Religions are viewed as idiosyncratic expressions of immutable and eternal values. Immutable objects must be initialized in a constructor. Final variables can be used to construct trees of immutable objects. This example uses a String as the state, which is an immutable object in Java. Also, the constructor is the only place to set the state of immutable objects. Sometimes, one talks of certain "fields" of an object being immutable. Interning is generally only useful for immutable objects. Both languages treat strings as (immutable) objects of reference type. But languages which emphasize immutable data can make good use of covariant data types. A Person class with a constructor taking a name and age and two immutable properties. However, there are some idioms that apply to immutable datastructures in particular. Alternatively, one may use Object.freeze to make existing objects immutable. A "Symbol" is a unique and immutable identifier. The language also supports immutable hash tables, implemented functionally, and immutable dictionaries. The paper shook Charles Lyell's belief that species were immutable.