Java: Summary - Data

Vanilla Java

The basic types you should know are: int, double, String, boolean, and char. For structuring data you need to know arrays and how to define a class.

Primitive types

Object types

There are 8 primitive types: boolean (true and false); char (Unicode characters); byte, short, int, and long (integers); float and double (floating point). Object types are created whenever a class is defined. Object type names start with an uppercase character (eg, String). The most commonly used pre-defined object type is String.

Integers - primitive types

Integers - object types

The integer types byte, short, int, and long are stored as two's complement, signed binary integers. char, which is technically also an integer type, is stored as an unsigned binary number. Expressions are computed as ints, so a cast is needed to store in a smaller type.

Type	Bytes	Range	Literals
byte	1	-128..+127	none
short	2	-32,768..+32,767	none
int	4	-2,147,483,648..+2,147,483,647	23, 0xAF
long	8	-9,223,372,036,854,775,808..	23L, 0xAFL

Operators: arithmetic, comparison, bitwise, assignment.
Utility methods: Math class.

The "wrapper" classes correspond to each primitive integer types: Integer (not Int), Short, Byte, Long. Objects of these classes are immutable -- they can not be changed after they are created. The wrapper classes contain utility methods such as conversions between String and numbers (eg, Integer.parseInt(s)), constants for the min and max values, ... .

Autoboxing. Conversion between the primitive and the wrapper classes is largely automatic as of Java 5.

java.math.BigInteger is very useful for arithmetic on unbounded integers.

Several other integer classes have limited or special utility: Number, AtomicInteger, and AtomicLong.

Floating-point - primitive types

Floating-point - object types

The floating-point types, float and double, are stored in IEEE-754 format. Calculations may produce NaN (Not a Number) or +/- infinity. Calculations are done as doubles, so a cast is needed to store in a float.

Type	Bytes	Range	Accuracy	Literals
float	4	-3.4E38..+3.4E38	6-7 digits	3.14F 6.02e23F
double	8	-1.7E308..+1.7E308	14-15 digits	3.14 6.02e23

Operators: arithmetic, comparison, assignment.
Utility methods: Math class.

The "wrapper" classes correspond to the primitive floating-point types: Double and Float. Objects of these classes are immutable -- they can not be changed after they are created. The wrapper classes contain utility methods such as conversions between String and numbers (eg, Integer.parseInt(s)), constants for the min and max values, ... .

Autoboxing. Conversion between the primitive and the wrapper classes is largely automatic as of Java 5.

java.math.BigDecimal is very useful for arithmetic on unbounded floating-point numbers.

Several other integer classes have limited or special utility: Number.

boolean - primitive type

Boolean - object type

boolean is used to store the values true or false. The storage is unspecified.
Operators: logical, ==, !=, assignment. Boolean is a wrapper class for boolean, but it has relatively little utility.

char - primitive types

Character, String, ... - object types

char type is a Unicode character stored as an unsigned number in two bytes (range 0..65,535 or '\u0000'..'\uFFFF'). char is an integer type.
char literals

'A' (single character)
Unicode '\uxxxx' where x is a hexadecimal digit. Eg '\u0041'
Octal '\nnn' where n is an octal digit.
Escape combinations: '\n' newline, '\\' backslash, '\'' single quote, '\"' double quote, '\r' carriage return, '\t' tab, '\b' backspace, '\f' form feed.

Character is the wrapper class for char. It contains many very useful methods for working with characters.

String is the extremely common immutable object type for storing multiple characters.

java.util.StringBuilder (as of Java 5) and java.util.StringBuffer (very slightly slower than the equivalent StringBuilder because it's synchronized) are useful for strings that can be changed.

java.util.regex.Pattern and java.util.regex.Matcher can be used for regular expression matching as of Java 1.4.. java.util.StringTokenizer is an earlier, and not as general way to break input strings into "tokens".