Java: Create a multithreaded program that reads all text files in a directory given by the user and creates a single output text file that counts the frequency of words found in all text files, combined. Ignore punctuation and symbols. Only words should be counted.

example: suppose in a /textfiles directory, there exists:

input1.txt : "hello world"

input2.txt : "hello everyone"

input3.txt: "This is world for everyone"

input4.txt: "We are the world"

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Then either in the same directory or in a different directory, an output.txt file must contain:

hello: 2

world: 3

everyone: 2

is: 1

for: 1

this: 1

are: 1