string对象是引用类型，为什么又说它是值传递方式呢，而不是引用传递呢？
另外string s=new String ("xyz");创建了几个String Object？它在内存中到底是怎么分配的。
有点搞不懂
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引用传递,编译原理中的call by reference或者pass by reference是专有术语，这里的引用和Java里的“引用类型”中的引用不是相同的概念。
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java中的参数传递方式确实是“值传递”，对于对象类型，传递的值是所引用对象的地址，所以其实是个引用，理解其原理即可，不要在术语上徘徊。
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看java虚拟机规范
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Pass-by-value
The actual parameter (or argument expression) is fully evaluated and the resulting value is copied into a location being used to hold the formal parameter's value during method/function execution. That location is typically a chunk of memory on the runtime stack for the application (which is how Java handles it), but other languages could choose parameter storage differently.
Pass-by-reference
The formal parameter merely acts as an alias for the actual parameter. Anytime the method/function uses the formal parameter (for reading or writing), it is actually using the actual parameter.