Preconditions作为Guava中的异常的前置检查，提供了一系列方法。用于API实现者检查API参数的合法性。
其中前置检查主要为

public static void checkArgument(boolean expression) {
if (!expression) {
throw new IllegalArgumentException();
}
}

public static void checkState(boolean expression) {
if (!expression) {
throw new IllegalStateException();
}
}

public static <T> T checkNotNull(T reference)
检查是否为空
public static int checkElementIndex(int index, int size)
检查元素的索引是否超出0～size
public static int checkPositionIndex(int index, int size)
判断索引错误或size小于0,返回format
public static void checkPositionIndexes(int start, int end, int size) {
if (start < 0 || end < start || end > size) {
throw new IndexOutOfBoundsException(badPositionIndexes(start, end, size));
}
}

private static String badElementIndex(int index, int size, String desc)

private static String badPositionIndexes(int start, int end, int size)

其中 Preconditions 中实现了format 构造字符串

static String format(String template, @Nullable Object... args) {
template = String.valueOf(template);
StringBuilder builder = new StringBuilder(template.length() + 16 * args.length);
int templateStart = 0;

    int i;
    int placeholderStart;
    for(i = 0; i < args.length; templateStart = placeholderStart + 2) {
        placeholderStart = template.indexOf("%s", templateStart);
        if (placeholderStart == -1) {
            break;
        }

        builder.append(template.substring(templateStart, placeholderStart));
        builder.append(args[i++]);
    }

    builder.append(template.substring(templateStart));
    if (i < args.length) {
        builder.append(" [");
        builder.append(args[i++]);

        while(i < args.length) {
            builder.append(", ");
            builder.append(args[i++]);
        }

        builder.append(']');
    }

    return builder.toString();
}

}