SE450: Java: Identifiers [13/22]

One of the major advantages of assembly language over machine language is the ability to refer to data memory using names, or identifiers.

Some important properties of identifiers:

• scope
• value versus reference

References are sometimes referred to as boxed values.

Plain values are then called unboxed.

A declaration is a statement that, when executed, binds a name to a data-memory location.

The scope of a declaration is the text of the program in which the binding has effect.

Lifetime is a dynamic property. Scope is a static property.

Value/Reference is really part of the type, but it is worth discussing separately.

A variable of value type T (synonym: unboxed type) stores a value of type T.

A variable of reference type T (synonyms: pointer type, boxed, type) stores a pointer to a value of type T.

In java, all base-type identifiers hold values; all object-type identifiers hold references.

int x = 0;         // value type (unboxed)
String s = "dog";  // reference type (boxed)

primitive types are

• Boolean type:boolean
• integer types: byte, short, int, long
• Character types: char
• floating point types: float, double

Java has wrapper classes in the java.lang package that wrap all the primitive types.

String in Java is not a primitive type, but a class. However it behaves like a special class, allowing string concatination using primitive operators.

In Java, objects are always boxed (ie, identified by reference).

In C++, objects of any class may be boxed or unboxed. For example:

string* sp = new string("dog"); // boxed (requires delete)
string  s  = string("cat");     // unboxed (no delete needed)