In Java:
How to create a class
How to create an object
How to create a constructor and Types of constructors
How to create a method and how to use the same method in different classes
Conditional and looping statements
Inheritance
Up-casting & Down-casting
Runtime polymorphism - generally we use in POM(Selenium) it is also called lazy initialization
File Input | File Output [How to read the data from the file and How to write the data into file]
If you know the above-mentioned topics then you are ready to kick start with selenium and later on you can learn the collection topic of Java.
All the best mate!
