Question

One of the important parts of object-oriented programming is encapsulation, but public properties / fields tend to break this encapsulation. Under what circumstances does a public property or field actually make sense?

Note: I only use the term ‘property’ or ‘field’ because terminology varies between languages. In general, I mean a variable that belongs to an object that can be accessed and set from outside the object.

Answer 1

Yes, there are sometimes good reasons. Information hiding is usually desirable. But there are some occasional exceptions.

For example, public fields are reasonable and useful for:

• A C++ pimpl – a struct/class holding the private implementation of another class. Its fields may be declared public syntatically, but are typically accessible only within one source file, by the class holding the pimpl.
• Constant fields. For example, Joshua Bloch writes in Effective Java: “Classes are permitted to expose constants via public static final fields.”
• Structs used for communication between C and C++.
• Types which represent only data, whose representation is unlikely to change. For example, javax.vecmath.Point3d, which represents an {x,y,z} coordinate.