Congratulations on considering to write at OpenGenus IQ 
for Java language.
This is a small list of topics that you can write about Java 
Programming Language.
To become a writer at OpenGenus IQ, check out this guide
For using this task list, check out this guide.
Following is the list of topics:
Memory management
- Minor, Major and Full Garbage Collection
- JVM Memory Model
- Mark Sweep Compact Copy algorithm
- Garbage Collection algorithms
- Garbage Collection Tuning and Optimization
Variable, Data-type and Operators
- Variable, Data-type and Operators (assigned to @yifan97)
- Primitive data types
- Arithmetic data types
- Operator types
- Operator precedence
- Type conversion
Input and Output
- Input and output with streams
- File stream
- Buffered stream
- Chaining streams
- Using default file system
- Using ZIP file system
Control Flow
- If else (assigned to @sherlockbeard)
- For loop (assigned)
- While loop (assigned)
- Do while loop (assigned)
- continue (assigned)
- break (assigned)
- Switch case
User defined data types
- Classes + Objects (assigned to @janvi04)
- this reference
- null reference
- Encapsulation (assigned to @janvi04)
- access modifier
- constructors
- class initializers
- blocks
Inheritence
- Inheritance basics
- Typed references
- Member hiding and overriding
- Super reference
- Using Final and abstract
- Multiple inheritance
Data types
- String class
- StringBuilder class
- Wrapper class
- Final field and enumeration types
- String equality
Exceptions and error handling
- Throwing exceptions (assigned to @Robotjelly123)
- Custom exceptions
- Error handling by exception
- Error handling by type
Java Collections
- List (assigned to @weichli)
- ArrayList
- Linked List
- Set
- Queue
- Deque
- Stack
- Map (assigned to @Robotjelly123)
- HashMap (assigned to @yilin)
- HashSet
- Collection Interface
- Iterable Interface
Generics
- Generic class
- Generic object
- Type bound
- Reflection introduction
- Class literals
- Reflecting types
- Reflecting generic information
Concurrency
- Thread (assigned to @Robotjelly123)
- Process
- CPU time sharing Thread scheduler
- Race condition
- Reentrant lock
- Deadlock
- Producer consumer problem using wait and notify
- Ordering read write operations on multi-core CPU
- Implementing a thread safe Singleton on multi-core CPU
Design patterns in Java
- Observer pattern
- Command Pattern
- Singleton Pattern
- Builder Pattern
- Factory Pattern
- Abstract Factory Pattern
- Null Pattern
- Command Pattern
- Singleton Pattern
- Builder Pattern
- Factory Pattern
- Abstract Factory Pattern
- Null Pattern
Others
- Packages
- Interface
- Generic interface
- Static members
- Inner classes
- Nested types
- Anonymous class
- Static initialization blocks