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.

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.time.*
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 Java, array initializers take either of these two forms:

int[] array = {1, 2, 3};             // Java array initializer shorthand syntax
int[] array2 = new int[] {4, 5, 6};  // Java array initializer long syntax

In Groovy, the { … } block is reserved for closures. That means that you cannot create array literals using Java’s array initializer shorthand syntax. You instead borrow Groovy’s literal list notation like this:

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

For Groovy 3+, you can optionally use the Java’s array initializer long syntax:

def array2 = new int[] {1, 2, 3} // Groovy 3.0+ supports the Java-style array initialization long syntax

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

Java 7 introduced ARM (Automatic Resource Management) blocks (also know as try-with-resources) blocks like this:

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();
}

Such blocks are supported from Groovy 3+. However, Groovy provides various methods relying on closures, which have the same effect while being more idiomatic. For example:

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
   }
}