继承在枚举类中的应用（Extending Enums in Java）

Extending an Enum Type

public enum BasicStringOperation {
    TRIM("Removing leading and trailing spaces."),
    TO_UPPER("Changing all characters into upper case."),
    REVERSE("Reversing the given string.");

    private String description;

    // constructor and getter
}

public enum ExtendedStringOperation extends BasicStringOperation {
    MD5_ENCODE("Encoding the given string using the MD5 algorithm."),
    BASE64_ENCODE("Encoding the given string using the BASE64 algorithm.");

    private String description;

    // constructor and getter
}

原因：
Cannot inherit from enum ‘BasicStringOperation’ → Inheritance Is Not Allowed for Enums

原因：

• It turns the enum into a subclass of the abstract class java.lang.Enum
• It compiles the enum as a final class

$ javap BasicStringOperation  
public final class com.baeldung.enums.extendenum.BasicStringOperation 
    extends java.lang.Enum<com.baeldung.enums.extendenum.BasicStringOperation> {
  public static final com.baeldung.enums.extendenum.BasicStringOperation TRIM;
  public static final com.baeldung.enums.extendenum.BasicStringOperation TO_UPPER;
  public static final com.baeldung.enums.extendenum.BasicStringOperation REVERSE;
 ...
}

Emulate Extending the Constants

public interface StringOperation {
    String getDescription();
}

public enum BasicStringOperation implements StringOperation {
    TRIM("Removing leading and trailing spaces."),
    TO_UPPER("Changing all characters into upper case."),
    REVERSE("Reversing the given string.");

    BasicStringOperation(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    private String description;
}

public enum ExtendedStringOperation implements StringOperation {
    MD5_ENCODE("Encoding the given string using the MD5 algorithm."),
    BASE64_ENCODE("Encoding the given string using the BASE64 algorithm.");

    ExtendedStringOperation(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    private String description;
}

public class Application {
    // 具体类型
    public String getOperationDescription(BasicStringOperation stringOperation) {
        return stringOperation.getDescription();
    }
    // 面相接口编程
    public String getOperationDescription1(StringOperation stringOperation) {
        return stringOperation.getDescription();
    }
}

Extending Functionalities

public interface StringOperation {
    String getDescription();
    String apply(String input);
}

public enum BasicStringOperation implements StringOperation {
    TRIM("Removing leading and trailing spaces.") {
        @Override
        public String apply(String input) {
            return input.trim();
        }
    },
    TO_UPPER("Changing all characters into upper case.") {
        @Override
        public String apply(String input) {
            return input.toUpperCase();
        }
    },
    REVERSE("Reversing the given string.") {
        @Override
        public String apply(String input) {
            return new StringBuilder(input).reverse().toString();
        }
    };

    BasicStringOperation(String description) {
        this.description = description;
    }

    @Override
    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    private String description;
}

public enum ExtendedStringOperation implements StringOperation {
    MD5_ENCODE("Encoding the given string using the MD5 algorithm.") {
        @Override
        public String apply(String input) {
            return DigestUtils.md5Hex(input);
        }
    },
    BASE64_ENCODE("Encoding the given string using the BASE64 algorithm.") {
        @Override
        public String apply(String input) {
            return new String(Base64.getEncoder().encodeToString(input.getBytes()));
        }
    };

    ExtendedStringOperation(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    private String description;
}

public class Application {
    public String applyOperation(StringOperation operation, String input) {
        return operation.apply(input);
    }
}

String input = " hello";
Application app = new Application();
System.out.println(app.applyOperation(BasicStringOperation.TRIM, input)); // "hello"
System.out.println(app.applyOperation(BasicStringOperation.TO_UPPER, input));// " HELLO"
System.out.println(app.applyOperation(BasicStringOperation.REVERSE, input));// "olleh"
System.out.println(app.applyOperation(ExtendedStringOperation.BASE64_ENCODE, input)); // "IGhlbGxv"
System.out.println(app.applyOperation(ExtendedStringOperation.MD5_ENCODE, input)); // "292a5af68d31c10e31ad449bd8f51263"

Associating Enum Constants and Interface Implementations

public class Application {
    public enum ImmutableOperation {
        REMOVE_WHITESPACES, TO_LOWER, INVERT_CASE
    }

    private static final Map<ImmutableOperation, Operator> OPERATION_MAP;

    static {
        OPERATION_MAP = new EnumMap<>(ImmutableOperation.class);
        OPERATION_MAP.put(ImmutableOperation.TO_LOWER, String::toLowerCase);
        OPERATION_MAP.put(ImmutableOperation.INVERT_CASE, StringUtils::swapCase);
        OPERATION_MAP.put(ImmutableOperation.REMOVE_WHITESPACES, input -> input.replaceAll("\\s", ""));
    }

    public String applyImmutableOperation(ImmutableOperation operation, String input) {
        return OPERATION_MAP.get(operation).apply(input);
    }
}

String input = " He ll O ";
Application app = new Application();
System.out.println(app.applyImmutableOperation(ImmutableOperation.TO_LOWER, input)); // " he ll o "
System.out.println(app.applyImmutableOperation(ImmutableOperation.INVERT_CASE, input));// " hE LL o "
System.out.println(app.applyImmutableOperation(ImmutableOperation.REMOVE_WHITESPACES, input));// "HellO"

Validating the EnumMap Object

public enum ImmutableOperation {
    REMOVE_WHITESPACES, TO_LOWER, INVERT_CASE
}

private static final Map<ImmutableOperation, Operator> OPERATION_MAP;

static {
    OPERATION_MAP = new EnumMap<>(ImmutableOperation.class);
    OPERATION_MAP.put(ImmutableOperation.TO_LOWER, String::toLowerCase);
    OPERATION_MAP.put(ImmutableOperation.INVERT_CASE, StringUtils::swapCase);
      OPERATION_MAP.put(ImmutableOperation.REMOVE_WHITESPACES, input -> input.replaceAll("\\s", ""));
    if (Arrays.stream(ImmutableOperation.values()).anyMatch(it -> !OPERATION_MAP.containsKey(it))) {
        throw new IllegalStateException("Unmapped enum constant found!");
    }
}

Application app = new Application();

Exception in thread "main" java.lang.ExceptionInInitializerError
Caused by: java.lang.IllegalStateException: Unmapped enum constant found!
	at javabasic.enumprac.extendenums.Application.<clinit>(Application.java:23)