java为什么泛型类型信息在类文件中可见？

Question

根据https://docs.oracle.com/javase/tutorial/java/generics/genTypes.html 编译器应将每个无边界类型替换为Object

我有一门课：

 public class Test<E> {

    void setE(E e) {
        return;
   }
}

在Idea中反编译后，结果如下：

 public class Test<E> {
    public Test() {
    }

    void setE(E e) {
    }
  }

为什么E没有被Object取代

Answer 1

本教程在这里有些误导。它试图强调类型擦除，但它做得并不优雅

生成的类的字节码如下所示。显然，这些信息并没有被丢弃

  Last modified 2 Mar 2020; size 332 bytes
  MD5 checksum e3b7faf33fd5666eae578bd516d0f903
  Compiled from "Test.java"
public class Test<E extends java.lang.Object> extends java.lang.Object
  minor version: 0
  major version: 53
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #2                          // Test
  super_class: #3                         // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 2
Constant pool:
   #1 = Methodref          #3.#15         // java/lang/Object."<init>":()V
   #2 = Class              #16            // Test
   #3 = Class              #17            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               setE
   #9 = Utf8               (Ljava/lang/Object;)V
  #10 = Utf8               Signature
  #11 = Utf8               (TE;)V
  #12 = Utf8               <E:Ljava/lang/Object;>Ljava/lang/Object;
  #13 = Utf8               SourceFile
  #14 = Utf8               Test.java
  #15 = NameAndType        #4:#5          // "<init>":()V
  #16 = Utf8               Test
  #17 = Utf8               java/lang/Object
{
  public Test();
    descriptor: ()V
    flags: (0x0001) 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

  void setE(E);
    descriptor: (Ljava/lang/Object;)V
    flags: (0x0000)
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 3: 0
    Signature: #11                          // (TE;)V
}
Signature: #12                          // <E:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "Test.java"

这些信息不能在字节码中丢弃的原因是，假设这个类是库的一部分；另一个项目取决于它。在第二个项目的编译时，他们的编译器需要能够断言泛型类型参数是否在边界内。如果它被完全扔掉了，他们就得写信了

Test test = new Test();

而在我的源项目中，我可以写

Test<String> test = new Test<>():