How do i refrence my validation method (true or false) when i makeing a conditional statment?

so i have an ArryList called "files" and ive created a validation method for validating if its when used you are callling on a valid index refrence:

// validation method of an ArrayList called "files"
public boolean validIndex(int index){
    if(index >= 0 && index < files.size()){
        return true;
    }
    else{
        return false;
    }
}

The insted of just calling on the get method i want to be able to refrence "validIndex" method when calling an on an item from the arrayList.

// Trying to make this one work:
Public void listFile(int index){
    if(validindex(index) = true){
        file.get(index);
    }
    else{
        System.out.println("Index: " + index + "is not valid!");
    }
}

please help