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The different kinds of variables in a Java program

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• Different kinds of information
  • Computer program needs information to operate correctly

    The information used by computer programs are stored inside variables

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  • We can divide the information broadly into:

    Short term information Long(er) term information

    As a result, computer programs use different kinds of variables specialized to store short term and long term information

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  • Furthermore, because variables can take up a lot of RAM memory, we organize the storage of the different kinds of variable in the RAM memory.

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• The different kinds of variables in a Java program
  • Java has 4 different kinds of variables (See: click here)

    Class variables Instance variables Local variables Parameter variables

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  • General information on variables:

    Each kind of variable has it's own characteristics (properties) The kind of variable is determined by: Where the variable is defined Whether the keyword static was used in the variable definition.

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• Kind and Type
  • I deliberately used the term "kind" of variable to distiguish this property from the "type" of the variable

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  • Kind and type:

    The type of a variable determines the encoding method used to interprete the information stored inside a variable The kind of a variable determines the life time and scope of a variable

    The life time and scope of variables will be discussed later.

    What I want to make clear here is:

    kind of variable ≠ type of variable

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• Properties of variables
  • Every variable has 2 properties:

    life time = the duration that a variable exists scope = the region in the program where the variable is accessible (can be used)

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  • Note:

    The different kinds of variables (class, instance, local and parameter variables) have different life time durations and different scopes

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• Why do programs have different kinds of variables
  • The reason is to accommodate different needs:

    Short term information: Some information need only be retained within a (one) method (We call this kind of information local information) Long term information: Some information must be retained across several methods

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  • Local variables and parameter variables are used to store local (or short term) information

  • Class variables and instance variables are used to store longer term information