Create your ENUM (yes from Java’)

04314

Java DIY (2 Part Series)

1 Create your ENUM (yes from Java’)
2 Java StringBuilder Demystified

If you are always in awe of the simple yet powerful Enums in java, follow along 😉

In Java, enum data type internally inherits from java.lang.Enum class.

Use cases to consider:

  • should have methods name(), ordinal() as final which no one can override
  • always maintain singleton status for each enum value
  • you shouldn’t be able to access enum constructor (make it private)
  • should have ordinal (consider sequential numbering idea)
  • the enum should have a values() which returns all enum values.

I wrote a simple yet fairly complete implementation, so I welcome any feedback. 

<span>import</span> <span>java.util.List</span><span>;</span>
<span>abstract</span> <span>class</span> <span>EnumBase</span> <span>implements</span> <span>Comparable</span><span><</span><span>EnumBase</span><span>>,</span> <span>Serializable</span> <span>{</span>
    <span>private</span> <span>final</span> <span>int</span> <span>ordinal</span><span>;</span>
    <span>private</span> <span>final</span> <span>String</span> <span>name</span><span>;</span>
    <span>protected</span> <span>EnumBase</span><span>(</span><span>int</span> <span>ordinal</span><span>,</span> <span>String</span> <span>name</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>ordinal</span> <span>=</span> <span>ordinal</span><span>;</span>
        <span>this</span><span>.</span><span>name</span> <span>=</span> <span>name</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>final</span> <span>int</span> <span>ordinal</span><span>()</span> <span>{</span>
        <span>return</span> <span>ordinal</span><span>;</span>
    <span>}</span>
    <span>/** * non overridable * * @return always gives ENUM Constant name */</span>
    <span>public</span> <span>final</span> <span>String</span> <span>name</span><span>()</span> <span>{</span>
        <span>return</span> <span>name</span><span>;</span>
    <span>}</span>
    <span>/** * It is desired to be overloaded * * @return default to enum constant name */</span>
    <span>@Override</span>
    <span>public</span> <span>String</span> <span>toString</span><span>()</span> <span>{</span>
        <span>return</span> <span>name</span><span>;</span>
    <span>}</span>
    <span>@Override</span>
    <span>public</span> <span>final</span> <span>boolean</span> <span>equals</span><span>(</span><span>Object</span> <span>o</span><span>)</span> <span>{</span>
        <span>return</span> <span>o</span> <span>==</span> <span>this</span><span>;</span>   <span>// as singleton always yield true</span>
    <span>}</span>
    <span>@Override</span>
    <span>public</span> <span>final</span> <span>int</span> <span>hashCode</span><span>()</span> <span>{</span>
        <span>return</span> <span>super</span><span>.</span><span>hashCode</span><span>();</span>    <span>// don't need it as there can only be ONE instance on an Enum</span>
    <span>}</span>
    <span>@Override</span>
    <span>public</span> <span>final</span> <span>int</span> <span>compareTo</span><span>(</span><span>EnumBase</span> <span>o</span><span>)</span> <span>{</span>
        <span>return</span> <span>Integer</span><span>.</span><span>compare</span><span>(</span><span>o</span><span>.</span><span>ordinal</span><span>,</span> <span>ordinal</span><span>);</span>
    <span>}</span>
    <span>/** * This guarantees that enums are never cloned, which is necessary * to preserve their "singleton" status. */</span>
    <span>protected</span> <span>final</span> <span>Object</span> <span>clone</span><span>()</span> <span>throws</span> <span>CloneNotSupportedException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>CloneNotSupportedException</span><span>();</span>
    <span>}</span>
    <span>/** * prevent default deserialization */</span>
    <span>private</span> <span>void</span> <span>readObject</span><span>(</span><span>ObjectInputStream</span> <span>in</span><span>)</span> <span>throws</span> <span>IOException</span><span>,</span>
