枚舉及其高級用法

Enum源碼：

Enum.java

/*
 * Copyright (c) 2003, 2009, 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.
 *
 * More information about enums, including descriptions of the
 * implicitly declared methods synthesized by the compiler, can be
 * found in section 8.9 of
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * <p> Note that when using an enumeration type as the type of a set
 * or as the type of the keys in a map, specialized and efficient
 * {@linkplain java.util.EnumSet set} and {@linkplain
 * java.util.EnumMap map} implementations are available.
 *
 * @param <E> The enum type subclass
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Class#getEnumConstants()
 * @see     java.util.EnumSet
 * @see     java.util.EnumMap
 * @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.)
     *
     * <p>Note that for a particular enum type {@code T}, the
     * implicitly declared {@code public static T valueOf(String)}
     * method on that enum may be used instead of this method to map
     * from a name to the corresponding enum constant.  All the
     * constants of an enum type can be obtained by calling the
     * implicit {@code public static T[] values()} method of that
     * type.
     *
     * @param <T> The enum type whose constant is to be returned
     * @param enumType the {@code Class} 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 {@code enumType} or {@code name}
     *         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 constant " + enumType.getCanonicalName() + "." + name);
    }

    /**
     * enum classes cannot have finalize methods.
     */
    protected final void finalize() { }

    /**
     * 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");
    }
}

咱們自定義一個枚舉類：

而且須要注意的是枚舉類不容許繼承別的類

public enum Week {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY
}

測試結果：

System.out.println(Week.TUESDAY);　　// TUESDAY
System.out.println(Week.TUESDAY.name());　　// TUESDAY
System.out.println(Week.TUESDAY.ordinal());　　// 1
System.out.println(Arrays.asList(Week.values()));　　// [MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]
System.out.println(Week.valueOf("TUESDAY"));　　// TUESDAY

能夠看到，咱們不用建立枚舉類的對象（也不容許建立實例），直接用枚舉類名便可訪問其枚舉元素，且Java自動給按照枚舉值出現的順序，從0開始分配了序號，

經過name()能夠得到枚舉值的名稱，經過ordinal能夠得到枚舉值的編號。

使用javap反編譯後的代碼爲

public final class enums.Week extends java.lang.Enum<enums.Week> {
  public static final enums.Week MONDAY;
  public static final enums.Week TUESDAY;
  public static final enums.Week WEDNESDAY;
  public static final enums.Week THURSDAY;
  public static final enums.Week FRIDAY;
  public static enums.Week[] values();
  public static enums.Week valueOf(java.lang.String);
  static {
　　MONDAY=new Week("MONDAY",0);
　　TUESDAY=new Week("TUESDAY",1);
　　WEDNESDAY=new Week("WEDNESDAY",2);
　　THURSDAY=new Week("THURSDAY",3);
　　FRIDAY=new Week("FRIDAY",4);
 }
}

一、能夠看到enum類型的類在通過編譯後生成了一個繼承了Enum的類

二、枚舉類是final的，這也是爲何其不能再繼承其餘類的緣由，Java不容許多繼承

三、咱們定義的每一個枚舉值都是該類中的一個靜態final成員，且成員類型爲enums.Week

四、默認提供了value()、valueOf()靜態方法。

五、在static方法內爲咱們定義的枚舉值都生成了枚舉類的對象【MONDAY=new Week("MONDAY",0)】

因此，咱們使用enum定義的枚舉類型，會在編譯後轉化爲一個繼承了java.lang.Enum的類，而咱們定義的每一個枚舉值都會在類的初始化階段被實例化爲咱們所定義的枚舉類的一個對象。

 

Enum抽象類常見方法

　　Enum抽象類的方法，會被定義的枚舉類繼承。而最終的枚舉值被實例化成了Enum對象，所以枚舉元素擁有如下的方法。

　　ordinal()方法：該方法獲取的是枚舉變量在枚舉類中聲明的順序，下標從0開始。位置發生變化，那麼ordinal()方法獲取到的值也隨之變化。

　　compareTo(E o)方法：比較枚舉的大小，注意其內部實現是根據每一個枚舉的ordinal大小進行比較的

　　name()方法與toString()：幾乎是等同的，都是輸出變量的字符串形式

　　getDeclaringClass()：返回該枚舉變量所在的枚舉類

　　默認的構造方法：

    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

 

枚舉類型的其餘應用：

一、擴展枚舉值與自定義構造函數

　　咱們能夠給每一個枚舉值賦予更多的屬性，而後覆蓋原來默認的構造方法便可。

　　最簡單的枚舉值的默認構造是枚舉值字符串和順序下標，使用這兩個值爲咱們定義的枚舉值生成枚舉類的對象，如【MONDAY=new Week("MONDAY",0)】

public enum Week {
    MONDAY("週一", "1"),
    TUESDAY("週二", "2"),
    WEDNESDAY("週三", "3"),
    THURSDAY("週四", "4"),
    FRIDAY("週五", "5");

    private String name;
    private String value;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    Week(String name, String value) {
        this.name = name;
        this.value = value;
    }

    @Override
    public String toString() {
        return "Week{" +
                "name='" + name + '\'' +
                ", value='" + value + '\'' +
                '}';
    }
}

二、覆蓋enum類的方法

　　枚舉類最終繼承了java.lang.Enum抽象類，咱們也能夠覆蓋其中的方法。

三、枚舉類中定義抽象方法

　　編譯期最終將每一個枚舉值聲明爲了枚舉類的實例，那麼在枚舉類中定義的抽象方法也可讓枚舉值去實現該方法。如：

　　

public enum Week {
    MONDAY("週一", "1") {
        @Override
        void sayHello() {
            System.out.println("11");
        }
    },
    TUESDAY("週二", "2") {
        @Override
        void sayHello() {
            System.out.println("22");
        }
    },
    WEDNESDAY("週三", "3") {
        @Override
        void sayHello() {
            System.out.println("33");
        }
    },
    THURSDAY("週四", "4") {
        @Override
        void sayHello() {
            System.out.println("44");
        }
    },
    FRIDAY("週五", "5") {
        @Override
        void sayHello() {
            System.out.println("55");
        }
    };
    private String name;
    private String value;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    Week(String name, String value) {
        this.name = name;
        this.value = value;
    }
    
    abstract void sayHello();

    @Override
    public String toString() {
        return "Week{" +
                "name='" + name + '\'' +
                ", value='" + value + '\'' +
                '}';
    }
}

四、枚舉實現單例

利用枚舉值是枚舉類的一個實例，因此只要定義一個枚舉值就好了。

public enum SingletonEnum {
    SingletonEnum;
}