继承在枚举类中的应用（Extending Enums in Java）

快来打我* 2024-03-23 17:37 76阅读 0赞

## 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
    }

报错：

Cannot inherit from enum BasicStringOperation

原因： Inheritance Is Not Allowed for Enums  
1、It turns the enum into a subclass of the abstract class java.lang.Enum  
2、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;
        }
        
    	@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."),
        BASE64_ENCODE("Encoding the given string using the BASE64 algorithm.");
    
        ExtendedStringOperation(String description) {
        
            this.description = description;
        }
    
    	@Override
        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)

\-----------------------------------------------------------------------------读书笔记摘自 文章：[Extending Enums in Java][]

[Extending Enums in Java]: https://www.baeldung.com/java-extending-enums