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Variables

Variables

Variables store data that your code can use.

There are two fundamental categories of variables, primitive data and references:

  • With primitive data, the compiler names a memory location and uses itto store the actual data – numeric values such as integers, floating point values, and the code values of single individual text characters are stored as primitives.
  • With references, the data is accessed indirectly – the compiler selects a memory location, associates it with the variable name, and stores in it a value that is effectively the memory address of the actual data – in Java, all objects and arrays are stored using references.

In the diagram below, the boxes are areas in memory:

Object storage and reference variable

Declaring Variables

Variables must be declared before they are used.

A declaration informs the compiler that you wish to:

  • Create an identifier that will be accepted by the compiler within a section of code (exactly what that section is depends on how and where the variable is declared; that is the concept of scope, which will be addressed later).
  • Associate that identifier with a specified type of data, and enforce restrictions related to that in the remainder of your code.
  • Create a memory location for the specified type of data.
  • Associate the identifier with that memory location.

Java uses many specific data types; each has different properties in terms of size required and handling by the compiler:

  • The declaration specifies the name and datatype.
  • Generally the declaration also defines the variable, meaning that it causes the compiler to allocate storage space in memory of the requested type’s size.
  • A declaration may also assign an initial value (to initialize the variable).
  • Multiple variables of the same type can be declared in a comma-separated list.
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