1. Primitive Data

Computer memory stores arbitrary bit patterns. Making sense of these patterns involves the use of data types.

Chapter Topics:

Data Types
Primitive Data vs. Objects
The Eight Primitive Data Types of Java
Numeric Data Types
Character and Boolean Data Types

Question 1:
Say that you came across the following torn slip of paper. What can it mean?

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap08/ch08_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 License.

2. Data Types

Answer:
Not easy to say, without knowing more.

Data Types

You would like to see the rest of the paper, or know where it came from. Without knowing the context, it is hard to say what MIX means. It could be 1009 in Roman numerals, or it could be the English word "mix" (which itself has several meanings), or it could be the last name of the old-time radio hero Tom Mix. It could be part of a label on a music CD, "MTV Dance MIX", or part of a label on a bottle, "BLOODY MARY MIX". Or maybe you are looking at it upside down and it should be "XIW". Of course, it might not be English at all. Without knowing the context, a string of letters has little meaning.

Computer memory stores arbitrary bit patterns.

As with a string of letters, the meaning of a string of bits depends on how it is used. The scheme being used for a particular string of bits is its data type.

For example

0000000001100111

is a pattern of 16 bits that might be found somewhere in computer memory. What does it represent?

Without knowing more about how the above pattern is being used, it is impossible to say what it represents. The type short is one of Java's data types. If the pattern is regarded as data type short, then it represents the value 103 (one hundred and three). If the pattern is regarded as data type char , then it represents the character 'g'.

Question 2:
What does the following 16 bit pattern represent?
0000000000000000

3. There are Many Data Types

Answer:
Without knowing more about how the pattern is being used, it is impossible to say.

There are Many Data Types

You might be to tempted say that the pattern 0000000000000000 represents "zero". But it doesn't necessarily. Even such an obvious pattern has no automatic meaning.

If you were told that the above pattern was of type short, then you would know that it represents the integer zero. Here is another pattern:

1111111110011001

As a short, this pattern represents -103 (negative one hundred three).

There are uncountably many types of data that can be represented in the memory of a computer. If specific patterns always had specific meanings, then only a few types of data could be represented. This would be much too restrictive. Many data types are invented by programmers as they write programs. Without this flexibility computers would be much less useful.

The main memory of a general purpose computer (such as a desktop or a laptop computer) contains very many bits. Programs can use this memory with whatever data types they need. For example, one program may use data type int with four bytes of memory, and data type double with another eight bytes of memory. A different program may use the same memory with different data types.

Not all machines use memory this way. A simple electronic calculator, for example, uses memory for one purpose only: to store floating point numbers. It uses only one data type, and can do only those few things with that data type that it has been wired to do. The engineers who designed the calculator decided how to represent numbers with bit strings, and then designed the electronics to work with just those strings. This is too restrictive for a general purpose computer.

Question 3:
Do you imagine that the computers of the 1960's were built to handle audio data?

4. Primitive Data Types

Answer:
No — certainly this was not a common use for computers then.

Primitive Data Types

boolean key words

But some 1960's computers did compute with audio data. The programmers invented a way to represent audio with bit patterns and then wrote programs that were consistent with that scheme.

Recall the fourth advantage of binary (from chapter 2): Anything that can be represented with some sort of pattern can be represented with patterns of bits. Music can certainly be represented with patterns of symbols, and so can be represented with patterns of bits. (This is obvious now, but was not quite so obvious in 1960.)

It would be awkward if every time you used data you had to invent your own scheme to represent it with bits. There are types of data that are so fundamental that ways to represent them are built into Java. These are the primitive data types. The eight primitive data types are: byte, short, int, long, float, double, char, and boolean.

Upper and lower case characters are important in these names. So "byte" is the name of a primitive data type, but "BYTE" is not. Computer languages where case is important are called case sensitive. Some languages are not case sensitive, especially old languages that were designed when data entry equipment did not have lower case characters.

In the phrase primitive data type the word primitive means "a fundamental component that is used to create other, larger parts." This word is used frequently in computer science. To solve a large problem, you look for the primitive operations that are needed, then use them to build the solution.

Question 4:
(Trick Question: ) Is Int a primitive data type?

5. Objects

Answer:
No — it is not on the list of primitive data types (the word that starts with a small "i" 
is on the list: int). Remember that Java is case sensitive.

