如何创建一个只能设置一次但在Java中不是最终的变量 [英] How to create a variable that can be set only once but isn't final in Java

问题描述

我想要一个可以使用一个变量unset（ id ）创建实例的类，然后稍后初始化此变量，并在初始化后使其不可变即可。实际上，我想要一个 final 变量，我可以在构造函数之外进行初始化。

I want a class that I can create instances of with one variable unset (the id), then initialise this variable later, and have it immutable after initialisation. Effectively, I'd like a final variable that I can initialise outside of the constructor.

目前，我使用一个抛出异常的setter即兴创建，如下所示：

Currently, I'm improvising this with a setter that throws an Exception as follows:

public class Example {

    private long id = 0;

    // Constructors and other variables and methods deleted for clarity

    public long getId() {
        return id;
    }

    public void setId(long id) throws Exception {
        if ( this.id == 0 ) {
            this.id = id;
        } else {
            throw new Exception("Can't change id once set");
        }
    }
}

这是一个很好的方法吗？谈论我正在做什么？我觉得我应该能够在初始化后设置一些不可变的东西，或者我可以使用一种模式来使它更优雅。

Is this a good way of going about what I'm trying to do? I feel like I should be able to set something as immutable after it's initialised, or that there is a pattern I can use to make this more elegant.

推荐答案

让我建议你更优雅的决定。
第一个变种（不抛出异常）：

Let me suggest you a little bit more elegant decision. First variant (without throwing an exception):

public class Example {

    private Long id;

    // Constructors and other variables and methods deleted for clarity

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = this.id == null ? id : this.id;
    }

}

第二个变种（抛出异常） ）：

Second variant (with throwing an exception):

     public void setId(long id)  {
         this.id = this.id == null ? id : throw_();
     }

     public int throw_() {
         throw new RuntimeException("id is already set");
     }

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