🍬byte b += 1 和 b = b + 1

先来看下问题，如下图。声明了一个变量 byte b = 100;，第5行代码编译正常，第6行代码编译器报错。

原因是，Java编译器在处理 += 时，会自动进行强转（窄化处理），这属于一个Java的语法糖。

下面是反编译的字节码指令：

$ javap -v -l Hello
Classfile /Users/liyan/Documents/java-class/learning-java/src/Hello.class
  Last modified 2022-5-12; size 271 bytes
  MD5 checksum d656ead69e7ab2b5502aa1530196ce9f
  Compiled from "Hello.java"
public class Hello
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#12         // java/lang/Object."<init>":()V
   #2 = Class              #13            // Hello
   #3 = Class              #14            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               main
   #9 = Utf8               ([Ljava/lang/String;)V
  #10 = Utf8               SourceFile
  #11 = Utf8               Hello.java
  #12 = NameAndType        #4:#5          // "<init>":()V
  #13 = Utf8               Hello
  #14 = Utf8               java/lang/Object
{
  public Hello();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: bipush        100
         2: istore_1
         3: iload_1
         4: iconst_1
         5: iadd
         6: i2b
         7: istore_1
         8: return
      LineNumberTable:
        line 4: 0
        line 5: 3
        line 6: 8
}
SourceFile: "Hello.java"