Difference between internal-external private-public in function

Question

According to the Solidity docs, by default, function types are internal, so the internal keyword can be omitted. In contrast, contract functions themselves are public by default, only when used as the name of a type, the default is internal.

I have a doubt here. How is it possible that a function can be internal and public at the same time? Can somebody tell me the difference between internal-external, public-private in regards to the functions??

Answer 1

External: These functions are part of the contract interface and can be called from other contracts and via transactions. An external function cannot be called internally. 

Public: These can be called internally or via messages. 

Internal: these functions and variables can only be accessed internally. (from the current or the derived contract) 

Private: these functions and state variables are only visible for the contract they are defined in and not the derived contracts.

Answer 2

When a function is public, anybody can access it. When it is private, only the contract in which the function exists can access it.

Answer 3

A function that calls a function in the same contract is called an internal function. And a function that calls a function that's from a different smart contract is called an external function.

Answer 4

An internal function is similar to protected access specifier in Object Oriented programming languages. When you specify a function as internal, only the functions of that contract and the functions derived from that contract can access that internal-specified function. This is to provide protection for the functions so that it cannot be accessed by other smart contracts. 

Answer 5

When a function is internal and public, it means that the function can be called only by the calls from within that smart contract or from the calls derived from that smart contract.