Java cast of ArrayList<Map>

Question

sorry to say, I'm not very experienced with Java. I'm using eclipse as IDE. Eclispe complains about unchecked cast which I can not understand. Please refer to the code snipet. How to make this right in general ?

public Liberty2Server(ArrayList<Map<String,String>> libertySourceFiles , String basePath) throws FileNotFoundException , IllegalArgumentException {
    // check input file path
    for( Object map : libertySourceFiles) {
        Map<String,String> mp = (Map<String,String>) map ;  // <==Eclipse complains here: Unchecked cast from Object to Map<String,String>

Any hint is welcome

Rolf

A hint perhaps unrelated to your immediate issue. As soon as I start passing around complex collection compositions (e.g. a list of maps) I create a specific object representing and containing that. It makes the code less verbose and you can implement the population/iteration and pre/post-conditions in one place — Brian Agnew
– Brian Agnew, Commented Feb 5, 2013 at 10:24

assylias · Accepted Answer · 2013-02-05 10:19:36Z

3

You can change your for loop and use the explicit type in the collection:

for(Map<String,String> map : libertySourceFiles) {
    Map<String,String> mp = map; //probably not needed any more
}

answered Feb 5, 2013 at 10:19

assylias

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Peter Lawrey · Accepted Answer · 2013-02-05 10:20:59Z

2

It means you are performing a cast which the compiler cannot check is safe.

It warns you that you have to check it's safe.

A better solution is to remove the need for the cast by using

for(Map<String, String> mp : libertySourceFiles) {

answered Feb 5, 2013 at 10:20

Peter Lawrey

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Joshua · Accepted Answer · 2013-02-05 10:21:34Z

0

Eclipse complains, because you want to cast an Object to a Map<String, String>. Change the type of your running variable map to Map<String, String>, and everything will be fine. You won't even need the variable mpthen.

answered Feb 5, 2013 at 10:21

Joshua

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Carlo Pellegrini · Accepted Answer · 2013-02-05 10:22:10Z

0

The problem here is that you upcast the ArrayList<Map<String,String>> element do Object and then downcast it to a generic type. The solution is easy:

public Liberty2Server(ArrayList<Map<String,String>> libertySourceFiles , String basePath) throws FileNotFoundException , IllegalArgumentException {
    // check input file path
    for(  Map<String,String> mp : libertySourceFiles) {
      //  Map<String,String> mp = (Map<String,String>) map ; Not needed anymore
      ....

answered Feb 5, 2013 at 10:22

Carlo Pellegrini

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