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Streams
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String methods and String buffer
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As list , List of API's
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Collection Framework
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Static keyword
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default methods in interfaces
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Algorithms(Binary search, Linear search, Bubble sort, Insertion sort, Selection sort, Find duplicate characters etc.)
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Generics
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SOLID Prenciple (Singleton,Liskov, Open/closed, interface seggreation etc.)
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Patterns (Builder, Factory, DI etc.)