/PrimitiveTest.groovy[tags=widening_vs_boxing,indent=0]
----
<1> This is the method that Java would call, since widening has precedence over unboxing.
<2> This is the method Groovy actually calls, since all primitive references use their wrapper class.

== Behaviour of `==`

In Java `==` means equality of primitive types or identity for objects.  In
Groovy `==` translates to `a.compareTo(b)==0`, if they are `Comparable`, and
`a.equals(b)` otherwise.  To check for identity, there is `is`.  E.g.
`a.is(b)`.

== Conversions

Java does automatic widening and narrowing
https://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html[conversions].

[cols=">2s,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a"]
.Java Conversions
|============================================================================================
|             8+|*Converts to*
| Converts from | *boolean* | *byte* | *short* | *char* | *int* | *long* | *float* | *double*
| boolean       | -         | N      | N       | N      | N     | N      | N       | N
| byte          | N         | -      | Y       | C      | Y     | Y      | Y       | Y
| short         | N         | C      | -       | C      | Y     | Y      | Y       | Y
| char          | N         | C      | C       | -      | Y     | Y      | Y       | Y
| int           | N         | C      | C       | C      | -     | Y      | T       | Y
| long          | N         | C      | C       | C      | C     | -      | T       | T
| float         | N         | C      | C       | C      | C     | C      | -       | Y
| double        | N         | C      | C       | C      | C     | C      | C       | -
|============================================================================================
^*^ 'Y' indicates a conversion Java can make, 'C' indicates a conversion Java can make when there is an explicit cast,
'T` indicates a conversion Java can make but data is truncated, 'N' indicates a conversion Java can't make.

Groovy expands greatly on this.

[cols=">2s,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a,^1a"]
.Groovy Conversions
|=================================================================================================================================================================================================================
|            18+|*Converts to*
| Converts from | *boolean* | *Boolean* | *byte* | *Byte* | *short* | *Short* | *char* | *Character* | *int* | *Integer* | *long* | *Long* | *BigInteger* | *float* | *Float* | *double* | *Double* | *BigDecimal*
| boolean       | -         | B         | N      | N      | N       | N       | N      | N           | N     | N         | N      | N      | N            | N       | N       | N        | N        | N
| Boolean       | B         | -         | N      | N      | N       | N       | N      | N           | N     | N         | N      | N      | N            | N       | N       | N        | N        | N
| byte          | T         | T         | -      | B      | Y       | Y       | Y      | D           | Y     | Y         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| Byte          | T         | T         | B      | -      | Y       | Y       | Y      | D           | Y     | Y         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| short         | T         | T         | D      | D      | -       | B       | Y      | D           | Y     | Y         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| Short         | T         | T         | D      | T      | B       | -       | Y      | D           | Y     | Y         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| char          | T         | T         | Y      | D      | Y       | D       | -      | D           | Y     | D         | Y      | D      | D            | Y       | D       | Y        | D        | D
| Character     | T         | T         | D      | D      | D       | D       | D      | -           | D     | D         | D      | D      | D            | D       | D       | D        | D        | D
| int           | T         | T         | D      | D      | D       | D       | Y      | D           | -     | B         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| Integer       | T         | T         | D      | D      | D       | D       | Y      | D           | B     | -         | Y      | Y      | Y            | Y       | Y       | Y        | Y        | Y
| long          | T         | T         | D      | D      | D       | D       | Y      | D           | D     | D         | -      | B      | Y            | T       | T       | T        | T        | Y
| Long          | T         | T         | D      | D      | D       | T       | Y      | D           | D     | T         | B      | -      | Y            | T       | T       | T        | T        | Y
| BigInteger    | T         | T         | D      | D      | D       | D       | D      | D           | D     | D         | D      | D      | -            | D       | D       | D        | D        | T
| float         | T         | T         | D      | D      | D       | D       | T      | D           | D     | D         | D      | D      | D            | -       | B       | Y        | Y        | Y
| Float         | T         | T         | D      | T      | D       | T       | T      | D           | D     | T         | D      | T      | D            | B       | -       | Y        | Y        | Y
| double        | T         | T         | D      | D      | D       | D       | T      | D           | D     | D         | D      | D      | D            | D       | D       | -        | B        | Y
| Double        | T         | T         | D      | T      | D       | T       | T      | D           | D     | T         | D      | T      | D            | D       | T       | B        | -        | Y
| BigDecimal    | T         | T         | D      | D      | D       | D       | D      | D           | D     | D         | D      | D      | D            | T       | D       | T        | D        | -
|=================================================================================================================================================================================================================
^*^ 'Y' indicates a conversion Groovy can make, 'D' indicates a conversion Groovy can make when compiled dynamically or
explicitly cast, 'T` indicates a conversion Groovy can make but data is truncated, 'B' indicates a boxing/unboxing
operation, 'N' indicates a conversion Groovy can't make.

The truncation uses <<core-semantics.adoc#Groovy-Truth,Groovy Truth>> when converting to `boolean`/`Boolean`. Converting
from a number to a character casts the `Number.intvalue()` to `char`. Groovy constructs `BigInteger` and `BigDecimal`
using `Number.doubleValue()` when converting from a `Float` or `Double`, otherwise it constructs using `toString()`.
Other conversions have their behavior defined by `java.lang.Number`.

== Extra keywords

There are a few more keywords in Groovy than in Java.  Don't use them for
variable names etc.

* `as`
* `def`
* `in`
* `trait`