“最终”如何? Java中的关键字有效吗? (我仍然可以修改一个对象。) [英] How does the "final" keyword in Java work? (I can still modify an object.)
问题描述
在Java中,我们使用 final
关键字和变量来指定其值不被更改。
但是我看到你可以改变类的构造函数/方法中的值。同样,如果变量是 static
那么这是一个编译错误。
In Java we use final
keyword with variables to specify its values are not to be changed.
But I see that you can change the value in the constructor / methods of the class. Again, if the variable is static
then it is a compilation error.
这是代码:
import java.util.ArrayList;
import java.util.List;
class Test {
private final List foo;
public Test()
{
foo = new ArrayList();
foo.add("foo"); // Modification-1
}
public static void main(String[] args)
{
Test t = new Test();
t.foo.add("bar"); // Modification-2
System.out.println("print - " + t.foo);
}
}
上面的代码工作正常,没有错误。
Above code works fine and no errors.
现在将变量更改为 static
:
private static final List foo;
现在是编译错误。这个 final
如何真正起作用?
Now it is a compilation error. How does this final
really work?
推荐答案
你总是被允许初始化 最终
变量。编译器确保您只能执行一次。
You are always allowed to initialize a final
variable. The compiler makes sure that you can do it only once.
请注意,对存储在 final
变量中的对象的调用方法与<$的语义无关C $ C>最终。换句话说: final
只是关于引用本身,而不是引用对象的内容。
Note that calling methods on an object stored in a final
variable has nothing to do with the semantics of final
. In other words: final
is only about the reference itself, and not about the contents of the referenced object.
Java没有对象不变性的概念;这是通过精心设计对象来实现的,并且是一项非常重要的工作。
Java has no concept of object immutability; this is achieved by carefully designing the object, and is a far-from-trivial endeavor.
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