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OOP (basics (variables and expressions (variable name or identifier…

- - - - 0-9
      - first character of variable name can't be a number
      - a-z
      - _
      - unlimited length of variable name
      - case sensitive
      - keywords
        
        abstract, boolean, break, byte, case, catch, char, class, continue, default, do, double, else, enum, extends, final, finally, float, for, if, implements, import, instanceof, int, interface, long, native, new, null, package, private, protected, public, return, short, static, strictfp, super, switch, synchronized, this, throw, throws, transient, try, void, volatile, while
        
        if
        
        implements
        
        for
        
        import
        
        float
        
        instanceof
        
        finally
        
        int
        
        final
        
        refererred to a varible means that the program can't modify the value of the variable
        
        interface
        
        extends
        
        long
        
        enum
        
        native
        
        else
        
        new
        
        double
        
        null
        
        do
        
        package
        
        default
        
        private
        
        continue
        
        protected
        
        class
        
        public
        
        referred to a variable means that the variable can be used everywhere
        
        char
        
        return
        
        catch
        
        short
        
        case
        
        static
        
        byte
        
        strictfp
        
        break
        
        super
        
        boolean
        
        switch
        
        abstract
        
        synchronized
        
        this
        
        while
        
        throw
        
        volatile
        
        throws
        
        void
        
        transient
        
        try
        
        not keywords
        
        main, println
      - variable declared but not initialized: int sum;
        
        primitive type: if the variable hasn't been initialized has a default value
        
        class type: if the variable hasn't been initialized has no value.
        
        declaration and initialization combination
        
        int sum = 0, difference;
      - variable declared and initialized: int sum = 0;
    - - class type
      - primitive types
        
        long
        
        there is a default value if the variable hasn't been initialized
        
        float
        
        there is a default value if the variable hasn't been initialized
        
        float notation types
        
        example: 0,00481 -> float sum = 4.81e-3; (4.81e-3 -> 4,81x10^-3)
        
        example: 0,00481 -> float sum = 0.00481; (0,00481 -> 0,00481)
        
        approximation: for example 0,3 with a periodic 3 it's stored with a limited number of repated 3 after decimal point.
        
        int
        
        there is a default value if the variable hasn't been initialized
        
        short
        
        there is a default value if the variable hasn't been initialized
        
        char
        
        there is a default value if the variable hasn't been initialized
        
        byte
        
        there is a default value if the variable hasn't been initialized
        
        boolean
        
        there is a default value if the variable hasn't been initialized
        
        double
        
        there is a default value if the variable hasn't been initialized
    - - int
      - long
      - short
      - float
      - byte
      - double
      - automatic conversion types
        
        example: double sum = 7; The value 7 will be automatically converted to a double type.
        
        example: int sum = 7; double total = sum; The value 7 will be automatically converted to a double type.