为什么在java泛型中仅在运行时检查String类型返回值

Question

我在阅读Thinking in java,Generics.在一个例子中(在"边界的行动"段落)

public class GenericHolder<T> {
    private T obj;
    public void set(T obj) { this.obj = obj; }
    public T get() { return obj; }
    public static void main(String[] args) {
        GenericHolder<String> holder =
        new GenericHolder<String>();
        holder.set("Item");
        String s = holder.get();
    }
} ///:~


public void set(java.lang.Object);
    0: aload_0
    1: aload_1
    2: putfield #2; //Field obj:Object;
    5: return
public java.lang.Object get();
    0: aload_0
    1: getfield #2; //Field obj:Object;
    4: areturn
public static void main(java.lang.String[]);
    470 Thinking in Java Bruce Eckel
    0: new #3; //class GenericHolder
    3: dup
    4: invokespecial #4; //Method "<init>":()V
    7: astore_1
    8: aload_1
    9: ldc #5; //String Item
    11: invokevirtual #6; //Method set:(Object;)V
    14: aload_1
    15: invokevirtual #7; //Method get:()Object;
    18: checkcast #8; //class java/lang/String
    21: astore_2
    22: return

根据反汇编代码行18,编译器添加校验广播代码.我想知道是否总是对通用的这种checkcast.我将String替换为Integer并再次尝试,但我最终没有找到checkcast代码.对象类型也是如此.

有人可以解释一下吗？是String在Java是一种特殊的对象？

Answer 1

我将String替换为Integer并再次尝试,但我最后没有找到checkcast代码.

对我来说,它产生checkcast也为Integer:

  0: new    #3; //class GenericHolder
  3: dup
  4: invokespecial  #4; //Method GenericHolder."<init>":()V
  7: astore_1
  8: aload_1
  9: iconst_5
 10: invokestatic   #5; //Method java/lang/Integer.valueOf:(I)
 13: invokevirtual  #6; //Method GenericHolder.set:(Ljava/lang/Object;)V
 16: aload_1
 17: invokevirtual  #7; //Method GenericHolder.get:()Ljava/lang/Object;
 20: checkcast  #8; //class java/lang/Integer
 23: astore_2
 24: return

正如您所看到的,该get方法具有签名,Ljava/lang/Object;即checkcast确保Object返回的确是一个Integer.

编辑:在您发布的代码中作为评论:

0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: new #3; //class b/TestG
6: dup
7: new #4; //class java/lang/Integer
10: dup
11: iconst_2
12: invokespecial #5; //Method java/lang/Integer."<init>":(I)V
15: invokespecial #6; //Method b/TestG."<init>":(Ljava/lang/Object;)V
18: invokevirtual #7; //Method b/TestG.getT:()Ljava/lang/Object;
21: invokevirtual #8; //Method java/io/PrintStream.print:(Ljava/lang/Objec t;)V 
24: return

传递PrintStream.print接受的返回值Object.因此,不需要转换返回的值.(checkcast java.lang.Object总是会经历!)