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putIfAbsent vs computeIfAbsent putIfAbsent putIfAbsent() 方法会先判断指定的键（key）是否存在，不存在则将键/值对插入到 HashMap 中。 如果所指定的 key 已经在 HashMap 中存在，返回和这个 key 值对应的 value, 如果所指定的 key 不在 HashMap 中存在，则返回 null, putIfAbsent 的重点不在于返回值, 而在于如果 key 不为空, 就不插入新值了

Using real life cases to understand when to use NoSQL or SQL?

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.

Rethink system design in a much fun way, as a former urban planner/landscape planner.