Generic enum iterator java

Question

My Enum

public enum ExamStausEnum {

    RESULTAWAITED("Result Awaiting"), 
    PASSED("Passed"), 
    FAILED("Failed");

    private String value;

    ExamStausEnum(String value) {
        this.value = value;
    }

    @JsonValue
    public String getValue() {
        return value;
    }
}

Generic Iterator Enum

static <E extends Enum <E>> void foo(Class<E> elemType) {
    for (E e : java.util.EnumSet.allOf(elemType)) {
        System.out.println(e);
    }
}

Result :

RESULTAWAITED
PASSED
FAILED

How can I print the constructor values ?

Result Awaiting
Passed
Failed

@khelwood, but this is generic for all enums, and getValue is for this enum called ExamStausEnum — mlecz
– mlecz, Commented Oct 11, 2017 at 11:30
you cannot do it generically. If you have a couple enums with value field, you can get that field using reflection. — mlecz
– mlecz, Commented Oct 11, 2017 at 11:31

Holger · Accepted Answer · 2017-10-11 11:41:51Z

6

It’s unavoidable to add another parameter to have an abstraction of the getValue() call:

static <E extends Enum <E>> void foo(Class<E> elemType, Function<? super E, ?> f) {
    for(E e : java.util.EnumSet.allOf(elemType)) {
        System.out.println(f.apply(e));
    }
}

Then, you may it invoke for arbitrary enum types not necessarily having that method, e.g.

foo(Thread.State.class, Object::toString);

or for your specific enum having the method:

foo(ExamStausEnum.class, ExamStausEnum::getValue);

Even more use cases are possible:

foo(Thread.State.class, Enum::name);

or

foo(ExamStausEnum.class, Enum::ordinal);

Of course, you may also let your ExamStausEnum type override the toString() method, eliminating the need for foo to call the getValue() method.

answered Oct 11, 2017 at 11:41

Holger

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Comments

Timothy Truckle · Accepted Answer · 2017-10-11 11:57:00Z

1

the more foreward way is to create an interface declaring the getValue() method implemented by all your enums.

interface EnumWithValue{
   @JsonValue
   String getValue();
}

public enum ExamStausEnum implements EnumWithValue {

    RESULTAWAITED("Result Awaiting"), 
    PASSED("Passed"), 
    FAILED("Failed");

    private String value;

    ExamStausEnum(String value) {
        this.value = value;
    }

    @Override
    public String getValue() {
        return value;
    }
}

then you can cast your enum in foo:

static <E extends Enum <E>> void foo(Class<E> elemType) {
    for (E e : java.util.EnumSet.allOf(elemType)) {
        System.out.println(((EnumWithValue)e).getValue());
    }
}

answered Oct 11, 2017 at 11:57

Timothy Truckle

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3 Comments

Holger Over a year ago

I’d rather change the declaration to <E extends Enum<E>&EnumWithValue> void foo(Class<E> elemType) instead of inserting a type cast that can fail at runtime.

Timothy Truckle Over a year ago

I think your suggestion will also fail at runtime in the same situation...

Holger Over a year ago

When you use the declaration <E extends Enum<E>&EnumWithValue>, the compiler will already reject Class objects of enums not implementing EnumWithValue. With your code, you will not even get a warning when passing a wrong type.

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