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public static int isValid(int n)
This returns an integer. Which you do here;
Great! Where are you storing that returned result??? There's problem #1
Also, I think you may need to consider your steps again.
Step 1 - Read Number
Step 2 - Test Number
Step 3 - Valid Number = Store It / Invalid Number alert user
Step 4 - Repeat until array is filled with valid numbers.
Also, are 50 and 100 valid entries or not?
Why did you use && in the if statement. Can a number be both <= 50 AND >= 100 at the same time?
You are passing the value of i into the method and testing that, is that the number you want to test if it is valid. What is the value of i for each iteration of the loop
With a name like isValid, I feel it would be more natural to return a boolean result. But both ways can work.