아이템 38. 확장할수있는 열거 타입이 필요하면 인터페이스를 사용하라.
이펙티브 자바
아이템 38. 확장할수있는 열거 타입이 필요하면 인터페이스를 사용하라.
package com.github.sejoung.codetest.enumtest.enuminterface;
// 코드 38-1 인터페이스를 이용해 확장 가능 열거 타입을 흉내 냈다. (232쪽)
public interface Operation {
double apply(double x, double y);
}
package com.github.sejoung.codetest.enumtest.enuminterface;
// 코드 38-1 인터페이스를 이용해 확장 가능 열거 타입을 흉내 냈다. - 기본 구현 (233쪽)
public enum BasicOperation implements Operation {
PLUS("+") {
@Override
public double apply(double x, double y) {
return x + y;
}
},
MINUS("-") {
@Override
public double apply(double x, double y) {
return x - y;
}
},
TIMES("*") {
@Override
public double apply(double x, double y) {
return x * y;
}
},
DIVIDE("/") {
@Override
public double apply(double x, double y) {
return x / y;
}
};
private final String symbol;
BasicOperation(String symbol) {
this.symbol = symbol;
}
@Override
public String toString() {
return symbol;
}
}
위에 처럼 인터페이스를 사용하여 Operation을 확장할수 있다. 다시 2개의 오퍼레이션을 확장 시키면
package com.github.sejoung.codetest.enumtest.enuminterface;
import java.util.Arrays;
import java.util.Collection;
// 코드 38-2 확장 가능 열거 타입 (233-235쪽)
public enum ExtendedOperation implements Operation {
EXP("^") {
@Override
public double apply(double x, double y) {
return Math.pow(x, y);
}
},
REMAINDER("%") {
@Override
public double apply(double x, double y) {
return x % y;
}
};
private final String symbol;
ExtendedOperation(String symbol) {
this.symbol = symbol;
}
@Override
public String toString() {
return symbol;
}
}
이렇게 확장을 하면 기존 연산에 쓰던 곳에서 계속 확장된것도 사용할수 있다.
테스트 코드를 다시 작성해서 돌려보면
package com.github.sejoung.codetest.enumtest.enuminterface;
public class Test {
// 열거 타입의 Class 객체를 이용해 확장된 열거 타입의 모든 원소를 사용하는 예 (234쪽)
public static void main(String[] args) {
double x = 2;
double y = 4;
test(ExtendedOperation.class, x, y);
}
private static <T extends Enum<T> & Operation> void test(
Class<T> opEnumType, double x, double y) {
for (Operation op : opEnumType.getEnumConstants())
System.out.printf("%f %s %f = %f%n",
x, op, y, op.apply(x, y));
}
}
실행결과
2.000000 ^ 4.000000 = 16.000000
2.000000 % 4.000000 = 2.000000
Process finished with exit code 0
위에 코드를 작성하면 테스트를 할수있는데 Class 객체를 넘기는거 아니고 한정적 와일드 카드를 넘기는 방법이 있다.
package com.github.sejoung.codetest.enumtest.enuminterface;
import java.util.Arrays;
import java.util.Collection;
public class Test {
// 컬렉션 인스턴스를 이용해 확장된 열거 타입의 모든 원소를 사용하는 예 (235쪽)
public static void main(String[] args) {
double x = 2;
double y = 4;
test(Arrays.asList(ExtendedOperation.values()), x, y);
}
private static void test(Collection<? extends Operation> opSet,
double x, double y) {
for (Operation op : opSet)
System.out.printf("%f %s %f = %f%n",
x, op, y, op.apply(x, y));
}
}
실행결과
2.000000 ^ 4.000000 = 16.000000
2.000000 % 4.000000 = 2.000000
Process finished with exit code 0
java sdk 에도 있는데 아래의 객체들이다.
/*
* Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.nio.file;
/**
* An object that configures how to open or create a file.
*
* <p> Objects of this type are used by methods such as {@link
* Files#newOutputStream(Path,OpenOption[]) newOutputStream}, {@link
* Files#newByteChannel newByteChannel}, {@link
* java.nio.channels.FileChannel#open FileChannel.open}, and {@link
* java.nio.channels.AsynchronousFileChannel#open AsynchronousFileChannel.open}
* when opening or creating a file.
*
* <p> The {@link StandardOpenOption} enumeration type defines the
* <i>standard</i> options.
*
* @since 1.7
*/
public interface OpenOption {
}
/*
* Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.nio.file;
/**
* An object that configures how to copy or move a file.
*
* <p> Objects of this type may be used with the {@link
* Files#copy(Path,Path,CopyOption[]) Files.copy(Path,Path,CopyOption...)},
* {@link Files#copy(java.io.InputStream,Path,CopyOption[])
* Files.copy(InputStream,Path,CopyOption...)} and {@link Files#move
* Files.move(Path,Path,CopyOption...)} methods to configure how a file is
* copied or moved.
*
* <p> The {@link StandardCopyOption} enumeration type defines the
* <i>standard</i> options.
*
* @since 1.7
*/
public interface CopyOption {
}
/*
* Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.nio.file;
/**
* Defines the options as to how symbolic links are handled.
*
* @since 1.7
*/
public enum LinkOption implements OpenOption, CopyOption {
/**
* Do not follow symbolic links.
*
* @see Files#getFileAttributeView(Path,Class,LinkOption[])
* @see Files#copy
* @see SecureDirectoryStream#newByteChannel
*/
NOFOLLOW_LINKS;
}
열거타입은 확장할수 없지만 인터페이스와 그 인터페이스를 구현하는 기본열거 타입을 함께 사용해 같은 효과를 낼수있다.