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java 枚举
C 语言中可以这样来定义枚举
- enum color {
- RED=0, GREEN, BLUE, YELLOW
- } col;
关键字 enum 定义枚举,在定义枚举的同时,声明该枚举变量 col.
注意:C 语言中枚举成员的值是根据上下文自动加 1 的(GREEN = 1, BLUE = 2 等)。
C 语言中 switch 语句支持枚举类型
- #include<stdio.h>
- int main() {
- enum color {
- RED=0, GREEN, BLUE, YELLOW
- } col;
- int cl;
- printf("0=red,1=green,2=blue,3=yellow. seclect:\n");
- scanf("%d",&cl);
- col=(enum color) cl;
- switch(col) {
- case RED:
- printf("the color is red\n");
- break;
- case GREEN:
- printf("the color is green\n");
- break;
- case BLUE:
- printf("the color is blue\n");
- break;
- case YELLOW:
- printf("the color is yellow\n");
- break;
- defalut:
- printf("no this color\n");
- break;
- }
- return 0;
- }
那么,Java 里面的枚举与其类似,但是又不是完全一样。
Java 语言中定义枚举也是使用关键字 enum
- public enum Color {
- RED, GREEN, BLUE, YELLOW;
- }
上述定义了一个枚举类型 Color(可以说是类,编译之后是 Color.class).
上面的定义,还可以改成下面的这种形式
- public enum Color {
- RED(), GREEN(), BLUE(), YELLOW();
- }
到这里你就会觉得迷茫(如果你是初学者的话),为什么这样子也可以?
其实,枚举的成员就是枚举对象,只不过他们是静态常量而已。
使用 javap 命令(javap 文件名<没有后缀.class>)可以反编译 class 文件,如下:
我们可以使用普通类来模拟枚举,下面定义一个 Color 类。
- public class Color {
- private static final Color RED = new Color();
- private static final Color GREEN = new Color();
- private static final Color BLUE = new Color();
- private static final Color YELLOW = new Color();
- }
对比一下,你就明白了。
如果按照这个逻辑,是否还可以为其添加另外的构造方法?答案是肯定的!
- public enum Color {
- RED("red color", 0), GREEN("green color", 1),
- BLUE("blue color", 2), YELLOW("yellow color", 3);
- Color(String name, int id) {
- _name = name;
- _id = id;
- }
- String _name;
- int _id;
- }
为 Color 声明了两个成员变量,并为其构造带参数的构造器。
如果你这样创建一个枚举
- public enum Color {
- RED("red color", 0), GREEN("green color", 1),
- BLUE("blue color", 2), YELLOW("yellow color", 3);
- }
编译器就会报错
- The constructor EnumDemo.Color(String, int) is undefined
到此,你就可以明白,枚举和普通类基本一致(但是不完全一样)。
对于类来讲,最好将其成员变量私有化,然后,为成员变量提供 get、set 方法。
按照这个原则,可以进一步写好 enum Color.
- public enum Color {
- RED("red color", 0), GREEN("green color", 1),
- BLUE("blue color", 2), YELLOW("yellow color", 3);
- Color(String name, int id) {
- _name = name;
- _id = id;
- }
- private String _name;
- private int _id;
- public void setName(String name) {
- _name = name;
- }
- public void setId(int id) {
- _id = id;
- }
- public String getName() {
- return _name;
- }
- public int getId() {
- return _id;
- }
- }
但是,java 设计 enum 的目的是提供一组常量,方便用户设计。
如果我们冒然的提供 set 方法(外界可以改变其成员属性),好像是有点违背了设计的初衷。
那么,我们应该舍弃 set 方法,保留 get 方法。
- public enum Color {
- RED("red color", 0), GREEN("green color", 1),
- BLUE("blue color", 2), YELLOW("yellow color", 3);
- Color(String name, int id) {
- _name = name;
- _id = id;
- }
- private String _name;
- private int _id;
- public String getName() {
- return _name;
- }
- public int getId() {
- return _id;
- }
- }
普通类,我们可以将其实例化,那么,能否实例化枚举呢?
