一、从动态数组到二维 ArrayList：核心概念 ArrayList 是数组（Array）的动态版本。原生数组在声明时必须预定义长度，而 ArrayList 内部维护一个可扩容的底层数组，当元素数量超过当前容量时会自动增长，因此使用者在多数场景下无需关心容量上限。

本文以 cosmos 仓库中 readme-2DArray.md 为主线，结合同一目录下的可运行示例 2d-array.java 与 2d-array-list-java.java ，系统讲解二维数组的定义、声明、初始化、动态创建、遍历输出，并延伸到二维 ArrayList 与常见陷阱。

To implement a program that tracks the first unique (non-repeating) number in a stream of integers using a LinkedHashMap.

ArrayList is a dynamic array. In other words, you don't need to specify the size at the beginning, you can store values one after another as you go.

This is something beginners encounter quite often when they start writing Java in training. Exception in thread "main" java.lang.NullPointerException at ...