Wah How to use "In case".

We use in case to express that an action is connected with something that might happen.

• The football referee wears two watches in case one of them stops.                               ( There was a possibility of one of the watches stopping. )
• I will take some food and drinks in case we get lost in the forest.                                     ( I will take some food and drinks before I go to the forest because I want to make sure that I don't get hungry or thirsty when I get lost in the forest. )
• It's a bit cloudy. You should take an umbrella in case it rains.
• You had better take a cellphone in case you need to contact me.
• I think I will wear gloves in case I get cold finger.
• We should leave early in case the traffic is bad.

Note: After in case we never use will even though we talk about the future.
         You should take an umbrella in case it will rains. is incorrect.

The different between in case and if.
    I will take an umbrella in case it rains. (Whether or not it rains, I will take it anyway.)
    I will take an umbrella if it rains. (If it does not rain, you won't take it.)

We use just in case to express that an action is connected with something that probably might not happen.

• I just want to sunbathe not swim. But I will take my swimsuit just in case.
• I might get hungry later so I will take something to eat just in case.
• I don't think I will be lost in Tokyo but I will take a map with me just in case.
• Get hold of the rail just in case.
• Make that file uncopyable just in case.

I usually wake up early, but I will set the alarm clock in case I oversleep.