如何打开在Java字符串枚举中定义的值

Question

I have the following code :

class MyClass {
    private Value value;
    public enum Value {
        INSERT_ONLY("INSERT_ONLY"), UPDATE_ONLY("UPDATE_ONLY"), UPSERT("UPSERT") ;
        private final String val ;
        private Value(final String v) {val = v ;}
        public String toString() {return val ;}     
        public String getVal() {
            return val;
        }
    } ;
    public Value getValue() {
        return value;
    }
    public void setValue(Value value) {
        this.value = value;
    }
}

public class one {
    public static void main(String[] args) {
        MyClass obj = new MyClass() ;
        obj.setValue(MyClass.Value.INSERT_ONLY) ;

        String s = obj.getValue().toString() ;
        String s1 = MyClass.Value.INSERT_ONLY.toString() ;
        switch(s) {
            case "INSERT_ONLY" : System.out.println("INSERT_ONLY") ;
                break ;
            case "s2" : System.out.println("s2") ;
                break ;
        }
    }
}

This code works. But what I want is that in switch-case I use the strings as defined in the enum Value . If I use s1 in case , it generates an error. What is the way out?

Answer 1

Enums have a method .name() that returns the enum as a String. All your "values" are equivalent to this - just switch on that.

You can delete the value field, the constructor and getter and call name() where you are currently calling getValue() .

Further, the default implementation of toString() returns name() so you can delete your toString() method without having any effect.

Finally, you can switch on the enum itself.

Answer 2

you string in the enum is actually the same as the enumertor constants... so it is a little redundant and completely unnescessary...

try this:

enum Value {
    INSERT_ONLY, UPDATE_ONLY, UPSERT;
}

public static void main(String[] args)  {
Week obj = new Week() ;
Value s = obj.getValue( ) ;
        switch(s) {
            case INSERT_ONLY : System.out.println("INSERT_ONLY") ;
                break ;
            case UPDATE_ONLY : System.out.println("UPDATE_ONLY") ;
                break ;
        }
    }
}

Answer 3

You could try it this way:

MyClass.Value value = obj.getValue() ;
switch(value) {
  case INSERT_ONLY : System.out.println("INSERT_ONLY") ;
    break ;
  case UPSERT : System.out.println("Upsert") ;
    break ;
  case UPDATE_ONLY : System.out.println("Update only") ;
   break ;
 }

Basically, the switch-statement uses the enum-names. So there is no need to apply the toString() -method on the Value returned by obj.getValue() .

If you would like to know why switch will not work in combination with strings, please have a look here . One more suggestion: add also the default-branch to the switch-statement.

Answer 4

If I understand correctly, you're trying to look up an enum constant by an unknown string s . The reason your case expression can't be s1 is because it must be a compile-time constant , which s1 is not. Since your example seems to be mostly theoretical, I'll suggest a few options and you can pick the most appropriate for your actual case:

1. Assuming the enum names are the same as their value s (in which case you can scrap the field entirely) and you're just trying to look up an enum by its name, just do this:

    MyClass.Value v = MyClass.Value.valueOf(s); 

   This will throw an IllegalArgumentException if no mapping is found for s1 .

2. Still assuming the names are the same, but you do need an actual switch with some additional cases and custom logic:

    try { MyClass.Value v = MyClass.Value.valueOf(s); switch (v) { case INSERT_ONLY : System.out.println("INSERT_ONLY") ; break ; } } catch (IllegalArgumentException e) { switch (s) case "s2" : System.out.println("s2") ; break ; } } 

3. If the names are not actually the same, you can add a static map of constants inside the enum class, to simulate valueOf() :

    public enum Value { ONLY_INSERT("ONLY_INSE"), ONLY_UPDATE("UPDATE_ONLY"), UPSERT("UPSERT") ; private static final Map<String, Value> byName = new HashMap<>(); static { for (Value v : values()) { byName.put(v.getVal(), v); } } public static Value byName(String name) { Value result = byName.get(name); if (result == null) { throw new IllegalArgumentException("Invalid name" + name); } return result; } private final String val ; private Value(final String v) {val = v ;} public String toString() {return val ;} public String getVal() { return val; } } ; 

   Now you can do the same as the previous solutions, using MyClass.Value.byName() .