Java字符串池和类型转换 [英] Java String pool and type casting
问题描述
我的问题是关于Java处理字符串文字的方式。从Java语言规范（JLS）中可以清楚地看出，字符串文字被隐式地嵌入了 - 换句话说，在堆的String常量池部分创建的对象，与调用<$时创建的基于堆的对象c $ c> new String（whatever）。
My question is in regard to the way Java handles String literals. It's quite clear from the Java Language Specs (JLS) that String literals are being implicitly interned - in other words, objects that are created in the String constant pool part of the heap, in contrast to the heap-based objects created when calling new String("whatever").
什么似乎不符合JLS说的是，创建一个新的字符串使用字符串连接与一个强制的常量String类型，这应该被视为一个常量字符串按照JLS，显然JVM正在创建一个新的String对象，而不是隐式隐藏它。我喜欢关于这个特定行为的任何解释，以及这是否是一个平台特定的行为。我在Mac OSX雪豹运行。
What doesn't seem to line up with what the JLS says is that when creating a new String using String concatenation with a casted constant String type, which should be considered as a constant String as per the JLS, apparently the JVM is creating a new String object rather than interning it implicitly. I appreciate any explanation about this particular behaviour and whether or not this is a platform-specific behaviour. I am running on a Mac OSX Snow Leopard.
public class Test
{
public static void main(String args[])
{
/*
Create a String object on the String constant pool
using a String literal
*/
String hello = "hello";
final String lo = "lo"; // this will be created in the String pool as well
/*
Compare the hello variable to a String constant expression
, that should cause the JVM to implicitly call String.intern()
*/
System.out.println(hello == ("hel" + lo));// This should print true
/*
Here we need to create a String by casting an Object back
into a String, this will be used later to create a constant
expression to be compared with the hello variable
*/
Object object = "lo";
final String stringObject = (String) object;// as per the JLS, casted String types can be used to form constant expressions
/*
Compare with the hello variable
*/
System.out.println(hello == "hel" + stringObject);// This should print true, but it doesn't :(
}
}
推荐答案
投放到对象在编译时常量表达式中是不允许的，只有 String 和原语JLS（Java SE 7 edition）section 15.28：
Casting to Object is not allowed in a compile time constant expression. The only casts permitted are to String and primitives. JLS (Java SE 7 edition) section 15.28:
（实际上有第二个原因。 object isn 't final 所以不能考虑a 常量变量。原始类型或类型的变量 String ，即 final 并使用编译时常量表达式（§15.28）初始化，称为常量变量。 - 第4.12.4节）
(There's actually a second reason. object isn't final so cannot possibly by considered a constant variable. "A variable of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28), is called a constant variable." -- section 4.12.4.)
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