[基础] 学习笔记(七)

 public class Demo{
	 public void out(int i){
		 
	 }
	 public void out(long l){
		 
	 }
	 
	 public static void main(String[] args){
		 
	 }
 }
 
	/*
		在转为class文件的时候，会转为字节码，根据字节码可以看出方法的描述符。
		重载：
			Java中方法的重载在编译期就已经产生了方法的描述符，在执行的时候根据方法的描述符和传入的参数
			选择方法执行。这样的形式成为编译期的多态。
			
		为什么Java中重载是以方法名和方法参数来作为方法的描述符的？
		回答：因为，在方法执行中，我们有时候只关心方法的执行过程而不关心方法的返回值，所以我们用方法名和方法的返回值作为
		方法的描述符。
		直白描述：返回值并不能确定和影响你的方法体执行的操作。
		
		多态： 一个对象多种形态。  向下转型必须拥有向上转型之后才可以向下转型。  不可以直接向下转型。
	*/
	
	
	/*
		Classfile /C:/Users/Administrator/Desktop/Test.class
  Last modified 2020-2-25; size 470 bytes
  MD5 checksum 64ab131b71c976b6a84ef554878ee62a
  Compiled from "Test.java"
public class Test extends A
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#18         // A."<init>":()V
   #2 = Fieldref           #19.#20        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #21            // i am Test
   #4 = Methodref          #22.#23        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #24            // Test
   #6 = Methodref          #5.#18         // Test."<init>":()V
   #7 = Methodref          #8.#25         // A.out:()V
   #8 = Class              #26            // A
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               LineNumberTable
  #13 = Utf8               out
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               SourceFile
  #17 = Utf8               Test.java
  #18 = NameAndType        #9:#10         // "<init>":()V
  #19 = Class              #27            // java/lang/System
  #20 = NameAndType        #13:#28        // out:Ljava/io/PrintStream;
  #21 = Utf8               i am Test
  #22 = Class              #29            // java/io/PrintStream
  #23 = NameAndType        #30:#31        // println:(Ljava/lang/String;)V
  #24 = Utf8               Test
  #25 = NameAndType        #13:#10        // out:()V
  #26 = Utf8               A
  #27 = Utf8               java/lang/System
  #28 = Utf8               Ljava/io/PrintStream;
  #29 = Utf8               java/io/PrintStream
  #30 = Utf8               println
  #31 = Utf8               (Ljava/lang/String;)V
{
  public Test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method A."<init>":()V
         4: return
      LineNumberTable:
        line 8: 0

  public void out();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String i am Test
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 10: 0
        line 11: 8

  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: new           #5                  // class Test
         3: dup
         4: invokespecial #6                  // Method "<init>":()V
         7: astore_1
         8: aload_1
         9: invokevirtual #7                  // Method A.out:()V
        12: return
      LineNumberTable:
        line 13: 0
        line 14: 8
        line 15: 12
}
SourceFile: "Test.java"

C:\Users\Administrator\Desktop>javac Test.java

C:\Users\Administrator\Desktop>java Test
123

C:\Users\Administrator\Desktop>javap -verbose Test
Classfile /C:/Users/Administrator/Desktop/Test.class
  Last modified 2020-2-25; size 405 bytes
  MD5 checksum b566d315b34bb54f3ca0ed08d60c8db2
  Compiled from "Test.java"
public class Test
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#17        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #18            // 123
   #4 = Methodref          #19.#20        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #21            // Test
   #6 = Class              #22            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               SourceFile
  #14 = Utf8               Test.java
  #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Class              #23            // java/lang/System
  #17 = NameAndType        #24:#25        // out:Ljava/io/PrintStream;
  #18 = Utf8               123
  #19 = Class              #26            // java/io/PrintStream
  #20 = NameAndType        #27:#28        // println:(Ljava/lang/String;)V
  #21 = Utf8               Test
  #22 = Utf8               java/lang/Object
  #23 = Utf8               java/lang/System
  #24 = Utf8               out
  #25 = Utf8               Ljava/io/PrintStream;
  #26 = Utf8               java/io/PrintStream
  #27 = Utf8               println
  #28 = Utf8               (Ljava/lang/String;)V
{
  public Test();
    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 2: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String 123
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 4: 0
        line 5: 8
}
SourceFile: "Test.java"

以上就是字节码：
	可以看出的有：
		1、所有字面量都是存在utf8的常量表中。
		2、用到这些字面量的时候会使用指针去指向地址拼接。
		3、// Field java/lang/System.out:Ljava/io/PrintStream;   是静态方法。
		4、3: ldc           #3                  // String 123  应该是取值的意思，取常量池三号位的String表中的
			指向#18号位的Utf8常量表中的字面量 123；
		5、之后调用方法  // Method java/io/PrintStream.println:(Ljava/lang/String;)V   打印出 123；
		6、返回
		
		具体细节，俺也不知道，只是知道一个大概- -，继续做笔记。。
	
	*/