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up vote 6 down vote favorite
2

I need convert HashMap to a String array, follow is my java code

import java.util.HashMap;
import java.util.Map;

public class demo {

    public static void main(String[] args) {

        Map<String, String> map1 = new HashMap<String, String>();

        map1.put("1", "1");
        map1.put("2", "2");
        map1.put("3", "3");

        String[] str = (String[]) map1.keySet().toArray();

        for(int i=0; i<str.length;i++) {
            System.out.println(str[i]);
        }
    }
}

when i run the code, but it shows 

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
at demo.main(demo.java:17)

i am confused with this,who can help me.

share|improve this question
up vote 15 down vote accepted

toArray() returns an Object[], regardless of generics. You could instead use the overloaded variant:

String[] str = map1.keySet().toArray(new String[map1.size()]);

Alternatively, since a Set's toArray method gives no gaurentee about the order, and all you're using the array for is printing out the values, you could iterate the keySet() directly:

for (String str: map1.keySet()) {
    System.out.println(str);
}
share|improve this answer
1  
String[] str = map1.keySet().toArray(new String[map1.size()]); you missed the parenthesis in map1.size() – ir2pid Apr 22 '15 at 20:14
    
@ir2pid - indeed, thanks for noticing! Fixed. – Mureinik Apr 23 '15 at 6:03
up vote 4 down vote

It is returning Object[] Not String[]. Try this:

Object[] obj = (Object[]) map1.keySet().toArray();
for(int i=0; i<obj.length;i++) {
    String someString = (String)obj[i];
    System.out.println(someString);
}
share|improve this answer
2  
And you cast Object[] to Object[] at the first line? – farukdgn Oct 3 '15 at 17:31
    
Indeed two casts (i.e. Object[] and String) cannot be done in one step. They need to be done step by step. Thanks for the great incite. – Ahmed Akhtar Apr 25 at 7:25

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