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up vote 1 down vote favorite

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.

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2 Answers 2

up vote 5 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
up vote 2 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);
}
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