Objects

dataSplit

All data in Java falls into one of two categories: primitive data and objects. There are only eight primitive data types. However, Java has many types of objects, and you can invent as many others as you need. Any data type you invent will be a type of object.

Much more will be said about objects in future chapters (since Java is an object oriented programming language). The following is all you need to know, for now:

A primitive data value uses a small, fixed number of bytes.
There are only eight primitive data types.
A programmer can not create new primitive data types.

An object is a big block of data. An object may use many bytes of memory.
An object usually consists of many internal pieces.
The data type of an object is called its class.
Many classes are already defined in Java.
A programmer can invent new classes to meet the particular needs of a program.

A (crude) analogy is that a primitive data value is like a nut or a bolt, but an object is like a whole machine. If you want to learn more about objects right away, it would not hurt to jump ahead to the Chapter On Objects and read a few pages.

Question 5:
The following are ALL legitimate data types in Java. Which ones are primitive, 
 and which ones are classes?
int
String
double
Scanner
short

6. Numeric Primitive Data Types

Answer:

int - primitive
String - class
double - primitive
Scanner - class
short - primitive

Numeric Primitive Data Types

Numbers are so important in Java that six of the eight primitive data types are numeric types.

There are both integer and floating point primitive types. Integer types have no fractional part; floating point types have a fractional part. On paper, integers have no decimal point, and floating point types do. But in main memory, there are no decimal points. Even floating point values are represented with bit patterns. There is a fundamental difference between the method used to represent integers and the method used to represent floating point numbers.

Each primitive type uses a fixed number of bits. This means that if you are using a particular data type then the same number of bits will be used no matter what value is represented.

For example, all values represented using the short data type use 16 bits. The value zero (as a short ) uses 16 bits and the value thirty thousand uses 16 bits.

All values represented using the long data type use 64 bits. The value zero (as a long ) uses 64 bits, the value thirty thousand uses 64 bits, and the value eight trillion uses 64 bits.

Values that are large in magnitude (negative or positive) need more bits to be represented. This is similar to writing out numbers on paper: large numbers need more digits. If a value needs more bits than a particular data type uses, then it cannot be represented using that data type.

In the tables, E means "ten to the power of". So 3.5E38 means 3.5 x 10^³⁸

Integer Primitive Data Types

Floating point primitive data points

Question 6:
Say that you want to deal with the number 1,023,004 in your computer program. 
 Would data type short be an appropriate choice?

7. More Bits for More Range

Answer:

No. Data of type short can be only in the range -32,768 to +32,767.

More Bits for More Range

Larger ranges of numeric values require more bits. The different sizes for integer data enable you to pick an appropriate size for the data you are working with. Usually you should pick a data type that has a range much greater than the range of numbers you expect to deal with. If a program uses only a few dozen variables it will run just as fast and take up about as much main memory no matter what size is used for its variables.

Why do the small sized data types exist, then? Well, many real-world programs deal with massive amounts of data (billions of data items) and then using the smaller sizes may save significant amounts of space and time. But we will not use that much data in these notes. Usually you should use int or double for your numeric data.

When you write a program you do not have to know how to represent a number in bits. You can type the number just as you would on a typewriter. This is called a literal. The word "literal" means that a value is explicitly shown in the program.

For example, 125 literally represents the value one hundred twenty five. Integer literals in a program are written as in a book, except there are no commas:

All of the above examples are 32 bit int literals. A 64 bit long literal has an upper case 'L' or lower case 'l' at the end. However, NEVER use the lower case 'l' because it is easily confused with a digit '1'.

The last two examples use lower case 'l' and are very confusing. (The author once spent several hours debugging a program where this was the problem.)

In Java 7 and above, you can place the underscore character _ between digits in a numeric literal. Doing this is optional. Usually you would do this to put digits into groups of three. In the US a comma is used for this; in Europe a space is used for this. Here are more correct literals:

12_340_000

84_034.87

-45_000L

Don't put underscore at the beginning or end of a literal, nor before nor after the decimal point.

Question 7:
Is the following an integer literal?
197.0

8. Floating Point Types

Answer:

No — it has a decimal point.

Floating Point Types

If you use the literal 197.0 in a program, the decimal point tells the compiler to represent the value using a floating point primitive data type. The bit pattern used for floating point 197.0 is very much different than that used for the integer 197. There are two floating point primitive types.

