Java 简明教程

Java - Numbers Class

通常，当我们使用数字时，我们会使用基本 data types ，如 byte、int、long、double 等。

Normally, when we work with Numbers, we use primitive data types such as byte, int, long, double, etc.

考虑以下示例：

Consider the below example:

int i = 5000;
float gpa = 13.65f;
double mask = 125;

有时，我们可能需要使用对象，而不是基本数据类型。为了实现这一点，Java 提供了 wrapper classes 。

Sometimes, there may be some situations where we need to use objects instead of primitive data types. To achieve this, Java provides wrapper classes.

所有包装类（Integer、Long、Byte、Double、Float、Short）都是抽象类 Number 的子类。

All the wrapper classes (Integer, Long, Byte, Double, Float, Short) are subclasses of the abstract class Number.

Java Number Class

Number 类是 java.lang 软件包中的抽象类。它是表示可转换为基本数据类型（如 byte、short、int、long、float 和 double）的数字值的类的超类。

The Number class is an abstract class in java.lang package. It is the superclass of the classes that represent numeric values convertible to primitive data types such as byte, short, int, long, float, and double.

包装类对象包含或包装其相应的基本数据类型。将基本数据类型转换为对象称为 boxing，这由编译器负责。因此，在使用包装类时只需要将基本数据类型的值传递给包装类的构造函数。

The object of the wrapper class contains or wraps its respective primitive data type. Converting primitive data types into object is called boxing, and this is taken care by the compiler. Therefore, while using a wrapper class you just need to pass the value of the primitive data type to the constructor of the Wrapper class.

而包装器对象将被转换回基本数据类型，此过程称为拆箱。Number 类属于 java.lang 包。

And the Wrapper object will be converted back to a primitive data type, and this process is called unboxing. The Number class is part of the java.lang package.

以下是装箱和拆箱的一个示例 −

Following is an example of boxing and unboxing −

Example

在此示例中，我们展示了使用包装类操作基元及其运算。在第一个语句中，我们为整数对象 x 分配了一个 int，此过程称为装箱。在第二个语句中，我们为 x 添加 10，这需要将 x 拆箱为 int 并执行加法操作，并将结果分配回变量 x 并打印。

In this example, we’ve showcase use of primitives and their operations using a wrapper class. In first statement we’ve assigned an int to an Integer object x which is termed as boxing. In second statment, we’re adding 10 to x which requires x to be unboxed as int and addition is performed and result is assigned back to the variable x and printed.

public class Test {

   public static void main(String args[]) {
      Integer x = 5; // boxes int to an Integer object
      x =  x + 10;   // unboxes the Integer to a int
      System.out.println(x);
   }
}

这会产生以下结果 −

This will produce the following result −

当为 x 分配整数值时，编译器将此整数装箱，这是因为 x 是整数对象。稍后，对 x 进行拆箱以便将其作为整数进行加法操作。

When x is assigned an integer value, the compiler boxes the integer because x is integer object. Later, x is unboxed so that they can be added as an integer.

Java Number Class Methods

以下为数字类的所有子类实现的实例方法列表 -

Following is the list of the instance methods that all the subclasses of the Number class implements −

Sr.No.	Method & Description
1	byteValue() This method returns the value of the specified number as a byte.
2	doubleValue() This method returns the value of the specified number as a double.
3	floatValue() This method returns the value of the specified number as a float.
4	intValue() This method returns the value of the specified number as a int.
5	longValue() This method returns the value of the specified number as a long.
6	compareTo()Compares this Number object to the argument.
7	equals()Determines whether this number object is equal to the argument.
8	valueOf()Returns an Integer object holding the value of the specified primitive.
9	toString()Returns a String object representing the value of a specified number.
10	parseInt()This method is used to get the primitive data type of a certain String.
11	min()Returns the smaller of the two arguments.
12	max()Returns the larger of the two arguments.

Java Number: Wrapper Classes

以下是 Number 类的所有子类的包装器类列表 −

Following is the list of the wrapper classes that all the subclasses of the Number class −

Sr.No.	Class & Description
1	Boolean The Java Boolean class wraps a value of the primitive type boolean in an object.
2	Byte The Java Byte class wraps a value of the primitive type byte in an object.
3	Character The Java Character class wraps a value of the primitive type char in an object.
4	Double The Java Double class wraps a value of the primitive type double in an object.
5	Float The Java Float class wraps a value of the primitive type float in an object.
6	Integer The Java Integer class wraps a value of the primitive type int in an object.
7	Long The Java Long class wraps a value of the primitive type long in an object.
8	Short The Java Short class wraps a value of the primitive type short in an object.