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.equals()-Regarding collections

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From the K&B 1.5 p. 526-27

Of Course,overriding the equals() for Car also allows the potential that more than one object representing a single unique car can exist, which might not be safe in your design. Fortunately the String and Wrapper classes work well as keys in hashtable-they override the equals() method.

I dont quite understand the above. What does the overriden equals() of String and wrapper class has so special about it that I couldnt do with my class car. If I override the equals() of car class how is that not as good as the overiden equals() of String.
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First Check why we need to override hashcode and why we need to override equals method.
Check how hashcode and equals are interlinked with each other.
Check proper functions of map.

Then in above code comment and uncomment hashcode or equals method
and see result carefully.

If you will understand above code then you will be able to answer any question in exam regarding hashcode and equals method.
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