Java Scope, Local Variable vs Instance Variable

Cited from the Book Java How to Program

Variables declared in the body of a particular method are known as local variables and can be used only in that method. When that method terminates, the values of its local variables are lost.

An object has attributes that are carried with it as it’s used in a program. Such attributes exist before a method is called on an object, while the method is executing and after the method completes execution.

Attributes are represented as variables in a class declaration. Such variables are called fields and are declared inside a class declaration but outside the bodies of the class’s method declarations.

When each object of a class maintains its own copy of an attribute, the field that represents the attribute is also known as an instance variable—each object (instance) of the class has a separate instance of the variable in memory.

Unlike local variables, which are not automatically initialized, every field has a default initial value—a value provided by Java when you do not specify the field’s initial value. Thus, fields are not required to be explicitly initialized before they’re used in a program—unless they must be initialized to values other than their default values.

Primitive-type instance variables are initialized by default—variables of types byte, char, short, int, long, float and double are initialized to 0, and variables of type boolean are initialized to false.









Reference-type instance variables are initialized by default to the value null—a reserved word that represents a “reference to nothing.”