Number抽象类源码注释解析

 1 package java.lang;
 2 
 3 /**
 4  * The abstract class {@code Number} is the superclass of platform
 5  * classes representing numeric values that are convertible to the
 6  * primitive types {@code byte}, {@code double}, {@code float}, {@code
 7  * int}, {@code long}, and {@code short}.
 8  *
 9  * The specific semantics of the conversion from the numeric value of
10  * a particular {@code Number} implementation to a given primitive
11  * type is defined by the {@code Number} implementation in question.
12  *
13  * For platform classes, the conversion is often analogous to a
14  * narrowing primitive conversion or a widening primitive conversion
15  * as defining in <cite>The Java&trade; Language Specification</cite>
16  * for converting between primitive types.  Therefore, conversions may
17  * lose information about the overall magnitude of a numeric value, may
18  * lose precision, and may even return a result of a different sign
19  * than the input.
20  *
21  * See the documentation of a given {@code Number} implementation for
22  * conversion details.
23  *
24  * @author      Lee Boynton
25  * @author      Arthur van Hoff
26  * @jls 5.1.2 Widening Primitive Conversions
27  * @jls 5.1.3 Narrowing Primitive Conversions
28  * @since   JDK1.0
29  */
30 public abstract class Number implements java.io.Serializable {
31     /**
32      * Returns the value of the specified number as an {@code int},
33      * which may involve rounding or truncation.
34      *
35      * @return  the numeric value represented by this object after conversion
36      *          to type {@code int}.
37      */
38     public abstract int intValue();
39 
40     /**
41      * Returns the value of the specified number as a {@code long},
42      * which may involve rounding or truncation.
43      *
44      * @return  the numeric value represented by this object after conversion
45      *          to type {@code long}.
46      */
47     public abstract long longValue();
48 
49     /**
50      * Returns the value of the specified number as a {@code float},
51      * which may involve rounding.
52      *
53      * @return  the numeric value represented by this object after conversion
54      *          to type {@code float}.
55      */
56     public abstract float floatValue();
57 
58     /**
59      * Returns the value of the specified number as a {@code double},
60      * which may involve rounding.
61      *
62      * @return  the numeric value represented by this object after conversion
63      *          to type {@code double}.
64      */
65     public abstract double doubleValue();
66 
67     /**
68      * Returns the value of the specified number as a {@code byte},
69      * which may involve rounding or truncation.
70      *
71      * <p>This implementation returns the result of {@link #intValue} cast
72      * to a {@code byte}.
73      *
74      * @return  the numeric value represented by this object after conversion
75      *          to type {@code byte}.
76      * @since   JDK1.1
77      */
78     public byte byteValue() {
79         return (byte)intValue();
80     }
81 
82     /**
83      * Returns the value of the specified number as a {@code short},
84      * which may involve rounding or truncation.
85      *
86      * <p>This implementation returns the result of {@link #intValue} cast
87      * to a {@code short}.
88      *
89      * @return  the numeric value represented by this object after conversion
90      *          to type {@code short}.
91      * @since   JDK1.1
92      */
93     public short shortValue() {
94         return (short)intValue();
95     }
96 
97     /** use serialVersionUID from JDK 1.0.2 for interoperability */
98     private static final long serialVersionUID = -8742448824652078965L;
99 }

这个抽象类Number是可以转换为基本数据类型(byte,double,float,int,long,short)的展示数值的类的父类

从特定Number实现类转换到一个给定的基础类型的方法是由Number类定义的（..Value()）

在java基本数据类型转换语言规范定义中，对类来说，转换通常类似于一个窄化或者扩大。因此，转换可能会丢失过大数值的信息，可能会丢失精度，甚至可能返回与输入不同的符号的结果。

将指定数字的值作为int/long/float/doube/byte/short返回，这可能涉及舍入/截断。