Computational Problem Solving with Java and Interactive Object Graphics

Section 2.13 Key Concepts

1. Java defines primitive types, which may be organized into four groups:

   integer number types	byte, short, int, long
   floating point number types	float, double
   boolean type	boolean
   a single character type	char

2. Java has an additional type called String, which is a class, not a primitive. What makes String unique is that Java provides a literal notation for creating new String objects. Merely writing a sequence of characters surrounded by a pair of double-quotes (") is enough to create a String in Java. This makes String objects easy to create and use in expressions. We’ll learn much more about String objects.
3. Using Java we can write down arithmetic, and arithmetic-like, expressions. Java will automatically evaluate these expressions, producing a final value with a well-defined type.
4. The rules for evaluating expressions are nearly the same as the standard rules used to evaluate mathematical expressions. For example, recall PEMDAS.
5. All data items in Java have a type, which informs Java about the memory required to store it. The value resulting from evaluating an expression also has a type, which informs Java how to store it.
6. Java implements all standard mathematical operators. These include +, -, * (multiplication), and / (division). Java also implements the % operator (modulo), which produces the remainder after Euclidian division (division with a remainder).
7. The operation performed by an operator on its operands may change, depending upon the types of the operands. For example, Java has two implemenations of the / operator: one that operates on float point operands producing a floating point result, and another that operates on int operands producing an integer result.
8. Another special operator is +. This operator works as you expect with any number type. But, if one operand is a String, then Java changes the meaning of + from addition to String concatenation (it joins the two Strings producing one new String). You must identify the operand types of a + operator to predict the result.
9. Java will operate only on operands having the same type. For example, Java will not add an int and long. Similarly, Java will not add a float and a double. Before evaluating an expression with different operand types, Java attempts to promote the type automatically of one of the operands to match the type of the other. These type changes are called widening conversions because more memory is used to store the converted data compared to the original data (the memory required widens).
10. There are rules for what types may be promoted automatically during expression evaluation. Types that may be promoted automatically during expression evaluation are summarized in the following table.

    FROM	TO
    byte	short, int, long, float, double
    short	int, long, float, double
    int	long, float, double
    long	float, double
    float	double
    char	int, long, float, double

11. There is a way to force data type conversions in the other direction. That is, to force data into a smaller amount of memory. These conversions are called narrowing conversions because less memory is used to store the converted data as compared to the original data (the amount of memory narrows).
12. One way to force a narrowing conversion is called a cast. We say that the type of one data value is cast to another.
13. There are rules for which primitive types may be cast to another. These are summarized in the following table.

    FROM	TO
    byte	char
    short	byte, char
    int	byte, short, char
    long	byte, short, char, int
    float	byte, short, char, int, long
    double	byte, short, char, int, long, float
    char	byte, short

14. To cast data from one type to another, precede the given value, variable, or expression with parentheses containing the new type. For example, to convert a long to an int, precede a variable, value or expression with (int).

    jshell> (int)123L
    $2 ==> 123
    | created scratch variable $2 : int
    To convert a double to a float, precede a variable,
    value, or expression with (float)
    jshell> (float)12.34
    $3 ==> 12.34
    | created scratch variable $3 : float
    

15. Java provides ways of writing down literal values with many (but not all) primitive types and a String. You can look at data in a Java expression and identify its type by the way it is written.
16. We may declare variables to hold data of any type. Notation for variable declaration is the data type followed by the new variable name. Variables may be used in any expression in place of values. Examples of variable declarations include:

    int i;
    double x;
    String name;
    boolean result;
    

17. Variables may be initialized with a value at the same time they are declared. For example:

    int i = 1;
    double x = 3.1415926;
    String name = "Roscoe";
    boolean result = true;
    

18. Comments are text that are ignored by Java. They are useful for describing a program. Java comments come in two forms. In the first form, Java ignores any text between /* and */. Comments of this form may span any number of lines. In second form of comment, Java ignores everything from the characters // through the end of a line.