在回答这个问题之前,先来看看,编译之后的 Color.class 文件
- public static enum Color {
- RED("red color", 0), GREEN("green color", 1),
- BLUE("blue color", 2), YELLOW("yellow color", 3);
- private String _name;
- private int _id;
- private Color(String name, int id) {
- this._name = name;
- this._id = id;
- }
- public String getName() {
- return this._name;
- }
- public int getId() {
- return this._id;
- }
- }
可以看出,编译器淘气的为其构造方法加上了 private,那么也就是说,我们无法实例化枚举。
所有枚举类都继承了 Enum 类的方法,包括 toString 、equals、hashcode 等方法。
因为 equals、hashcode 方法是 final 的,所以不可以被枚举重写(只可以继承)。
但是,可以重写 toString 方法。
关于 Enum 源码,详见附录!
那么,使用 Java 的不同类来模拟一下枚举,大概是这个样子
- package mark.demo;
- import java.util.ArrayList;
- import java.util.List;
- public class Color {
- private static final Color RED = new Color("red color", 0);
- private static final Color GREEN = new Color("green color", 1);
- private static final Color BLUE = new Color("blue color", 2);
- private static final Color YELLOW = new Color("yellow color", 3);
- private final String _name;
- private final int _id;
- private Color(String name, int id) {
- _name = name;
- _id = id;
- }
- public String getName() {
- return _name;
- }
- public int getId() {
- return _id;
- }
- public static List<Color> values() {
- List<Color> list = new ArrayList<Color>();
- list.add(RED);
- list.add(GREEN);
- list.add(BLUE);
- list.add(YELLOW);
- return list;
- }
- @Override
- public String toString() {
- return "the color _name=" + _name + ", _id=" + _id;
- }
- }
附录
Enum.java
- /*
- * %W% %E%
- *
- * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
- * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
- */
- package java.lang;
- import java.io.Serializable;
- import java.io.IOException;
- import java.io.InvalidObjectException;
- import java.io.ObjectInputStream;
- import java.io.ObjectStreamException;
- /**
- * This is the common base class of all Java language enumeration types.
- *
- * @author Josh Bloch
- * @author Neal Gafter
- * @version %I%, %G%
- * @since 1.5
- */
- public abstract class Enum<E extends Enum<E>>
- implements Comparable<E>, Serializable {
- /**
- * The name of this enum constant, as declared in the enum declaration.
- * Most programmers should use the {@link #toString} method rather than
- * accessing this field.
- */
- private final String name;
- /**
- * Returns the name of this enum constant, exactly as declared in its
- * enum declaration.
- *
- * <b>Most programmers should use the {@link #toString} method in
- * preference to this one, as the toString method may return
- * a more user-friendly name.</b> This method is designed primarily for
- * use in specialized situations where correctness depends on getting the
- * exact name, which will not vary from release to release.
- *
- * @return the name of this enum constant
- */
- public final String name() {
- return name;
- }
- /**
- * The ordinal of this enumeration constant (its position
- * in the enum declaration, where the initial constant is assigned
- * an ordinal of zero).
- *
- * Most programmers will have no use for this field. It is designed
- * for use by sophisticated enum-based data structures, such as
- * {@link java.util.EnumSet} and {@link java.util.EnumMap}.
- */
- private final int ordinal;
- /**
- * Returns the ordinal of this enumeration constant (its position
- * in its enum declaration, where the initial constant is assigned
- * an ordinal of zero).
- *
- * Most programmers will have no use for this method. It is
- * designed for use by sophisticated enum-based data structures, such
- * as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
- *
- * @return the ordinal of this enumeration constant
- */
- public final int ordinal() {
- return ordinal;
- }
- /**
- * Sole constructor. Programmers cannot invoke this constructor.
- * It is for use by code emitted by the compiler in response to
- * enum type declarations.