Data type float is sometimes called "single-precision floating point". Data type double has twice as many bits and is sometimes called "double-precision floating point". These phrases come from the language FORTRAN, at one time the dominant programming language.

In source programs, floating point literals always have a decimal point in them, and no commas although you can used underscore to separate thousands:

     123.0       

    -123.5     

 -198234.234   

-198_234.234

 0.00000381

 0.000_003_81

Note: Literals written like the above will automatically be of type double. Almost always, if you are dealing with floating point numbers you should use variables of type double. Then the data type of literals like the above will match the data type of your variables. Data type float should be used only for special circumstances (such as when you need to process a file of data containing 32 bit floats).

Floating point

Question 8:
(Thought question: ) Do you think that using float instead of double saves a significant amount of computer memory?

9. Floating Point Literals

Answer:

No. For most programs using variables of type double will cost only a few extra bytes.
This is insignificant in a program thousands of bytes long.

Floating Point Literals

Sometimes you need to explicitly ask for a single-precision float literal. Do this by putting a lower case f or upper case F at the end, like this:

      123.0f

     -123.5F

-198234.234f

 0.00000381F

Sometimes you need to explicitly ask for a double literal. Do this by putting a lower case d or upper case D at the end, like this:

      123.0d

     -123.5D

-198234.234d

 0.00000381D

Remember, that without any letter at the end, a floating point literal will automatically be of type double .

Question 9:

Do you think that the following is legal?

8912D

10. Scientific Notation

Answer:
Yes. The 'D' will make the literal a double (even though it lacks a decimal point). 
 However, to avoid confusion, always include a decimal point in a floating point literal, even where it is not required.
Common Practice: To avoid errors, always put digits on either side of the decimal point (in programs and in writing.) So
0.1      not .1
12.0     not 12.
8912.0D  not 8912D

Scientific Notation

You will sometimes see scientific notation. The following are all double-precision literals:

 1.23E+02 
-1.235E+02 
-1.98234234E+05 
 3.81E-06

Another way to say this is that the integer that follows "E" says in which direction and for how many places to shift the decimal point. Positive integers mean right shifts; negative integers mean left shifts.

Question 10:

What is the usual way to write this number: 1.9345E+03

11. Precision of Floating Point Numbers

Answer:
1934.5
The +03 says to move the decimal point three places right.

Precision of Floating Point Numbers

Consider writing the value 1/3 in decimal notation:

0.333333333333333333

There is no limit to the number of 3's required for complete accuracy. However, with a limited amount of paper, you can not be completely accurate. With a data type, there is a limited number of bits. Those bits cannot accurately represent a value that requires more than that number of bits.

The data type float has 24 bits of precision. This is equivalent to only about 7 decimal places. (The rest of the 32 bits are used for the sign and size of the number.)

The number of places of precision for float is the same no matter what the size of the number. Data type float can represent numbers as big as about 3.4E+38. But the precision of these large numbers will also be about 7 decimal digits.

Remember: data type float has about the range and precision of a cheap pocket calculator. This is usually not sufficient.

Question 11:

What is wrong with the following constant, expected to be of type float?

1230.00089F

12. Precision of Double

Answer:
There are nine decimal places of precision. Data type float can't handle that. 
 (The compiler will round the number into a value that can fit in a float).

Precision of Double

You might wish to argue that there are only five places used in the above number: the places used by the digits 1, 2, 3, 8, and 9. However, the four 0's in the middle do count. It takes bits to represent them, even if they are zeros.

Primitive data type double uses 64 bits, and has a much greater range, -1.7E+308 to +1.7E+308. It also has a much greater precision: about 15 significant decimal digits.

Because of this, if you write a literal like 2.345 in a Java program, it will automatically be regarded as a double, even though a float might be good enough. The other numbers in the program might need to be double, so we might as well make them all double.

Question 12:

Do you suspect that characters are important enough to be one of the eight primitive data types?

13. The CHAR Primitive Data Type

Answer:
Yes

The char Primitive Data Type

Computer programs frequently work with character data. The primitive data type for characters in Java is named char. The char type represents a character using 16 bits. In many programming languages, only 8 bits are used for this purpose. Java uses 16 bits so that a very large number of characters can be represented, nearly all of the characters in all of the World's languages. The method used is called Unicode.

