# Differences with Java

version 2.5.21

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.

## 1. 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.*

## 2. 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:

    int method(String arg) {
        return 1;
    }
    int method(Object arg) {
        return 2;
    }
    Object o = "Object";
    int result = method(o);

In Java, you would have:

    assertEquals(2, result);

Whereas in Groovy:

    assertEquals(1, result);

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.

## 3. Array initializers

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

    int[] array = { 1, 2, 3}

You actually have to use:

    int[] array = [1,2,3]

## 4. Package scope visibility

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

    class Person {
        String name
    }

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`:

    class Person {
        @PackageScope String name
    }

## 5. 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:

    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:

    new File('/path/to/file').eachLine('UTF-8') {
       println it
    }

or, if you want a version closer to Java:

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

## 6. Inner classes

| 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.

### 6.1. Static inner classes

Here’s an example of static inner class:

    class A {
        static class B {}
    }

    new A.B()

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.

### 6.2. Anonymous Inner Classes

    import java.util.concurrent.CountDownLatch
    import java.util.concurrent.TimeUnit

    CountDownLatch called = new CountDownLatch(1)

    Timer timer = new Timer()
    timer.schedule(new TimerTask() {
        void run() {
            called.countDown()
        }
    }, 0)

    assert called.await(10, TimeUnit.SECONDS)

### 6.3. Creating Instances of Non-Static Inner Classes

In Java you can do this:

    public class Y {
        public class X {}
        public X foo() {
            return new X();
        }
        public static X createX(Y y) {
            return y.new X();
        }
    }

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

    public class Y {
        public class X {}
        public X foo() {
            return new X()
        }
        public static X createX(Y y) {
            return new X(y)
        }
    }

| 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.

## 7. Lambdas

Java 8 supports lambdas and method references:

    Runnable run = () -> System.out.println("Run");
    list.forEach(System.out::println);

Java 8 lambdas can be more or less considered as anonymous inner classes. Groovy doesn’t support that syntax, but has closures instead:

    Runnable run = { println 'run' }
    list.each { println it } // or list.each(this.&println)

## 8. 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.

## 9. 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.

    assert 'c'.getClass()==String
    assert "c".getClass()==String
    assert "c${1}".getClass() in GString

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.

    char a='a'
    assert Character.digit(a, 16)==10 : 'But Groovy does boxing'
    assert Character.digit((char) 'a', 16)==10

    try {
      assert Character.digit('a', 16)==10
      assert false: 'Need explicit cast'
    } catch(MissingMethodException e) {
    }

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.

    // for single char strings, both are the same
    assert ((char) "c").class==Character
    assert ("c" as char).class==Character

    // for multi char strings they are not
    try {
      ((char) 'cx') == 'c'
      assert false: 'will fail - not castable'
    } catch(GroovyCastException e) {
    }
    assert ('cx' as char) == 'c'
    assert 'cx'.asType(char) == 'c'

## 10. Primitives and wrappers

Because Groovy uses Objects for everything, it 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`

    int i
    m(i)

    void m(long l) {           (1)
      println "in m(long)"
    }

    void m(Integer i) {        (2)
      println "in m(Integer)"
    }

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.

## 11. 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)`.

## 12. Conversions

Java does automatic widening and narrowing conversions.

Table 1. Java Conversions
| 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.

Table 2. Groovy Conversions
| 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 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`.

## 13. 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`