[英]Java type erasure - why can I see the type when I look at the bytecode?
I am trying to understand why writing both methods in a class is not allowed 我试图理解为什么不允许在类中编写这两个方法
public bool plus(List<String>) {return true;}
public bool plus(List<Integer>) {return true;}
I try to figure how it is related to type erasure but when I decompile the following code 我试图弄清楚它与类型擦除有什么关系,但是当我反编译以下代码时
public class Test<T> {
boolean plus2(List<T> ss) {return false;}
boolean plus(List<String> ss) {return false;}
boolean plus(Set<Integer> ss) {return false;}
}
I get the same when I decompile it with Java decompiler (jd) 当我用Java反编译器(jd)反编译它时,我得到了同样的结果
Even when I print the byte code I can clearly see the types. 即使我打印字节代码,我也可以清楚地看到类型。
(Looking at SO answer that declares 'but rest assure the types are erased in the bytecode' ) (看看那些声明'但是确保在字节码中删除类型'的答案 ')
Compiled from "Test.java"
public class com.example.Test<T> {
public com.example.Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
boolean plus2(java.util.List<T>);
Code:
0: iconst_0
1: ireturn
boolean plus(java.util.List<java.lang.String>);
Code:
0: iconst_0
1: ireturn
boolean plus(java.util.Set<java.lang.Integer>);
Code:
0: iconst_0
1: ireturn
}
Your compiler needs to be able to check the generic type information based on information in the byte code. 您的编译器需要能够根据字节代码中的信息检查泛型类型信息。
Java 5.0+ records the generic information in byte code, but it doesn't record it in the object instance. Java 5.0+以字节代码记录通用信息,但它不会将其记录在对象实例中。
eg there is no way to get the generic type of this List 例如,无法获得此列表的泛型类型
// list.getClass() is just ArrayList which doesn't record the generic type.
List list = new ArrayList<String>();
however this does 不过这样做
// list.getClass() is not ArrayList
// list is an instance of a class which extends ArrayList<String>
List list = new ArrayList<String>() { };
ParameterizedType t = (ParameterizedType) list.getClass().getGenericSuperclass();
assert t.getRawType() == ArrayList.class;
assert t.getActualTypeArguments()[0] == String.class;
This is because the byte code of the sub-class of ArrayList records the generic used in it's parent. 这是因为ArrayList的子类的字节代码记录了它的父类中使用的泛型。
Note: this also works for fields, constructor/method arguments as return types and the actual type is recorded in the byte code. 注意:这也适用于字段,构造函数/方法参数作为返回类型,实际类型记录在字节代码中。
However, none of these mean the generic type of an instance is available (though the generic type of a parent class/interface could be available if recorded in byte code. 但是,这些都不意味着实例的泛型类型可用(尽管如果以字节代码记录,父类/接口的泛型类型可用。
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