如何通过值查找枚举

Question

public enum Foo {
   one ("one"), two ("two")
}

How to find a enumeration by its value (say Foo.getFoo("two") ) should return Foo.two .

Answer 1

Each enum has a valueOf(String) method that looks up an instance given the name.

Foo.valueOf("one") == Foo.one

This is based on the name of the enum value. There is no concept of value as such for enums -- only names and ordinals.

Answer 2

You have some options...

Know the enum, want to look up by name

If you know the exact enum type and its name, as others have pointed out, you can use it directly:

Foo myFoo = Foo.valueOf("one");

Keep in mind that the string has to match the enum name exactly, including case. Enum values are usually all-caps, so if you're looking up by some String s , it's often a good idea to do Foo.valueOf(s.toUpperCase()) .

Have the enum's Class, want to look up by name

If you don't have that type, but you have a class representing the enum type, you can use the static method Enum.valueOf(Class<T>, String) to get the enum for its name:

Class<T> myEnumClass = whatever();
T myEnum = Enum.valueOf(myEnumClass, "one");

As above, the string is case sensitive.

Want to look up by some non-name attribute

If you want to look up the enum by some other (presumably unique) value, one approach would be to loop over the enum values. You can get them either from Foo.values() , or, if you just have a Class<T> , you can use the instance method Class.getEnumConstants() :

T[] values = myEnumClass.getEnumConstants();
for (T value : values) ...

same, but potentially faster

If you need to do this often, and speed is a concern, it may be useful to pre-load these into a map (but see below for caveats):

Map<EnumVal, Foo> valuesToEnum = new HashMap<EnumVal, Foo>();
for (Foo foo : Foo.values()) {
    EnumVal val = foo.getValue();
    Foo oldFoo = valuesToEnum.put(val, foo);
    assert oldFoo == null : "multiple Foos have a value " + val;
        // ^^^ this means we can't meaningfully look up by EnumVals.
}
// now you can do something like
EnumVal lookupBy = whatever2();
Foo foo = valuesToEnum.get(lookupBy);
if (foo == null)
    throw new IllegalArgumentException("no Foo has the value " + lookupBy);

Note that this requires calculating the EnumVal 's hash, if it's not pre-computed, and then a couple lookups. If you have a lot of values in the enum, this lookup could be worth it. Otherwise, a loop over a handful of items is likely to be about as fast, plus or minus -- and either way, it'll be negligible.

Answer 3

特别是在您的情况下， Foo.valueOf(String)可以做到。

Answer 4

The valueOf(String) only works on the declared name s of Enum . If you want to find an enum by some custom value, the best way is to provide a find method:

public static Foo fromValue(String value) {
    for (Foo foo : Foo.values()) {
        if (foo.value.equals(value)) {
            return foo;
        }
    }
}