这与自动装箱相比真的扩大了吗? [英] Is this really widening vs autoboxing?

问题描述

我在有关另一个问题的答案中看到了这一点，其中提到了Java规范的缺点:

I saw this in an answer to another question, in reference to shortcomings of the Java spec:

还有更多缺点，这是一个微妙的话题.检查此:

public class methodOverloading{
     public static void hello(Integer x){
          System.out.println("Integer");
     }

     public static void hello(long x){
          System.out.println("long");
     }

     public static void main(String[] args){
         int i = 5;
         hello(i);
     }
}

长"字表示会被打印(我自己没有检查过)，因为编译器选择加宽而不是自动装箱.使用自动装箱时要小心，否则请不要使用它！

Here "long" would be printed (haven't checked it myself), because the compiler chooses widening over auto-boxing. Be careful when using auto-boxing or don't use it at all!

我们确定这实际上是加宽而不是自动装箱的示例，还是完全其他的东西?

在我的初始扫描中，我同意输出为长"输出的陈述.基于 i 被声明为原始而不是对象.但是，如果您更改了

On my initial scanning, I would agree with the statement that the output would be "long" on the basis of i being declared as a primitive and not an object. However, if you changed

hello(long x)

到

hello(Long x)

输出将显示"Integer"

the output would print "Integer"

这到底是怎么回事?我对Java的编译器/字节码解释器一无所知...

What's really going on here? I know nothing about the compilers/bytecode interpreters for java...

推荐答案

在第一种情况下，转换范围不断扩大.在编译的类上运行"javap"实用程序(包含在JDK中)时，可以看到以下内容:

In the first case, you have a widening conversion happening. This can be see when runinng the "javap" utility program (included w/ the JDK), on the compiled class:

public static void main(java.lang.String[]);
  Code:
   0:   iconst_ 5
   1:   istore_ 1
   2:   iload_ 1
   3:   i2l
   4:   invokestatic    #6; //Method hello:(J)V
   7:   return

}

很显然，您看到了I2L，它是扩展的Integer-To-Long字节码指令的助记符.请参见此处.

Clearly, you see the I2L, which is the mnemonic for the widening Integer-To-Long bytecode instruction. See reference here.

在另一种情况下，将"long x"替换为对象"Long x"签名，您将在main方法中使用以下代码:

And in the other case, replacing the "long x" with the object "Long x" signature, you'll have this code in the main method:

public static void main(java.lang.String[]);
  Code:
   0:   iconst_ 5
   1:   istore_ 1
   2:   iload_ 1
   3:   invokestatic    #6; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   6:   invokestatic    #7; //Method hello:(Ljava/lang/Integer;)V
   9:   return

}

因此，您会看到编译器已创建了Integer.valueOf(int)指令，以将原始容器装箱在包装器中.

So you see the compiler has created the instruction Integer.valueOf(int), to box the primitive inside the wrapper.

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