在构造函数或字段声明中初始化List [英] Initializing List in constructor or field declaration

问题描述

我想知道在初始化对象（如ArrayList<>）和字段声明或构造函数中的内容是否存在差异。

I was wondering whether there is a difference in initializing objects like ArrayList<> and stuff in field declaration or constructor.

内存使用情况，性能或其他类似情况是否存在差异或完全相同？

Is there a difference in memory usage, performance or anything like that or is it completely the same?

选项1：

class MyClass {
     private List<String> strings = new ArrayList<String>();
}

选项2：

class MyClass {
    private List<String> strings;
    public MyClass() {
        strings = new ArrayList<String>();
    }
}

这可能是一个愚蠢的问题，或者是一个非常基本的问题一，但我喜欢从一开始就建立，我喜欢理解我所看到的一切。

It may be a stupid question, or a very basic one, but I like to build from the start, I like to understand all that I see.

推荐答案

存在差异：初始化时。字段首先被初始化，然后构造函数被激活。

There is a difference: when initialization occurs. Fields are initialized first, then the constructor fires.

在你的简单例子中，没有实际的区别，但如果另一个字段依赖于List字段进行初始化，构造函数版本会随NPE爆炸。

In your trivial example, there would be no practical difference, but if another field depended on the List field for initialization, the constructor version would explode with a NPE.

考虑：

 private List<String> strings = Arrays.asList("foo", "bar");
 private String stringsDescription = strings.toString();

如果您将字符串的初始化移至构造函数， stringsDescription 的初始化会随NPE爆炸。

If you moved initialization of strings to the constructor, the initialization of stringsDescription would explode with a NPE.

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