For example, here is a 16 bit pattern:

0000000001100111

If you know that these 16 bits are of data type char, then you could look in a table and discover that they represent the character 'g'. If you have a really good memory, you might recall that the same 16 bits represent the integer 103 if they are regarded as data type short. Knowing the data type of a pattern is necessary to make sense of it.

Upper and lower case characters are represented by different patterns. Punctuation and special characters are also char data. There are also special characters, like the space character that separates words.

Control characters are bit patterns that show the end of a line or where to start pages. Other control characters represent the mechanical activities of old communications equipment (such as Teletypes) that are rarely used these days. Many of these control characters are no longer used for their original purpose.

Primitive type char represents a SINGLE character. It does not include any font information. When you want to deal with more than one character at a time (almost always), you need to use objects that have been built out of char data.

Question 13:

Is each of the following a different character?

0       O         o 
(Depending on the fonts your browser is using, you may have to look carefully.)

14. Character Literals

Answer:
Yes. Each of the above (zero, Capital 'O', and lower case 'o') is a different character 
 and has its own 16 bit code.

Character Literals

A character literal is a single character with an apostrophe on each side:

'm'       'y'         'A'

A control character is represented with a special sequence of characters:

'\n'       '\t'

Each of these represents a single control character. The first one represents the newline character and the second one represents the tabulation character. You will rarely use any control characters other than these two. Several others are listed in the Java documentation.

Warning: The following is not a character literal:

"Hello"

This is a String, which is not primitive data. It is, in fact, an object. Strings are surrounded by double quote marks ", not by apostrophes.

Question 14:

What is wrong with the following char literal:

"W"

15. Primitive Data Type BOOLEAN

Answer:
The character is not surrounded by apostrophes. It should be: 'W' .
With double quotes, "W", you get a String that contains a single character. This is not the same 
 as a primitive single character. A String is represented as an object and may consist of 
 several hundred bytes. A primitive character data item is always only two bytes.

Primitive Data Type boolean

Another of the primitive data types is the type boolean. It is used to represent a single true/false value. A boolean value can have only one of two values:

true     false

In a Java program, the words true and false always mean these boolean values. The data type boolean is named after George Boole, a nineteenth century mathematician, who discovered that a great many things can be done with true/false values. Although the amount of information in a boolean primitive is logically one bit, for convenience Java uses more than that to represent it.

Question 15:

Would you like a data type named after you someday?

  true     false

16. End of Chapter

Answer:
Maybe.

End of Chapter

You have reached the end of the chapter. You may wish to review the following. Click on a subject that interests you to go to where it was discussed.

data type Data type.
data type, primitive Primitive data type.
case sensitive Case sensitive computer languages.
data, two kinds Objects and primitive data types.
data type, numeric Numeric data types.
literal, integer Integer literals.
literal, floating point Floating point literals.
The precision precision of a floating point number.
Data type characters char.
Data type boolean boolean

The next chapter will discuss variables and assignment statements.

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Primitive Data

Description

Table of contents

1. Primitive Data

Question 1:

2. Data Types

Answer:

Data Types

Question 2:

3. There are Many Data Types

Answer:

There are Many Data Types

Question 3:

4. Primitive Data Types

Answer:

Primitive Data Types

Question 4:

5. Objects

Answer:

Objects

Question 5:

6. Numeric Primitive Data Types

Answer:

Numeric Primitive Data Types

Question 6:

7. More Bits for More Range

Answer:

More Bits for More Range

Question 7:

8. Floating Point Types

Answer:

Floating Point Types

Question 8:

9. Floating Point Literals

Answer:

Floating Point Literals

Question 9:

10. Scientific Notation

Answer:

Scientific Notation

Question 10:

11. Precision of Floating Point Numbers

Answer:

Precision of Floating Point Numbers

Question 11:

12. Precision of Double

Answer:

Precision of Double

Question 12:

13. The CHAR Primitive Data Type

Answer:

The char Primitive Data Type

Question 13:

14. Character Literals

Answer:

Character Literals

Question 14:

15. Primitive Data Type BOOLEAN

Answer:

Primitive Data Type boolean

Question 15:

16. End of Chapter

Answer:

End of Chapter