JAVA Essentials for Competitive Coding

Can anyone suggest some really cool/Interesting tutorial to learn advance concepts in JAVA that is very much useful for competitive coding, as I prefer using JAVA.
but, the problem is I really get stuck many times like while using COMPARATOR for sorting maps and other such advance concepts or topics in java…
like for Hashmap / HashSet / comparators / etc… I get so many links but all of them are too complex to understand also I don’t know which tutorial gives the most efficient implementation.
So, please help!!!

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