The given statement can be agreed on all terms and conditions. Because happiness and sadness are two extreme emotions that oppose each other, hence only when one has seen the worst can one experience real happiness. This can be clearly understood with suitable examples which support the given statement.
Let us take the example of the great and skilful inventor of the Light Bulb i.e., Thomas Alva Edison. When he was a child his teacher kept criticizing Edison that his attempts to invent the light bulb will be futile and worthless. Despite constant criticism and humility before others, he didn't lose hope of what he wanted to achieve. Yes, he failed numerous times while inventing the light bulb which made him feel sorrowful, but he saw himself as even more dedicated and industrious towards his goal and the fire in him made to think why he failed multiple times despite a lot of effort put to work. But when he finally achieved his invention of the light bulb the amount of happiness he must have felt could be unmeasurable. This can be seen as an example of how a sad person became happy and contented from the constant defamation and vilification.
Another bolstering instance/ example can be taken as the case of the water shortage problems to many households in many places during the peak summertime. Whenever the summer season arrives gradually, many people face water shortages and many other problems related to water shortages. Although, when the rain comes, most of them feel happy and relieved due to the increase in water levels in their storage. The happiness they feel when the weather reports on television state that the monsoon is nearing and is liable to relieve the country from its water problems, cannot be determined.
The last example can be supported with the following instance when a student achieves a gold medal in a particular activity be it in games, education or co-curricular activities, etc. the joy and jubilation the student can get from achieving the goal is very great. Even winning a gold medal is not as easy as it looks. Many students feel unhappy and downcasted for not achieving the gold medal they wanted to have around their neck. But when they keep practising any particular activity tirelessly and constantly think about their goal, there will surely arrive a time when they can achieve that "gold medal" and once they wear it the happiness on their faces are fathomless.
Thus, from the above-stated examples by supporting very crucial points in those examples, one can never feel the true and real meaning of happiness when they cannot feel the essence of sadness. As cited above sadness and happiness are two extremes where one has to face multiple failures to succeed to greater heights. Hence the given statement "Only once one has known real sadness can one feel true happiness." is quite genial and has got real meaning to it.