If I declare a class as 'protected', can other classes in the same package access the class?

What is "inversion of control" design pattern? What is "dependency injection" ?

Suppose U have a data collection (say a "arraylist"). U want to make sure that no one else can modify it. i.e. U want to make it immutable. How will U do it?

What would U do if U need a "Multi-Map" collection? i.e. U have a collection where one key may map to many values.

Can a Java class be private or protected?

What is the advantage of Inner classes over encapsulation?
I can make a class as "package" access. Will this not suffice ? Why do I need Inner classes?

How will U debug a Java program that has several threads running?
If U place a break-point somewhere, will all threads be suspended or only a single thread?