用if、Exception还是assert？ ——看JavaEye论坛帖子：《面试题:用Exception异常还是if判断》的感想

package com.google.common.base;

public final class Preconditions {
  /**
   * Ensures the truth of an expression involving one or more parameters to the
   * calling method.
   *
   * @param expression a boolean expression
   * @param errorMessageTemplate a template for the exception message should the
   *     check fail. The message is formed by replacing each {@code %s}
   *     placeholder in the template with an argument. These are matched by
   *     position - the first {@code %s} gets {@code errorMessageArgs[0]}, etc.
   *     Unmatched arguments will be appended to the formatted message in square
   *     braces. Unmatched placeholders will be left as-is.
   * @param errorMessageArgs the arguments to be substituted into the message
   *     template. Arguments are converted to strings using
   *     {@link String#valueOf(Object)}.
   * @throws IllegalArgumentException if {@code expression} is false
   * @throws NullPointerException if the check fails and either {@code
   *     errorMessageTemplate} or {@code errorMessageArgs} is null (don't let
   *     this happen)
   */
  public static void checkArgument(boolean expression,
      @Nullable String errorMessageTemplate,
      @Nullable Object... errorMessageArgs) {
    if (!expression) {
      throw new IllegalArgumentException(
          format(errorMessageTemplate, errorMessageArgs));
    }
  }
  
  /**
   * Ensures that an object reference passed as a parameter to the calling
   * method is not null.
   *
   * @param reference an object reference
   * @return the non-null reference that was validated
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> T checkNotNull(T reference) {
    if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }
}