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

Question

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)

Mureinik · Accepted Answer · 2017-03-16 08:50:03Z

48

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);

edited Mar 16, 2017 at 8:50

answered Mar 29, 2014 at 13:33

Mureinik

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3 Comments

ir2pid Over a year ago

String[] str = map1.keySet().toArray(new String[map1.size()]); you missed the parenthesis in map1.size()

Oleksii Morenets Over a year ago

String[] str = map1.keySet().toArray(new String[0]);

andy Over a year ago

If you want to use Java streams: map.keySet().stream.toArray(String[]::new)

Sabuj Hassan · Accepted Answer · 2014-03-29 13:30:17Z

5

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);
}

answered Mar 29, 2014 at 13:30

Sabuj Hassan

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3 Comments

omerfarukdogan Over a year ago

And you cast Object[] to Object[] at the first line?

Ahmed Akhtar Over a year ago

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.

mohsen.nour Over a year ago

(Object[]) map1.keySet().toArray(); is not necessary to cast object array................ ..... ........ ....... Object [] obj =map1.keySet().toArray();

Muhammed_G · Accepted Answer · 2022-10-04 21:05:54Z

3

You can cast Object[] to String[] like this:

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

answered Oct 4, 2022 at 21:05

Muhammed_G

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Comments

Mr.Q · Accepted Answer · 2017-01-06 07:21:54Z

toArray()method is defined in List interface so every where there is an instance of List, you also have access to this method.

At first you might think that you can cast an array of Objects which its elements are all of type String to a String array but java specs says otherwise Link, in short it says:

bArr = new B[]; A[] aArr = (A[]) bArr;

"works" at runtime if and only if B is a subtype of A (or A itself). Whether B actually only contains As is irrelevant and the compile-time type of bArr is not used either (what matters is the runtime type):

In your code by calling : image_urls.toArray() you will get an array of Object and since Object is not SubType of String, you get exception. To do this write, use other overload of toArray() which gets an array of certain type (for type reference) as "Mureinik" mentioned.

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