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: By default, the int data type is a 32-bit signed two's complement integer, which has a minimum value of -231 and a maximum value of 231-1. In Java SE 8 and later, you can use the int data type to represent an unsigned 32-bit integer, which has a minimum value of 0 and a maximum value of 232-1. Use the Integer class to use int data type as an unsigned integer. See the section The Number Classes for more information. Static methods like compareUnsigned, divideUnsigned etc have been added to the Integer class to support the arithmetic operations for unsigned integers.
long: The long data type is a 64-bit two's complement integer. The signed long has a minimum value of -263 and a maximum value of 263-1. In Java SE 8 and later, you can use the long data type to represent an unsigned 64-bit long, which has a minimum value of 0 and a maximum value of 264-1. Use this data type when you need a range of values wider than those provided by int. The Long class also contains methods like compareUnsigned, divideUnsignedetc to support arithmetic operations for unsigned long.
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 the Floating-Point Types, Formats, and Values section 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 the Floating-Point Types, Formats, and Values section 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.
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)
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;
An integer literal is of type long if it ends with the letter L or l; otherwise it is of type int. It is recommended that you use the upper case letter L because the lower case letter l is hard to distinguish from the digit 1.
Values of the integral types byte, short, int, and long can be created from int literals. Values of type long that exceed the range of int can be created from long literals. Integer literals can be expressed by these number systems:
Decimal: Base 10, whose digits consists of the numbers 0 through 9; this is the number system you use every day
Hexadecimal: Base 16, whose digits consist of the numbers 0 through 9 and the letters A through F
Binary: Base 2, whose digits consists of the numbers 0 and 1 (you can create binary literals in Java SE 7 and later
// The number 26, in decimal
int decVal = 26;
// The number 26, in hexadecimal
int hexVal = 0x1a;
// The number 26, in binary
int binVal = 0b11010;
Floating-Point Literals
A floating-point literal is of type float if it ends with the letter F or f; otherwise its type is double and it can optionally end with the letter D or d.
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;
//same value as d1, but in scientific notation
double d2 = 1.234e2;
float f1  = 123.4f;
Character and String Literals
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 Señor 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 (which are highlighted) in numeric literals:
// Invalid: cannot put underscores
// adjacent to a decimal point
float pi1 = 3_.1415F;
// Invalid: cannot put underscores
float pi2 = 3._1415F;
// prior to an L suffix
long socialSecurityNumber1 = 999_99_9999_L;
// OK (decimal literal)
int x1 = 5_2;
// At the end of a literal
int x2 = 52_;
int x3 = 5_______2;
// in the 0x radix prefix
int x4 = 0_x52;
// at the beginning of a number
int x5 = 0x_52;
// OK (hexadecimal literal)
int x6 = 0x5_2;
// at the end of a number
int x7 = 0x52_;
More on Classes
By default, the int data type is a 32-bit signed two's complement integer, which has a minimum value of -231 and a maximum value of 231-1.
The double data type is a double-precision 64-bit IEEE 754 floating point..
The long data type is a 64-bit two's complement integer.. The signed long has a minimum value of -263 and a maximum value of 263-1.. In Java SE 8 and later, you can use the long data type to represent an unsigned 64-bit long, which has a minimum value of 0 and a maximum value of 264-1..
The float data type is a single-precision 32-bit IEEE 754 floating point.. Use a float(instead of double) if you need to save memory in large arrays of floating point numbers. .
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 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)..
Is a 16-bit signed two's complement integer
In computer science, an object can be a variable, a data structure, a function, or a method, and as such, is a value in memory referenced by an identifier.. A typical Java program creates many objects, which as you know, interact by invoking methods. Through these object interactions, a program can carry out various tasks, such as implementing a GUI, running an animation, or sending and receiving information over a network. Once an object has completed the work for which it was created, its resources are recycled for use by other objects.
A primitive type is predefined by the language and is named by a reserved keyword.
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 type is commonly used to store yes/no values: true means "yes, correct", and false means "no, incorrect".
A Java compiler is a compiler for the programming language Java. . The most common form of output from a Java compiler is Java class files containing platform-neutral Java bytecode, but there are also compilers that gives optimized native machine code for a particular hardware/operating system combination..
In computer programming, a logic error is a bug in a program that causes it to operate incorrectly, but not to terminate abnormally (or crash).. These are exceptional conditions that are external to the application, and that the application usually cannot anticipate or recover from.. Errors are not subject to the Catch or Specify Requirement. Errors are those exceptions indicated by Error and its subclasses..
Are declared in methods, constructors, or blocks.. Are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor, or block.. Access modifiers cannot be used for local variables.. Are visible only within the declared method, constructor, or block..