Java Notes

Local/Instance/Class Variables

There are three kinds of Java variables:

  1. Local variables are declared in a method, constructor, or block. When a method is entered, an area is pushed onto the call stack. This area contains slots for each local variable and parameter. When the method is called, the parameter slots are initialized to the parameter values. When the method exits, this area is popped off the stack and the memory becomes available for the next called method. Parameters are essentially local variables which are initialized from the actual parameters. Local variables are not visible outside the method.
  2. Instance variables are declared in a class, but outside a method. They are also called member or field variables. When an object is allocated in the heap, there is a slot in it for each instance variable value. Therefore an instance variable is created when an object is created and destroyed when the object is destroyed. Visible in all methods and constructors of the defining class, should generally be declared private, but may be given greater visibility.
  3. Class/static variables are declared with the static keyword in a class, but outside a method. There is only one copy per class, regardless of how many objects are created from it. They are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.

characteristic Local variable Instance variable Class variable
Where declared In a method, constructor, or block. In a class, but outside a method. Typically private. In a class, but outside a method. Must be declared static. Typically also final.
Use Local variables hold values used in computations in a method. Instance variables hold values that must be referenced by more than one method (for example, components that hold values like text fields, variables that control drawing, etc), or that are essential parts of an object's state that must exist from one method invocation to another. Class variables are mostly used for constants, variables that never change from their initial value.
Lifetime Created when method or constructor is entered.

Destroyed on exit.

Created when instance of class is created with new.
Destroyed when there are no more references to enclosing object (made available for garbage collection).
Created when the program starts.
Destroyed when the program stops.
Scope/Visibility Local variables (including formal parameters) are visible only in the method, constructor, or block in which they are declared. Access modifiers (private, public, ...) can not be used with local variables. All local variables are effectively private to the block in which they are declared. No part of the program outside of the method / block can see them. A special case is that local variables declared in the initializer part of a for statement have a scope of the for statement. Instance (field) variables can been seen by all methods in the class. Which other classes can see them is determined by their declared access:

private should be your default choice in declaring them. No other class can see private instance variables. This is regarded as the best choice. Define getter and setter methods if the value has to be gotten or set from outside so that data consistency can be enforced, and to preserve internal representation flexibility.

Default (also called package visibility) allows a variable to be seen by any class in the same package. private is preferable.

public. Can be seen from any class. Generally a bad idea.

protected variables are only visible from any descendant classes. Uncommon, and probably a bad choice.

Same as instance variable, but are often declared public to make constants available to users of the class.
Declaration Declare before use anywhere in a method or block. Declare anywhere at class level (before or after use). Declare anywhere at class level with static.
Initial value None. Must be assigned a value before the first use. Zero for numbers, false for booleans, or null for object references. May be assigned value at declaration or in constructor. Same as instance variable, and it addition can be assigned value in special static initializer block.
Access from outside Impossible. Local variable names are known only within the method. Instance variables should be declared private to promote information hiding, so should not be accessed from outside a class. However, in the few cases where there are accessed from outside the class, they must be qualified by an object (eg, myPoint.x). Class variables are qualified with the class name (eg, Color.BLUE). They can also be qualified with an object, but this is a deceptive style.
Name syntax Standard rules. Standard rules, but are often prefixed to clarify difference from local variables, eg with my, m, or m_ (for member) as in myLength, or this as in this.length. static public final variables (constants) are all uppercase, otherwise normal naming conventions. Alternatively prefix the variable with "c_" (for class) or something similar.