            <span>ClassNotFoundException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>InvalidObjectException</span><span>(</span><span>"can't deserialize enum"</span><span>);</span>
    <span>}</span>
    <span>private</span> <span>void</span> <span>readObjectNoData</span><span>()</span> <span>throws</span> <span>ObjectStreamException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>InvalidObjectException</span><span>(</span><span>"can't deserialize enum"</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>MyEnum</span> <span>{</span>
    <span>private</span> <span>static</span> <span>int</span> <span>ordinalCounter</span> <span>=</span> <span>0</span><span>;</span>
    <span>/** * Utility class where we fix the ordinal as sequential integers */</span>
    <span>private</span> <span>abstract</span> <span>static</span> <span>class</span> <span>AbstractEnum</span> <span>extends</span> <span>EnumBase</span> <span>{</span>
        <span>private</span> <span>AbstractEnum</span><span>(</span><span>String</span> <span>name</span><span>)</span> <span>{</span>
            <span>super</span><span>(++</span><span>ordinalCounter</span><span>,</span> <span>name</span><span>);</span>
        <span>}</span>
    <span>}</span>
    <span>// singleton instance</span>
    <span>public</span> <span>static</span> <span>final</span> <span>FIRST</span> <span>FIRST</span> <span>=</span> <span>new</span> <span>FIRST</span><span>();</span>
    <span>static</span> <span>class</span> <span>FIRST</span> <span>extends</span> <span>AbstractEnum</span> <span>{</span>
        <span>private</span> <span>FIRST</span><span>()</span> <span>{</span>
            <span>super</span><span>(</span><span>FIRST</span><span>.</span><span>class</span><span>.</span><span>getSimpleName</span><span>());</span>
        <span>}</span>
    <span>}</span>
    <span>public</span> <span>static</span> <span>final</span> <span>SECOND</span> <span>SECOND</span> <span>=</span> <span>new</span> <span>SECOND</span><span>();</span>
    <span>static</span> <span>class</span> <span>SECOND</span> <span>extends</span> <span>AbstractEnum</span> <span>{</span>
        <span>private</span> <span>SECOND</span><span>()</span> <span>{</span>
            <span>super</span><span>(</span><span>SECOND</span><span>.</span><span>class</span><span>.</span><span>getSimpleName</span><span>());</span>
        <span>}</span>
    <span>}</span>
    <span>public</span> <span>static</span> <span>List</span><span><</span><span>EnumBase</span><span>></span> <span>values</span><span>()</span> <span>{</span>
        <span>return</span> <span>List</span><span>.</span><span>of</span><span>(</span><span>FIRST</span><span>,</span> <span>SECOND</span><span>);</span>
    <span>}</span>
<span>}</span>
<span>class</span> <span>MyEnumDriver</span> <span>{</span>
    <span>public</span> <span>static</span> <span>void</span> <span>main</span><span>(</span><span>String</span><span>[]</span> <span>args</span><span>)</span> <span>{</span>
        <span>// FIRST 1</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>FIRST</span><span>.</span><span>name</span><span>()</span> <span>+</span> <span>" "</span> <span>+</span> <span>MyEnum</span><span>.</span><span>FIRST</span><span>.</span><span>ordinal</span><span>());</span>
        <span>// true</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>FIRST</span> <span>==</span> <span>MyEnum</span><span>.</span><span>FIRST</span><span>);</span>
        <span>// FIRST 2</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>SECOND</span><span>.</span><span>name</span><span>()</span> <span>+</span> <span>" "</span> <span>+</span> <span>MyEnum</span><span>.</span><span>SECOND</span><span>.</span><span>ordinal</span><span>());</span>
        <span>// values</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>values</span><span>());</span>
    <span>}</span>
<span>}</span>
<span>import</span> <span>java.util.List</span><span>;</span>

