Java program data is kept in variables. Your Java program first declares the variables, then read data into them, execute operations on the variables, and then write the variables (or data based on the variables) somewhere again. Variables are explained in more detail in the text on Java variables.
Each variable has a data type. The data type determines what kind of data the variable can contain, and what operations you can execute on it. For instance, a variable could be a number. Numbers can be added, subtracted, multiplied, divided etc. Or, a variable could be a string (text). String's can be divided into substrings, searched for characters, concatenated with other strings etc. Java comes with a set of built-in data types. These data types are described in more detail in the text on Java data types.
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