# Preconditions:方法入参校验工具

## 为什么需要Preconditions

在日常开发中，我们经常会对方法的输入参数做一些数据格式上的验证，以便保证方法能够按照正常流程执行下去。对于可预知的一些数据上的错误，我们一定要做事前检测和判断，来避免程序流程出错，而不是完全通过错误处理来保证流程正确执行，毕竟错误处理是比较消耗资源的方式，并且谨慎编程所持有的一个态度就是：**永远不要相信收到的数据是合法的。**

平常写代码时我们可能会采用如下方式进行参数判断：

```java
private void add(boolean bool, int[] array, int position) {
    if (!bool) {
        throw new IllegalArgumentException("preCondition not allow!!");
    }
    if (array == null) {
        throw new NullPointerException("array is null!!");
    }
    if (array.length == 0) {
        throw new IllegalArgumentException("array length is 0!!");
    }
    if (position > array.length || position < 0) {
        throw new ArrayIndexOutOfBoundsException("position error!!");
    }

    //ok, do something...
}
```

采用这种方式，参数的判断都需要自己来逐个写方法判断，代码量多并且无法复用。

## Preconditions

Preconditions是Guava类库中提供了的一个作参数检查的工具类， 该类可以大大地简化我们代码中对于参数的预判断和处理，让我们对方法输入参数的验证实现起来更加简单。

下面，使用Preconditions对上述代码进行改造：

```java
private void testPreconditions(boolean bool, int[] array, int position) {
   Preconditions.checkArgument(bool);
   Preconditions.checkNotNull(array);
   Preconditions.checkElementIndex(position, array.length);
   //ok, do something...
}
```

还可以自定义错误信息：

```java
private static void testPreconditions(boolean bool, int[] array, int position) {
    Preconditions.checkArgument(bool, "not allow!!");
    Preconditions.checkNotNull(array, "array is null!!");
    Preconditions.checkElementIndex(position, array.length, "position error!!");
    //ok, do something...
}
```

**参考**

[Google Guava中Preconditions的用法，让前置条件判断变得更优雅](https://blog.csdn.net/zivensonice/article/details/51912188)

[Guava学习笔记：Preconditions优雅的检验参数](https://www.cnblogs.com/peida/p/Guava_Preconditions.html)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://maxwell.gitbook.io/way-to-architect/java-yu-yan/yi-chang/fang-fa-ru-canxiao-yan-gong-ju-preconditions.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.