<span>abstract</span> <span>class</span> <span>EnumBase</span> <span>implements</span> <span>Comparable</span><span><</span><span>EnumBase</span><span>>,</span> <span>Serializable</span> <span>{</span>

    <span>private</span> <span>final</span> <span>int</span> <span>ordinal</span><span>;</span>
    <span>private</span> <span>final</span> <span>String</span> <span>name</span><span>;</span>

    <span>protected</span> <span>EnumBase</span><span>(</span><span>int</span> <span>ordinal</span><span>,</span> <span>String</span> <span>name</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>ordinal</span> <span>=</span> <span>ordinal</span><span>;</span>
        <span>this</span><span>.</span><span>name</span> <span>=</span> <span>name</span><span>;</span>
    <span>}</span>

    <span>public</span> <span>final</span> <span>int</span> <span>ordinal</span><span>()</span> <span>{</span>
        <span>return</span> <span>ordinal</span><span>;</span>
    <span>}</span>

    <span>/** * non overridable * * @return always gives ENUM Constant name */</span>
    <span>public</span> <span>final</span> <span>String</span> <span>name</span><span>()</span> <span>{</span>
        <span>return</span> <span>name</span><span>;</span>
    <span>}</span>

    <span>/** * It is desired to be overloaded * * @return default to enum constant name */</span>
    <span>@Override</span>
    <span>public</span> <span>String</span> <span>toString</span><span>()</span> <span>{</span>
        <span>return</span> <span>name</span><span>;</span>
    <span>}</span>

    <span>@Override</span>
    <span>public</span> <span>final</span> <span>boolean</span> <span>equals</span><span>(</span><span>Object</span> <span>o</span><span>)</span> <span>{</span>
        <span>return</span> <span>o</span> <span>==</span> <span>this</span><span>;</span>   <span>// as singleton always yield true</span>
    <span>}</span>

    <span>@Override</span>
    <span>public</span> <span>final</span> <span>int</span> <span>hashCode</span><span>()</span> <span>{</span>
        <span>return</span> <span>super</span><span>.</span><span>hashCode</span><span>();</span>    <span>// don't need it as there can only be ONE instance on an Enum</span>
    <span>}</span>

    <span>@Override</span>
    <span>public</span> <span>final</span> <span>int</span> <span>compareTo</span><span>(</span><span>EnumBase</span> <span>o</span><span>)</span> <span>{</span>
        <span>return</span> <span>Integer</span><span>.</span><span>compare</span><span>(</span><span>o</span><span>.</span><span>ordinal</span><span>,</span> <span>ordinal</span><span>);</span>
    <span>}</span>

    <span>/** * This guarantees that enums are never cloned, which is necessary * to preserve their "singleton" status. */</span>
    <span>protected</span> <span>final</span> <span>Object</span> <span>clone</span><span>()</span> <span>throws</span> <span>CloneNotSupportedException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>CloneNotSupportedException</span><span>();</span>
    <span>}</span>

    <span>/** * prevent default deserialization */</span>
    <span>private</span> <span>void</span> <span>readObject</span><span>(</span><span>ObjectInputStream</span> <span>in</span><span>)</span> <span>throws</span> <span>IOException</span><span>,</span>
            <span>ClassNotFoundException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>InvalidObjectException</span><span>(</span><span>"can't deserialize enum"</span><span>);</span>
    <span>}</span>

    <span>private</span> <span>void</span> <span>readObjectNoData</span><span>()</span> <span>throws</span> <span>ObjectStreamException</span> <span>{</span>
        <span>throw</span> <span>new</span> <span>InvalidObjectException</span><span>(</span><span>"can't deserialize enum"</span><span>);</span>
    <span>}</span>
<span>}</span>

<span>public</span> <span>class</span> <span>MyEnum</span> <span>{</span>
    <span>private</span> <span>static</span> <span>int</span> <span>ordinalCounter</span> <span>=</span> <span>0</span><span>;</span>

    <span>/** * Utility class where we fix the ordinal as sequential integers */</span>
    <span>private</span> <span>abstract</span> <span>static</span> <span>class</span> <span>AbstractEnum</span> <span>extends</span> <span>EnumBase</span> <span>{</span>
        <span>private</span> <span>AbstractEnum</span><span>(</span><span>String</span> <span>name</span><span>)</span> <span>{</span>
            <span>super</span><span>(++</span><span>ordinalCounter</span><span>,</span> <span>name</span><span>);</span>
        <span>}</span>
    <span>}</span>

    <span>// singleton instance</span>
    <span>public</span> <span>static</span> <span>final</span> <span>FIRST</span> <span>FIRST</span> <span>=</span> <span>new</span> <span>FIRST</span><span>();</span>

    <span>static</span> <span>class</span> <span>FIRST</span> <span>extends</span> <span>AbstractEnum</span> <span>{</span>
        <span>private</span> <span>FIRST</span><span>()</span> <span>{</span>
            <span>super</span><span>(</span><span>FIRST</span><span>.</span><span>class</span><span>.</span><span>getSimpleName</span><span>());</span>
        <span>}</span>
    <span>}</span>

    <span>public</span> <span>static</span> <span>final</span> <span>SECOND</span> <span>SECOND</span> <span>=</span> <span>new</span> <span>SECOND</span><span>();</span>

