所有原始包装类都是不可变对象吗？ [英] Are all primitive wrapper classes immutable objects?

问题描述

Java中的所有原始包装类都是不可变对象吗？字符串是不可变的。什么是其他不可变对象？

Are all primitive wrapper classes in Java immutable objects? String is immutable. What are the other immutable objects?

推荐答案

任何不能为您提供更改数据的方法都是不可变的- 就这么简单。是的，所有原始包装类型都是不可变的 ¹ ，就像 String 一样。 UUID ，网址和 URI 是其他示例。

Any type which doesn't give you any means to change the data within it is immutable - it's as simple as that. Yes, all the primitive wrapper types are immutable¹, as is String. UUID, URL and URI are other examples.

虽然内置Java API中的 Calendar 和 Date 是可变的， Joda Time 中的许多类型都是不可变的 - 在我看来，这是一个为什么Joda Time更容易使用。如果一个对象是不可变的，你可以在代码中的其他地方保留对它的引用，而不必担心其他一些代码是否会进行更改 - reason 更容易你的代码。

Although Calendar and Date in the built-in Java API are mutable, many of the types within Joda Time are immutable - and to my mind, this is one reason why Joda Time is easier to work with. If an object is immutable, you can keep a reference to it somewhere else in your code and not have to worry about whether or not some other piece of code is going to make changes - it's easier to reason about your code.

¹ 我的意思是 java.lang .Integer 等如其他地方所述， Atomic * 类是可变的，实际上有是为了达到他们的目的。在标准的原始包装类集和包装原始值的类集之间我的想法有所不同。

¹ by which I mean java.lang.Integer etc. As noted elsewhere, the Atomic* classes are mutable, and indeed have to be in order to serve their purpose. There's a difference in my mind between "the standard set of primitive wrapper classes" and "the set of classes which wrap primitive values".

你可以编写自己的可变包装器很容易上课：

You can write your own mutable wrapper class very easily:

public class MutableInteger
{
    private int value;

    public MutableInteger(int value) 
    {
         this.value = value;
    }

    public int getValue()
    {
        return value;
    }

    public void setValue(int value)
    {
        this.value = value;
    }
}

所以你可以看到，没有什么本质上不可变的包装类 - 只是标准的设计是不可变的，因为没有提供任何改变包装值的方法。

So as you can see, there's nothing inherently immutable about wrapper classes - it's just that the standard ones were designed to be immutable, by virtue of not providing any way to change the wrapped value.

请注意，这允许在装箱时重复使用相同的对象，对于常见值：

Note that this allows for the same object to be used repeatedly when boxing, for common values:

Integer x = 100;
Integer y = 100;
// x and y are actually guaranteed to refer to the same object

Integer a = 1000;
Integer b = 1000;
// a and b *could* refer to the same object, but probably won't

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