JsInterop.java

// in JavaScript
com = { acme: {} };
com.acme.Foo = function() {
  this.x = 40;
  this.y = 2;
};
com.acme.Foo.prototype.sum = function() { return this.x + this.y; };

// in Java
package com.acme;

@JsType(isNative = true)
class Foo {
  public int x;
  public int y;

  public native int sum();
}

class FooMain {
  public static void main() {
    Foo foo = new Foo();
    foo.sum(); // will return 42!
    foo.x = 50;
    foo.y = 5;
    foo.sum(); // will return 55!
  }
}