？？？（？）优化？？：三元运算符 [英] Map.get() optimization in ?: ternary operator

问题描述

考虑以下代码：

  java.util.Map< String，String> map = new java.util.HashMap< String，String>（）; 
 ... 
 String key =A; 
 String value = map.get（key）== null？ DEFAULT_VALUE：map.get（key）; //（1）
  

将编译器优化行（1）类似于

  String tmp = map.get（key） ; 
字符串值= tmp == null？ DEFAULT_VALUE：tmp; 
  

（或到：

  String value = map.get（key）; 
 if（value == null）value =DEFAULT_VALUE; 
  

）
？

解决方案

不知道如果你问这对应于编译器将对原始表达式做出什么，在这种情况下，答案是：

两者 -

在这个例子中，你调用了map.get（）两次;编译器不知道没有副作用，所以当找到一个值时它会调用它两次。

可能最接近

  String tmp = map.get（key）; 
字符串值; 
 if（tmp == null）
 value =DEFAULT_VALUE; 
 else 
 value = map.get（key）; 
  

或者如果你问哪个替代方案是最有效的，那么答案是：

第二个选择可能稍微好一些，因为它不需要额外的局部变量。一个额外的本地变量在JVM上施加轻微的开销，但是在JIT通过它之后，它可能在运行时无关紧要。

Consider the following code:

java.util.Map<String, String> map = new java.util.HashMap<String, String>();
...
String key = "A";
String value = map.get(key) == null? "DEFAULT_VALUE" : map.get(key); // (1)

Would the compiler optimize the line (1) something similar to:

String tmp = map.get(key);
String value = tmp == null? "DEFAULT_VALUE" : tmp;

(or to:

String value = map.get(key);
if(value == null) value = "DEFAULT_VALUE";

) ?

解决方案

Not sure if you are asking which corresponds to what the compiler will make of the original expression, in which case the answer is:

Neither--

In the example you are calling map.get() twice; the compiler can't know that there are no side effects, so it will call it twice when a value is found.

Probably closest to

String tmp = map.get(key);
String value;
if (tmp == null)
    value= "DEFAULT_VALUE";
else
    value = map.get(key);

Or if you are asking which alternative is most efficient, in which case the answer is:

The second alternative is probably slightly better because it does not require an additional local variable. An additional local variable imposes a slight overhead on the JVM, but it will probably amount to nothing at runtime once the JIT gets through with it.

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