返回对象的函数保持不变。
如果对象不能被改变，那么哈希码也永远不变。
If the object cannot be altered, then the hash code will never change either.
通常对变量的赋值只有一次，并被当作不变的对象。
Often a variable is only assigned to once and is used as an (immutable) definition.
一个不变的“变量”可以用val关键字来声明(想象一个值对象)。
An immutable "variable" is declared with the keyword val (think value object).
对象不变式在面向对象程序的证明中具有重要作用。
Object invariant is important in the object-oriented program proof.
026如果世界要有一个不变的形式，就必须要有对象。
There must be objects, if the world is to have unalterable form.
026如果世界要有一个不变的形式，就必须要有对象。
There must be objects, if the world is to have unalterable form.
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