我们可以说指向不变对象的参考变量是常量吗？

Question

I have a doubt from one of the answers in stackoverflow to the following question:

How many String objects are created by the following code?

String x = new String("xyz");
String y = "abc";
x = x + y; 

In Stephen's answer he mentioned x and y are not constant variables.

My doubt- String is a final class, and it's instance will be a constant because String is an immutable class. Why is the reference variable of this constant class not a constant variable? - I do agree with Stephen though as x = x + y; points at "xyzabc" created in the heap memory.

Answer 1

There are a few concepts that you need to understand.

Marking a class as final does not make it immutable. It just makes it un-inheritable.

JLS §8.1.1.2

A class can be declared final if its definition is complete and no subclasses are desired or required.

It is a compile-time error if the name of a final class appears in the extends clause (§8.1.4) of another class declaration; this implies that a final class cannot have any subclasses.

A class is said to be immutable when the values that it stores cannot be changed after initialisation.

A constant variable is a variable marked with final .

JLS §4.12.4

A variable can be declared final. A final variable may only be assigned to once. Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors.

It is a compile-time error if a final variable is assigned to unless it is definitely unassigned (§16) immediately prior to the assignment.

x and y here are not constants because they are not marked final . That's it .

"But strings can't change, so they are constants, right?" you might ask.

String objects themselves can't change, but string variables can . I'll show you:

String s = "Hello";
s = "Goodbye";

The variable's value is changed so that it refers to another string. The original string "Hello" is not changed.