OOPS? Hashmap, Map? Why string is immutable? String and StringBuilder difference Arrays and Collection Difference? Multithreading ?

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Questions on OOP concepts, data structures, and multithreading.

OOPS stands for Object-Oriented Programming System.
HashMap and Map are both interfaces in Java. HashMap is a class that implements the Map interface.
String is immutable in Java because it enhances security, concurrency, and performance.
String is a sequence of characters, while StringBuilder is a mutable sequence of characters.
Arrays are fixed in size, while Collections are dynamic in size.
Multithreading is the ability of a CPU to execute multiple threads concurrently.

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