Glasses

A lens library for Java

var lens = Lens.create(Library.class)
    .select(Library::setAddress, Address.class)
    .select(Address::setCity, String.class);
var library = new Library()
    .setAddress(new Address()
        .setCity("Des Plaines")
    );

var city = lens.focus(library);
System.out.println(city.value());  // prints 'Des Plaines'
System.out.println(city.path());   // prints '$.address.city'
    
library.setAddress(null);
city = lens.focus(library);
System.out.println(city.value());  // prints 'null'

city.override("Chicago");
System.out.println(library.getAddress().getCity()); // prints 'Chicago'

Purpose

Lenses are a means to query and modify deep object trees in a type-safe way by relying on class's setters methods to define the path between properties.

Can I Use Lenses?

Maybe! As long as every class in the object model:

• Has a setter for every field
  • TODO Possibly optional for list fields if they are lazy initialized
• Has a no-argument constructor

The reason for needing a setter for every field is this is used by the library to find the associated java field for a getter/setter combo by invoking the setter with a dummy value and then finding that dummy value reflectively

The reason for the no-argument constructor is that many operations in the Lens library require a no-arg constructor on the class for reasons like

• Default constructing intermediate nodes in an object graph when overwriting a property whose parent is not yet created
• Injecting a dummy object into the field of another object and then using reflection to determine which field it got set to. Each dummy must be unique

Essentially you need mutable data classes with no business logic.