Primitive Data Types
The Java programming language is statically-typed, which means that all variables must first be declared before they can be used. This involves stating the variable's type and name, as you've already seen:

int gear = 1;

Doing so tells your program that a field named "gear" exists, holds numerical data, and has an initial value of "1". A variable's data type determines the values it may contain, plus the operations that may be performed on it. In addition to int, the Java programming language supports seven other primitive data types. A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values. The eight primitive data types supported by the Java programming language are:

byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation.

short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.

int: The int data type is a 32-bit signed two's complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead.

long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided by int.

float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead. Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform.

double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.

boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.

char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).

In addition to the eight primitive data types listed above, the Java programming language also provides special support for character strings via the java.lang.String class. Enclosing your character string within double quotes will automatically create a new String object; for example, String s = "this is a string";. String objects are immutable, which means that once created, their values cannot be changed. The String class is not technically a primitive data type, but considering the special support given to it by the language, you'll probably tend to think of it as such. You'll learn more about the String class in Simple Data Objects

Default Values
It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.

The following chart summarizes the default values for the above data types.

Data Type Default Value (for fields)
byte 0
short 0
int 0
long 0L
float 0.0f
double 0.0d
char '\u0000'
String (or any object)   null
boolean false

Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.

Literals
You may have noticed that the new keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. As shown below, it's possible to assign a literal to a variable of a primitive type:

     boolean result = true;
     char capitalC = 'C';
     byte b = 100;
     short s = 10000;
     int i = 100000;

The integral types (byte, short, int, and long) can be expressed using decimal, octal, hexadecimal, or binary number systems. (You can create binary literals in Java SE 7 and later.) Decimal is the number system you already use every day; it's based on 10 digits, numbered 0 through 9. The octal number system is base 8, consisting of the digits 0 through 7. The hexadecimal system is base 16, whose digits are the numbers 0 through 9 and the letters A through F. The binary system is base 2, whose digits are the numbers 0 and 1. For general-purpose programming, the decimal system is likely to be the only number system you'll ever use. However, if you need octal, hexadecimal, or binary numbers, the following example shows the correct syntax. The prefix 0 indicates octal, 0x indicates hexadecimal, and 0b indicates binary:

     int decVal = 26; 	   // The number 26, in decimal
     int octVal = 032; 	   // The number 26, in octal
     int hexVal = 0x1a;	   // The number 26, in hexadecimal
     int binVal = 0b11010; // The number 26, in binary

The floating point types (float and double) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal) and D or d (64-bit double literal; this is the default and by convention is omitted).

     double d1 = 123.4;
     double d2 = 1.234e2; // same value as d1, but in scientific notation
     float f1  = 123.4f;

Literals of types char and String may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as '\u0108' (capital C with circumflex), or "S\u00ED se\u00F1or" (Sí Senor in Spanish). Always use 'single quotes' for char literals and "double quotes" for String literals. Unicode escape sequences may be used elsewhere in a program (such as in field names, for example), not just in char or String literals.

The Java programming language also supports a few special escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage return), \" (double quote), \' (single quote), and \\ (backslash).

There's also a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. There's little you can do with a null value beyond testing for its presence. Therefore, null is often used in programs as a marker to indicate that some object is unavailable.

Finally, there's also a special kind of literal called a class literal, formed by taking a type name and appending ".class"; for example, String.class. This refers to the object (of type Class) that represents the type itself.

Using Underscore Characters in Numeric Literals
In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example. to separate groups of digits in numeric literals, which can improve the readability of your code.

For instance, if your code contains numbers with many digits, you can use an underscore character to separate digits in groups of three, similar to how you would use a punctuation mark like a comma, or a space, as a separator.

The following example shows other ways you can use the underscore in numeric literals:

long creditCardNumber = 1234_5678_9012_3456L;
long socialSecurityNumber = 999_99_9999L;
float pi = 	3.14_15F;
long hexBytes = 0xFF_EC_DE_5E;
long hexWords = 0xCAFE_BABE;
long maxLong = 0x7fff_ffff_ffff_ffffL;
byte nybbles = 0b0010_0101;
long bytes = 0b11010010_01101001_10010100_10010010;
You can place underscores only between digits; you cannot place underscores in the following places:

At the beginning or end of a number
Adjacent to a decimal point in a floating point literal
Prior to an F or L suffix
In positions where a string of digits is expected
The following examples demonstrate valid and invalid underscore placements in numeric literals:

float pi1 = 3_.1415F;      // Invalid; cannot put underscores adjacent to a decimal point
float pi2 = 3._1415F;      // Invalid; cannot put underscores adjacent to a decimal point
long socialSecurityNumber1
  = 999_99_9999_L;         // Invalid; cannot put underscores prior to an L suffix

int x1 = _52;              // This is an identifier, not a numeric literal
int x2 = 5_2;              // OK (decimal literal)
int x3 = 52_;              // Invalid; cannot put underscores at the end of a literal
int x4 = 5_______2;        // OK (decimal literal)

int x5 = 0_x52;            // Invalid; cannot put underscores in the 0x radix prefix
int x6 = 0x_52;            // Invalid; cannot put underscores at the beginning of a number
int x7 = 0x5_2;            // OK (hexadecimal literal)
int x8 = 0x52_;            // Invalid; cannot put underscores at the end of a number

int x9 = 0_52;             // OK (octal literal)
int x10 = 05_2;            // OK (octal literal)
int x11 = 052_;            // Invalid; cannot put underscores at the end of a number