Java 中的 Java.lang.Enum 类
枚举类存在于 java.lang 包中。它是所有 Java 语言枚举类型的公共基类。有关枚举的信息，请参考 java 中的枚举
班级申报T0】
我们可以看到，Enum 是一个抽象类，所以我们不能创建类 Enum 的对象。
枚举类中的方法
Enum 类提供了 10 种有用的方法。大部分都是从对象类覆盖的。这些方法在枚举类中声明为 final，因此程序员不能修改任何枚举常量。
-
final String name():此方法返回此枚举常量的名称，该名称与其枚举声明中声明的名称完全相同。
```java Syntax : public final String name() Parameters : NA Returns : the name of this enum constant
```
```java // Java program to demonstrate name() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.RED; System.out.print("Name of enum constant: ");
// name method System.out.println(c1.name()); } } ```
输出:
```java Name of enum constant: RED
```
-
最终 int 序数():这个方法返回这个枚举常量的索引。
```java Syntax : public final int ordinal() Parameters : NA Returns : the ordinal of this enumeration constant
```
```java // Java program to demonstrate ordinal() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.GREEN; System.out.print("ordinal of enum constant "+c1.name()+" : ");
// ordinal method System.out.println(c1.ordinal()); } } ```
输出:
```java ordinal of enum constant GREEN : 1
```
-
String ToString():这个方法返回一个代表这个枚举常量的 String 对象。此方法与 name()方法相同。
```java Syntax : public String toString() Parameters : NA Returns : a string representation of this enumeration constant Overrides : toString in class Object
```
```java // Java program to demonstrate toString() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.GREEN;
// getting string representation of enum // using toString() method String str = c1.toString();
System.out.println(str); } } ```
输出:
```java GREEN
```
-
最终布尔等于(Object obj) :如果指定的对象等于这个枚举常量，这个方法返回 true，否则返回 false。
```java Syntax : public final boolean equals(Object obj) Parameters : obj - the object too be compared for equality with this enum. Returns : true if the specified object is equal to this enum constant. false otherwise Overrides : equals in class Object
```
```java // Java program to demonstrate equals() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.RED; Color c2 = Color.GREEN; Color c3 = Color.RED;
// checking equality between enums // using equals() method
boolean b1 = c1.equals(c2); boolean b2 = c1.equals(c3); boolean b3 = c2.equals(null);
System.out.println("is c1 equal to c2 : " + b1); System.out.println("is c1 equal to c3 : " + b2); System.out.println("is c2 equal to null : " + b3); } } ```
输出:
```java is c1 equal to c2 : false is c1 equal to c3 : true is c2 equal to null : false
```
-
最终 int hashCode() :此方法返回此枚举常量的哈希代码。实际上这个方法只包含一个语句，那就是“return super.hashCode()”，它依次调用 Object 类 hashCode()方法。
```java Syntax : public final int hashCode() Parameters : NA Returns : a hash code for this enum constant. Overrides : hashCode in class Object
```
```java // Java program to demonstrate hashCode() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.RED; System.out.print("hashcode of enum constant "+ c1.name() +" : ");
// hashcode method System.out.println(c1.hashCode());
Color c2 = Color.GREEN; System.out.print("hashcode of enum constant "+ c2.name() +" : ");
// hashcode method System.out.println(c2.hashCode()); } } ```
输出:
```java hashcode of enum constant RED : 366712642 hashcode of enum constant GREEN : 1829164700
```
-
最终 int compareTo(E obj) :此方法将此枚举与订单的指定对象进行“比较”。枚举常量只能与相同枚举类型的其他枚举常量相比较。
```java Syntax : public int compareTo(E obj) Parameters : obj - the object to be compared. Returns : a negative integer if this object is at less ordinal than the specified object zero if this object is at equal ordinal than the specified object a positive integer if this object is at greater ordinal than the specified object Throws : NullPointerException - if the argument is null
```
```java // Java program to demonstrate compareTo() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { Color c1 = Color.RED; Color c2 = Color.GREEN; Color c3 = Color.RED; Color c4 = Color.BLUE;
System.out.print("Comparing "+c1.name()+" with "+ c2.name() +" : ");
// compareTo method System.out.println(c1.compareTo(c2));
System.out.print("Comparing "+c1.name()+" with "+ c3.name() +" : ");
// compareTo method System.out.println(c1.compareTo(c3));
System.out.print("Comparing "+c4.name()+" with "+ c2.name() +" : ");
// compareTo method System.out.println(c4.compareTo(c2));
// The following statement throw NullPointerException // as argument of compareTo method is null // System.out.println(c4.compareTo(null));
