5.0 Conversions and Contexts

类型转换，因为java是强类型语言。

数值类型：
不同类型的字节长度，拓宽可以自动做，缩窄需要强制转换。

//Example 5.0-2. Conversions In Various Contexts
jshell> int i = (int)12.5f
i ==> 12

jshell> float f = i
f ==> 12.0

jshell> f * i
$69 ==> 144.0

jshell> double d = Math.sin(f)
d ==> -0.5365729180004349

5.1 Kinds of Conversion

5.1.2 Widening Primitive Conversion

扩展的原始转换不会丢失有关整体的信息在下列情况下数值的大小，其中数值为完全保留. (拓宽）
反之，不能保证精度。

jshell> int big = 1234567890
big ==> 1234567890

jshell> float bigF = big
bigF ==> 1.23456794E9

//following will use float for big and bigF automactically
jshell> big - bigF
$73 ==> 0.0

//(int) will lost  precision
jshell> big - (int)bigF
$74 ==> -46

5.1.3 Narrowing Primitive Conversion

精度下降的强制类型转换，可能会损失信息。（类似有损压缩）
我觉得最快的是直接在jshll试试，不用记，而且最好不要这样做。

//Example 5.1.3-1. Narrowing Primitive Conversion
jshell> Float.NEGATIVE_INFINITY
$1 ==> -Infinity

jshell> Float.POSITIVE_INFINITY
$2 ==> Infinity

jshell> Float.POSITIVE_INFINITY + Float.NEGATIVE_INFINITY
$3 ==> NaN

//Example 5.1.3-2. Narrowing Primitive Conversions that lose information

(short)0x12345678==0x5678
(byte)255==-1
(int)1e20f==2147483647
(int)NaN==0
(float)-1e100==-Infinity
(float)1e-50==0.0

5.2 Assignment Contexts

注意类型是否兼容。

//Example 5.2-1. Assignment for Primitive Types
jshell> short s = 123
s ==> 123

jshell> char c =  s
|  错误:
|  不兼容的类型: 从short转换到char可能会有损失
|  char c =  s;
|            ^

jshell> s = c
|  错误:
|  找不到符号
|    符号:   变量 c
|    位置: 类
|  s = c

//Example 5.2-2. Assignment for Reference Types
class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
    public String toString() { return "("+x+","+y+")"; }
}
interface Colorable { void setColor(int color); }
class ColoredPoint extends Point implements Colorable {
    int color;
    ColoredPoint(int x, int y, int color) {
        super(x, y); setColor(color);
    }
    public void setColor(int color) { this.color = color; }
    public String toString() {
        return super.toString() + "@" + color;
    }
} 

//test
jshell> Point[] pa = new ColoredPoint(
jshell> Point[] pa = new ColoredPoint[4]
pa ==> ColoredPoint[4] { null, null, null, null }

jshell> pa[0] = new ColoredPoint(2,2,12)
$19 ==> (2,2)@12

jshell> pa[1] = new ColoredPoint(4,5,24)
$20 ==> (4,5)@24

jshell> ColoredPoint[] cpa = (ColoredPoint[])pa;
cpa ==> ColoredPoint[4] { (2,2)@12, (4,5)@24, null, null }

5.6.1 Unary Numeric Promotion

一些运算符将单一数字提升应用于单个操作数，这必须是
生成数值类型的值。

学习中 2018.7.9
《The Java® Language Specification Java SE 10 Edition》
absolute page 111 - 149

#jsl3# Conversions and Contexts 转换和上下文-chapter 5