Flink中使用lambda表达式出现的问题

A1 报错

A2 原因

lambda表达式编写方式不自动识别返回类型，需要手动跟上returns指定类型。

源码：

/**
	 * Adds a type information hint about the return type of this operator. This method
	 * can be used in cases where Flink cannot determine automatically what the produced
	 * type of a function is. That can be the case if the function uses generic type variables
	 * in the return type that cannot be inferred from the input type.
	 *
	 * <p>In most cases, the methods {@link #returns(Class)} and {@link #returns(TypeHint)}
	 * are preferable.
	 *
	 * @param typeInfo type information as a return type hint
	 * @return This operator with a give