Can only initialize a map within context of a function? ( can't initialize within constructor too)

Question

So my code basically looks like below

public with sharing class test_exists {

    Public Map<Id, Case> cases;

    public static void start_creation(Map<String, Account> acc){
        cases = New Map<Id, Case>();
        for (String s: acc.keyset()){
            Account c_acc = acc.get(s);
            Case new_case = new Case(AccountId = c_acc.Id, Name=c_acc.Name);
            cases.put(c_acc.Id, new_case);
        }
    }
}

I'm getting two errors

1. Variable does not exist: cases

2. Method does not exist or incorrect signature: void.put - and I've chalked this up to cases not actually being initialized and thus it's still null.

I've also tried

public with sharing class test_exists {

    Public Map<Id, Case> cases{get;set;}

    public test_exists(){
        cases = New Map<Id, Case>();
    }

    public static void start_creation(Map<String, Account> acc){
        for (String s: acc.keyset()){
            Account c_acc = acc.get(s);
            Case new_case = new Case(AccountId = c_acc.Id, Name=c_acc.Name);
            cases.put(c_acc.Id, new_case);
        }
    }
}

but I'm still getting the same error of "Method does not exist or incorrect signature: void.put"

Might the fact that I'm calling New within the constructor be the source of the problem? I've done similar things such as assigning to a previously declared variable without problems.

Edit:

The class above is being called from a trigger which has access to a bunch of other classes.

I removed the static keyword as suggested below and now from within the trigger code there is an error of 'Non static method cannot be referenced from a static context: void test_exists.start_creation(Map)'

Does that mean that acc as is being passed in has to be a static?

Answer 1

Remove static keyword from your function as non-static properties cannot be used in a static method.

---

update for additional question.

Change your code so that it returns a map:

public with sharing class test_exists {

    public static void start_creation(Map<String, Account> acc){
        Map<Id, Case> cases = New Map<Id, Case>();
        for (String s: acc.keyset()){
            Account c_acc = acc.get(s);
            Case new_case = new Case(AccountId = c_acc.Id, Name=c_acc.Name);
            cases.put(c_acc.Id, new_case);
        }
        return cases;
    }
}

It may be static or not, it would only change how the method is being called.