} } ```
输出:
```java Comparing RED with GREEN : -1 Comparing RED with RED : 0 Comparing BLUE with GREEN : 1
```
-
静态< T 扩展枚举> T valueOf(Class enumType，String name) :此方法返回具有指定名称的指定枚举类型的枚举常量。该名称必须与用于声明此类型的枚举常量的标识符完全匹配。
```java Syntax : public static T valueOf(Class enumType,String name) TypeParameters : T - The enum type whose constant is to be returned Parameters : enumType - the Class object of the enum type from which to return a constant name - the name of the constant to return Returns : the enum constant of the specified enum type with the specified name Throws : IllegalArgumentException - if the specified enum type has no constant with the specified name or the specified class object does not represent an enum type NullPointerException - if enumType or name is null
```
```java // Java program to demonstrate valueOf() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) { // getting value of enum with specified String // using valueOf method Color c1 = Color.valueOf("RED"); Color c2 = Color.valueOf("GREEN");
// name method System.out.println(c1.name()); System.out.println(c2.name());
// The following statement throw IllegalArgumentException // as GrEEN is not an enum constant // Color c3 = Color.valueOf("GrEEN");
// The following statement throw NullPointerException // as argument of valueOf method is null // Color c4 = Color.valueOf(null); } } ```
输出:
```java RED GREEN
```
-
最终类getdeclaring Class():该方法返回该枚举常量的枚举类型对应的 Class 对象。如果此方法为两者返回相同的类对象，则任意两个枚举常数 e1 和 e2 属于相同的枚举类型。
```java Syntax : public final Class getDeclaringClass() Parameters : NA Returns : the Class object corresponding to this enum constant's enum type
```
```java // Java program to demonstrate getDeclaringClass() method enum Color { RED, GREEN, BLUE; }
enum Day { MONDAY, TUESDAY ; }
public class Test { // Driver method public static void main(String[] args) { // getting value of enum with specified String // using valueOf method Color c1 = Color.valueOf("RED"); Color c2 = Color.valueOf("GREEN"); Day d1 = Day.valueOf("MONDAY"); Day d2 = Day.valueOf("TUESDAY");
System.out.print("Class corresponding to "+ c1.name() +" : ");
// getDeclaringClass method System.out.println(c1.getDeclaringClass());
System.out.print("Class corresponding to "+ c2.name() +" : ");
// getDeclaringClass method System.out.println(c2.getDeclaringClass());
System.out.print("Class corresponding to "+ d1.name() +" : ");
// getDeclaringClass method System.out.println(d1.getDeclaringClass());
System.out.print("Class corresponding to "+ d2.name() +" : ");
// getDeclaringClass method System.out.println(d2.getDeclaringClass()); } } ```
输出:
```java Class corresponding to RED : class Color Class corresponding to GREEN : class Color Class corresponding to MONDAY : class Day Class corresponding to TUESDAY : class Day
```
-
最终对象克隆():这种方法保证了枚举永远不会被克隆，这是保持其“单例”状态所必需的。编译器在内部使用它来创建枚举常量。
```java Syntax : public final Object clone() throws CloneNotSupportedException Parameters : NA Returns : NA Overrides : clone in class Object Throws : CloneNotSupportedException-if the object's class does not support the Cloneable interface.
```
```java // Java program to demonstrate clone() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) throws CloneNotSupportedException { System.out.println("Enums are never cloned"); Test t = new Test() { // final clone method protected final Object clone() throws CloneNotSupportedException { return new CloneNotSupportedException(); } };
System.out.println(t.clone()); } } ```
输出:
```java Enums are never cloned java.lang.CloneNotSupportedException
```
-
最终 void finalize() :此方法保证枚举类不能有 finalize 方法。
```java Syntax : protected final void finalize() Parameters : NA Returns : NA Overrides : finalize in class Object
```
```java // Java program to demonstrate finalize() method enum Color { RED, GREEN, BLUE; }
public class Test { // Driver method public static void main(String[] args) throws Throwable { System.out.println("enum classes cannot have finalize methods"); Test t = new Test() { // final finalize method protected final void finalize() throws Throwable { // empty implementation }; }; } } ```
输出:
```java enum classes cannot have finalize methods
```
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