Guava记录 - MoreObjects.firstNonN

在guava提倡的Using and avoiding null中，有关于此方法的相关描述:

Whenever you want a null value to be replaced with some default value instead, use MoreObjects.firstNonNull(T, T). As the method name suggests, if both of the inputs are null, it fails fast with a NullPointerException. If you are using an Optional, there are better alternatives -- e.g. first.or(second).

意思是当您希望用默认值替换空值时且为非Optional时，请使用MoreObject。
如果使用optional的话，可以使用optional的or()方法;

此方法要求传递两个参数。从命名上看仿佛描述了一个first参数不为空的处理，其实此方法类似于咱常常使用的三目运算。其源码如下:

/**
   * Returns the first of two given parameters that is not {@code null}, if either is, or otherwise
   * throws a {@link NullPointerException}.
   *
   * <p>To find the first non-null element in an iterable, use {@code
   * Iterables.find(iterable, Predicates.notNull())}. For varargs, use {@code
   * Iterables.find(Arrays.asList(a, b, c, ...), Predicates.notNull())}, static importing as
   * necessary.
   *
   * <p><b>Note:</b> if {@code first} is represented as an {@link Optional}, this can be
   * accomplished with {@link Optional#or(Object) first.or(second)}. That approach also allows for
   * lazy evaluation of the fallback instance, using {@link Optional#or(Supplier)
   * first.or(supplier)}.
   *
   * @return {@code first} if it is non-null; otherwise {@code second} if it is non-null
   * @throws NullPointerException if both {@code first} and {@code second} are null
   * @since 18.0 (since 3.0 as {@code Objects.firstNonNull()}).
   */
  public static <T> T firstNonNull(@Nullable T first, @Nullable T second) {
    return first != null ? first : checkNotNull(second);
  }

checkNotNull方法的源码如下:

/**
   * 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
   */
  @CanIgnoreReturnValue
  public static <T> T checkNotNull(T reference) {
    if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

以上可以看出：
1、其用法为如果两个参数都不为空，则返回第一个；
2、如果都为空，则抛出空指针异常
3、如果其中一个为空，返回不为空的那个

具体测试代码如下:

 private static void testMoreObjectsFirstNonNull() {
        List<String> testList1 = Lists.newArrayList("a", "b", "c");
        List<String> testList2 = Lists.newArrayList("1", "2", "3");
        List<String> testList3 = null;

        //大致意思就是如果其中一个为null就返回另外一个，都为null则抛出空指针
        //step1 test all not null
        System.out.println("test all not null,return first object : "+JsonMoreUtils.toJson(MoreObjects.firstNonNull(testList1, testList2)));
        //step2 test first is null
        System.out.println("test first is null,return another : "+JsonMoreUtils.toJson(MoreObjects.firstNonNull(testList3, testList2)));
        //step3 test second is null
        System.out.println("test second is null,return another : "+JsonMoreUtils.toJson(MoreObjects.firstNonNull(testList1, testList3)));
        //step4 test all is null
        System.out.println("test all is null: "+JsonMoreUtils.toJson(MoreObjects.firstNonNull(testList3, testList3)));
    }

输出结果: