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

[laredotornado]

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

 

[Lew]

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")?

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]

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

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

How can i [sic] 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")?

<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