FieldDeclaration

A FieldDeclaration is a construct used in object-oriented programming languages to declare one or more member variables of a type within a class, struct, or interface. It specifies the variable type, one or more variable names, and optional initializer expressions, along with optional modifiers such as access control and storage semantics. In many languages, several fields can be declared in a single declaration, separated by commas.

The exact syntax varies by language, but common elements include an optional set of modifiers, a type, and one or more declarators. For example, in Java a field declaration might look like private int x; or public static final int MAX_SIZE = 100; and you can declare multiple fields in one line, such as int a = 1, b = 2; In C#, a typical field declaration is private static int count = 0; with modifiers such as private, public, protected, static, readonly, or const. Some languages also support attributes or annotations applied to a field and allow initialization at the point of declaration.

Semantics typically distinguish instance fields from static fields, affecting lifetime and storage. Instance fields belong to an object instance, while static fields are shared by all instances of the type. Access modifiers control visibility, and language-specific qualifiers (like final in Java or readonly in C#) constrain mutability. Default initialization rules also vary: many languages automatically initialize fields to a default value, whereas local variables often require explicit initialization.

Field declarations are a foundational element of a type’s state and are often complemented by properties or accessors to enforce encapsulation and invariants. They are distinct from local variable declarations and from method parameters, though they share similar syntax concepts.