Creating Objects

In Flix, we can create objects using syntax similar to Java.

For example:

import java.io.File

def main(): Unit \ IO = 
    let f = new File("foo.txt");
    println("Hello World!")

Here we import the java.io.File class and instantiate a File object by calling one of its constructors using the new keyword.

The File class has multiple constructors, so we can also write:

import java.io.File

def main(): Unit \ IO = 
    let f1 = new File("foo.txt");
    let f2 = new File("bar", "foo.txt");
    println("Hello World!")

Flix resolves the constructor based on the number of arguments and their types.

As another example, we can write:

import java.io.File
import java.net.URI

def main(): Unit \ IO = 
    let f1 = new File("foo.txt");
    let f2 = new File("bar", "foo.txt");
    let f3 = new File(new URI("file://foo.txt"));
    println("Hello World!")

We can use a renaming import to resolve a clash between a Java name and a Flix module:

import java.lang.{String => JString}

def main(): Unit \ IO = 
    let s = new JString("Hello World");
    println("Hello World!")

Here JString refers to the Java class java.lang.String whereas String refers to the Flix module. Note that internally Flix and Java strings are the same.

Calling Super Constructors

When creating an anonymous subclass of a Java class, we can define a constructor that calls the parent constructor using super:

import java.lang.Thread

def main(): Unit \ IO =
    let t = new Thread {
        def new(): Thread \ IO = super("my-thread")
        def run(_this: Thread): Unit \ IO =
            println("Hello from ${Thread.currentThread().getName()}")
    };
    t.start()

Here we extend Thread and pass "my-thread" as the thread name to the parent constructor. The constructor is defined with def new() and its body must be exactly a super(...) call. At most one constructor can be defined per new expression.

If no constructor is defined, Flix automatically calls the parent’s no-argument constructor.

Note: Any interaction with Java code always has the IO effect.

Note: In Flix, Java classes must be imported before they can be used. In particular, we cannot write new java.io.File(...).