Reading a properties file so that keys can be retrieved in order

Discussion in 'Java' started by laredotornado, May 12, 2011.

  1. Hi,

    I'm using Java 1.6. Let's say I have a properties file ...

    a=1
    b=2
    c=3
    ....

    How can i read the properties file in Java such that when I retrieve
    the key set, the iterator structure would return the keys in the order
    they are found in the file (in the above example, "a", "b", "c")?

    Thanks, - Dave
     
    laredotornado, May 12, 2011
    #1
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  2. laredotornado

    Lew Guest

    You'll need custom code. The 'java.util.Properties' type inherits from
    'java.util.Hashtable' and there is no guarantee that the table will retain its
    order even after you've loaded the properties, let alone the order specified
    by the file, depending on its use.

    You should explain why the order matters, because the need influences what
    sort of data structures and algorithm you'll use.

    If the order is determined at compile time, you can use an enum to specify the
    property names and retrieve the values by iterating through the enum
    'values()' to get the keys, and using those keys to retrieve the properties.

    for ( Orderer ord : Orderer.values() )
    {
    doSomethingWith( properties.get( ord.toString() ));
    }

    If the order is dynamic, you'll have to go through more effort to create a
    'List orderers'.


    List <String> orderers = loadOrderers();

    for ( String ord : orderers )
    {
    doSomethingWith( properties.get( ord ));
    }

    Generally with properties or preferences, they should be "load once into a
    custom structure", not "load every time you need a property". So load the
    properties into an ordered structure at program start or at classload time.
    Don't keep the 'Properties' itself around. Then iterate through that custom
    structure when you need a property.
     
    Lew, May 12, 2011
    #2
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  3. laredotornado

    Lew Guest

    <sscce source="com/lewscanon/eegee/Properteer.java">
    /* Properteer.java
    * $Id$
    */
    package com.lewscanon.eegee;

    import java.io.Closeable;
    import java.io.IOException;
    import java.io.InputStreamReader;
    import java.io.Reader;
    import java.util.Map;
    import java.util.Properties;

    /**
    * Properteer.
    */
    public class Properteer
    {
    private static final String PROPFILE = "properteer.properties";

    /**
    * main.
    * @param args String [] command arguments.
    */
    public static void main( String [] args )
    {
    Reader propReader = new InputStreamReader(
    Properteer.class.getResourceAsStream( PROPFILE ));

    assert propReader != null;

    Properties props = new Properties();
    try
    {
    props.load( propReader );
    }
    catch ( IOException exc )
    {
    final String msg = "Unable to load properties: "
    + exc.getLocalizedMessage();
    System.err.println( msg );
    IllegalStateException rethrow = new IllegalStateException( msg,
    exc );
    throw rethrow;
    }
    finally
    {
    close( propReader );
    }

    System.out.println( "size "+ props.size() +": " );
    System.out.println( "------" );
    System.out.println( "native order" );
    for ( Map.Entry <?, ?> entry : props.entrySet() )
    {
    System.out.println( entry.getValue().toString() );
    }
    System.out.println( "------" );

    System.out.println( "forced order" );
    for ( Orderer ord : Orderer.values() )
    {
    System.out.println( props.get( ord.toString() ));
    }
    System.out.println( "------" );
    System.out.println();

    }

    static void close( Closeable closeable )
    {
    assert closeable != null; // package-private - cannot accept null
    try
    {
    closeable.close();
    }
    catch ( IOException exc )
    {
    final String msg = "Unable: "+ exc.getLocalizedMessage();
    System.err.println( msg );
    }
    }

    enum Orderer
    {
    FOO("foo"), BAR("bar"), BAZ("baz"), QUX("qux"), ;

    private final String handle;

    Orderer( String han )
    {
    this.handle = han;
    }

    @Override
    public final String toString()
    {
    return this.handle;
    }

    /**
    * fromString - convert from {@code String} to {@code Orderer}.
    * Tries custom strings first, then defaults to {@code valueOf()}.
    * @param han String to convert.
    * @return Orderer matching the input, or {@code null} if no match.
    */
    public static Orderer fromString( String han )
    {
    if ( han == null )
    {
    return null;
    }
    for ( Orderer inst : values() )
    {
    if ( han.equals( inst.toString() ) )
    {
    return inst;
    }
    }
    return valueOf( han );
    }
    }

    }
    </sscce>

    properteer.properties (in same subdirectory as Properteer.class):
    ----------------------
    foo = The Fonz
    bar = Richie Cunningham
    baz = Joanie Cunningham
    qux = Chachie
    ----------------------

    output:
    ----------------------
    size 4:
    ------
    native order
    Chachie
    Richie Cunningham
    Joanie Cunningham
    The Fonz
    ------
    forced order
    The Fonz
    Richie Cunningham
    Joanie Cunningham
    Chachie
     
    Lew, May 13, 2011
    #3
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