What are enums and why are they useful?

Question

Today I was browsing through some questions on this site and I found a mention of an enum being used in singleton pattern about purported thread safety benefits

Answer 1

I would use enums as a useful mapping instrument, avoiding multiple if-else provided that some methods are implemented.

public enum Mapping {

    ONE("1"),
    TWO("2");

    private String label;

    private Mapping(String label){
        this.label = label;
    }

    public static Mapping by(String label) {

        for(Mapping m: values() {
            if(m.label.equals(label)) return m;
        }

        return null;
    }

}

So the method by(String label) allows you to get the Enumerated value by non-enumerated. Further, one can invent mapping between 2 enums. Could also try '1 to many' or 'many to many' in addition to 'one to one' default relation

In the end, enum is a Java class. So you can have main method inside it, which might be useful when needing to do some mapping operations on args right away.