- *
- * @param name - The name of this enum constant, which is the identifier
- * used to declare it.
- * @param ordinal - The ordinal of this enumeration constant (its position
- * in the enum declaration, where the initial constant is assigned
- * an ordinal of zero).
- */
- protected Enum(String name, int ordinal) {
- this.name = name;
- this.ordinal = ordinal;
- }
- /**
- * Returns the name of this enum constant, as contained in the
- * declaration. This method may be overridden, though it typically
- * isn‘t necessary or desirable. An enum type should override this
- * method when a more "programmer-friendly" string form exists.
- *
- * @return the name of this enum constant
- */
- public String toString() {
- return name;
- }
- /**
- * Returns true if the specified object is equal to this
- * enum constant.
- *
- * @param other the object to be compared for equality with this object.
- * @return true if the specified object is equal to this
- * enum constant.
- */
- public final boolean equals(Object other) {
- return this==other;
- }
- /**
- * Returns a hash code for this enum constant.
- *
- * @return a hash code for this enum constant.
- */
- public final int hashCode() {
- return super.hashCode();
- }
- /**
- * Throws CloneNotSupportedException. This guarantees that enums
- * are never cloned, which is necessary to preserve their "singleton"
- * status.
- *
- * @return (never returns)
- */
- protected final Object clone() throws CloneNotSupportedException {
- throw new CloneNotSupportedException();
- }
- /**
- * Compares this enum with the specified object for order. Returns a
- * negative integer, zero, or a positive integer as this object is less
- * than, equal to, or greater than the specified object.
- *
- * Enum constants are only comparable to other enum constants of the
- * same enum type. The natural order implemented by this
- * method is the order in which the constants are declared.
- */
- public final int compareTo(E o) {
- Enum other = (Enum)o;
- Enum self = this;
- if (self.getClass() != other.getClass() && // optimization
- self.getDeclaringClass() != other.getDeclaringClass())
- throw new ClassCastException();
- return self.ordinal - other.ordinal;
- }
- /**
- * Returns the Class object corresponding to this enum constant‘s
- * enum type. Two enum constants e1 and e2 are of the
- * same enum type if and only if
- * e1.getDeclaringClass() == e2.getDeclaringClass().
- * (The value returned by this method may differ from the one returned
- * by the {@link Object#getClass} method for enum constants with
- * constant-specific class bodies.)
- *
- * @return the Class object corresponding to this enum constant‘s
- * enum type
- */
- public final Class<E> getDeclaringClass() {
- Class clazz = getClass();
- Class zuper = clazz.getSuperclass();
- return (zuper == Enum.class) ? clazz : zuper;
- }
- /**
- * Returns the enum constant of the specified enum type with the
- * specified name. The name must match exactly an identifier used
- * to declare an enum constant in this type. (Extraneous whitespace
- * characters are not permitted.)
- *
- * @param enumType the <tt>Class</tt> object of the enum type from which
- * to return a constant
- * @param name the name of the constant to return
- * @return 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
- * @throws NullPointerException if <tt>enumType</tt> or <tt>name</tt>
- * is null
- * @since 1.5
- */
- public static <T extends Enum<T>> T valueOf(Class<T> enumType,
- String name) {
- T result = enumType.enumConstantDirectory().get(name);
- if (result != null)
- return result;
- if (name == null)
- throw new NullPointerException("Name is null");
- throw new IllegalArgumentException(
- "No enum const " + enumType +"." + name);
- }
- /**
- * prevent default deserialization
- */
- private void readObject(ObjectInputStream in) throws IOException,
- ClassNotFoundException {
- throw new InvalidObjectException("can‘t deserialize enum");
- }
- private void readObjectNoData() throws ObjectStreamException {
- throw new InvalidObjectException("can‘t deserialize enum");
- }
- /**
- * enum classes cannot have finalize methods.
- */
- protected final void finalize() { }
- }
java 枚举