    <span>static</span> <span>class</span> <span>SECOND</span> <span>extends</span> <span>AbstractEnum</span> <span>{</span>
        <span>private</span> <span>SECOND</span><span>()</span> <span>{</span>
            <span>super</span><span>(</span><span>SECOND</span><span>.</span><span>class</span><span>.</span><span>getSimpleName</span><span>());</span>
        <span>}</span>
    <span>}</span>

    <span>public</span> <span>static</span> <span>List</span><span><</span><span>EnumBase</span><span>></span> <span>values</span><span>()</span> <span>{</span>
        <span>return</span> <span>List</span><span>.</span><span>of</span><span>(</span><span>FIRST</span><span>,</span> <span>SECOND</span><span>);</span>
    <span>}</span>
<span>}</span>

<span>class</span> <span>MyEnumDriver</span> <span>{</span>
    <span>public</span> <span>static</span> <span>void</span> <span>main</span><span>(</span><span>String</span><span>[]</span> <span>args</span><span>)</span> <span>{</span>
        <span>// FIRST 1</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>FIRST</span><span>.</span><span>name</span><span>()</span> <span>+</span> <span>" "</span> <span>+</span> <span>MyEnum</span><span>.</span><span>FIRST</span><span>.</span><span>ordinal</span><span>());</span>
        <span>// true</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>FIRST</span> <span>==</span> <span>MyEnum</span><span>.</span><span>FIRST</span><span>);</span>
        <span>// FIRST 2</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>SECOND</span><span>.</span><span>name</span><span>()</span> <span>+</span> <span>" "</span> <span>+</span> <span>MyEnum</span><span>.</span><span>SECOND</span><span>.</span><span>ordinal</span><span>());</span>
        <span>// values</span>
        <span>System</span><span>.</span><span>out</span><span>.</span><span>println</span><span>(</span><span>MyEnum</span><span>.</span><span>values</span><span>());</span>
    <span>}</span>
<span>}</span>
import java.util.List;

abstract class EnumBase implements Comparable<EnumBase>, Serializable {

    private final int ordinal;
    private final String name;

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

    public final int ordinal() {
        return ordinal;
    }

    /** * non overridable * * @return always gives ENUM Constant name */
    public final String name() {
        return name;
    }

    /** * It is desired to be overloaded * * @return default to enum constant name */
    @Override
    public String toString() {
        return name;
    }

    @Override
    public final boolean equals(Object o) {
        return o == this;   // as singleton always yield true
    }

    @Override
    public final int hashCode() {
        return super.hashCode();    // don't need it as there can only be ONE instance on an Enum
    }

    @Override
    public final int compareTo(EnumBase o) {
        return Integer.compare(o.ordinal, ordinal);
    }

    /** * This guarantees that enums are never cloned, which is necessary * to preserve their "singleton" status. */
    protected final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    /** * 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");
    }
}

public class MyEnum {
    private static int ordinalCounter = 0;

    /** * Utility class where we fix the ordinal as sequential integers */
    private abstract static class AbstractEnum extends EnumBase {
        private AbstractEnum(String name) {
            super(++ordinalCounter, name);
        }
    }

    // singleton instance
    public static final FIRST FIRST = new FIRST();

    static class FIRST extends AbstractEnum {
        private FIRST() {
            super(FIRST.class.getSimpleName());
        }
    }

    public static final SECOND SECOND = new SECOND();

    static class SECOND extends AbstractEnum {
        private SECOND() {
            super(SECOND.class.getSimpleName());
        }
    }

    public static List<EnumBase> values() {
        return List.of(FIRST, SECOND);
    }
}

class MyEnumDriver {
    public static void main(String[] args) {
        // FIRST 1
        System.out.println(MyEnum.FIRST.name() + " " + MyEnum.FIRST.ordinal());
        // true
        System.out.println(MyEnum.FIRST == MyEnum.FIRST);
        // FIRST 2
        System.out.println(MyEnum.SECOND.name() + " " + MyEnum.SECOND.ordinal());
        // values
        System.out.println(MyEnum.values());
    }
}

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Java DIY (2 Part Series)

1 Create your ENUM (yes from Java’)
2 Java StringBuilder Demystified

原文链接：Create your ENUM (yes from Java’)

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