Java Properties and lambda collect

Question

I have a method that convert Properties into hashmap in this way (i know it's wrong)

Map<String, String> mapProp = new HashMap<String, String>();
Properties prop = new Properties();
prop.load(new FileInputStream( path ));     

prop.forEach( (key, value) -> {mapProp.put( (String)key, (String)value );} );

return mapProp;

My idea is that mapping in a way like this:

Properties prop = new Properties();
prop.load(new FileInputStream( path ));

Map<String, String> mapProp = prop.entrySet().stream().collect( /*I don't know*/ );

return mapProp;

How write a lambda expression for do that?

Thanks all in advance

Andrea.

Lukasz Wiktor · Accepted Answer · 2014-05-27 11:15:23Z

19

Use Collectors.toMap

Properties prop = new Properties();
prop.load(new FileInputStream( path ));

Map<String, String> mapProp = prop.entrySet().stream().collect(
    Collectors.toMap(
        e -> (String) e.getKey(),
        e -> (String) e.getValue()
    ));

edited May 27, 2014 at 11:15

answered May 27, 2014 at 11:08

Lukasz Wiktor

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2 Comments

Andrea Catania Over a year ago

So easy... I had tried in this way: Collectors.toMap( e -> (String) e.getKey(), (String) e.getValue()) but ofcourse not work... Very very thanks, I have last question, have you a guide for lambda expression?

Lukasz Wiktor Over a year ago

@AndreaCatania I started with javadoc and tutorials from Oracle: docs.oracle.com/javase/tutorial/java/javaOO/… and docs.oracle.com/javase/tutorial/collections/streams/index.html This article was also very helpful: techempower.com/blog/2013/03/26/everything-about-java-8

OldCurmudgeon · Accepted Answer · 2014-05-27 11:17:57Z

6

Not actually an answer but may be useful to know for others passing by.

Since Properties extends Hashtable<Object,Object> which implements Map<K,V> you should not need to do anything other than:

    Properties p = new Properties();
    Map<String,String> m = new HashMap(p);

Not quite sure why no warnings are offered for this code as it implies a cast from Map<Object,Object> to Map<String,String> is acceptable but I suspect that is a separate question.

answered May 27, 2014 at 11:17

OldCurmudgeon

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4 Comments

Andrea Catania Over a year ago

what is the efficient way between constructor or lambda?

OldCurmudgeon Over a year ago

@AndreaCatania - I'd say let the system do it if you can, you should never optimise until you have proved that optimisation is necessary, so use the constructor.

Marko Topolnik Over a year ago

You wrote new HashMap(p) (raw type), escaping generic type analysis; this is why your code only produces a warning and not an error, at least in Eclipse: "The expression of type HashMap needs unchecked conversion to conform to Map<String,String>"

Andrea Catania Over a year ago

so the best solution is to use HashMap( p )?

niraj.nijju · Accepted Answer · 2017-12-27 12:36:29Z

1

for those who want to use forEach

HashMap<String, String> propMap = new HashMap<>();
prop.forEach((key, value)-> {
   propMap.put( (String)key, (String)value);
});

edited Dec 27, 2017 at 12:36

answered Dec 27, 2017 at 11:35

niraj.nijju

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