Java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

arraysdictionaryjava

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, I get the following ClassCastException. 

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

Best Answer

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

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

Alternatively, since a Set's toArray method gives no guarantee 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);
}

EDIT: Just to complete the picture, in Java 8, the foreach method can be used to make the code more elegant:

map1.keySet().forEach(System.out::println);
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