A field variable is a variable that is declared as a member of a class.
A local variable is a variable that is declared local to a method.
In Core Java, the main difference between a field variable (also known as an instance variable or member variable) and a local variable lies in their scope and where they are declared within a class or method:
- Scope:
- Field Variable: Declared at the class level, outside of any method, constructor, or block. It is accessible throughout the entire class and is associated with an instance of the class.
- Local Variable: Declared within a method, constructor, or block. It is only accessible within the scope where it is defined and has a limited lifetime, existing only as long as the method, constructor, or block is executing.
- Lifetime:
- Field Variable: Exists as long as the instance of the class exists. It is created when an object of the class is created and is destroyed when the object is garbage collected.
- Local Variable: Exists only for the duration of the method, constructor, or block in which it is declared. It is created when the method is called and is destroyed when the method exits.
- Initialization:
- Field Variable: May be automatically initialized with default values (e.g., 0 for numeric types, null for objects) if not explicitly initialized by the programmer.
- Local Variable: Must be explicitly initialized before use. The compiler does not provide default values for local variables.
Here’s a simple example illustrating the difference:
java
public class Example {
// Field variable
private int fieldVariable;// Local variable
int localVariable = 10;
fieldVariable = 20;
System.out.println(localVariable);
}
}
In this example, fieldVariable is a field variable, and localVariable is a local variable. The field variable is associated with an instance of the class, while the local variable exists only within the exampleMethod method.