How do you generate unique alphanumeric character in java?

Problem statement:
write an algorithm to generate alphanumeric characters using java.

public class GenerateUUID {
    public static void main(String args[]) {
        for (int i = 0; i < 1000000; i++) {
            UUID uuid1 = UUID.randomUUID();
            System.out.println(uuid1);  // 8-4-4-4-12 (36 char/32 + 4 )
        }
    }
}
Output:
889255d3-8179-4fd0-b870-fa7246e7b306
c46cc028-503d-469f-8aac-defe8b80a4df
d0fbf928-2c01-466f-86c6-6bf4c492799c
fa7bbabc-194b-4331-95fc-8676414858d5
dddca4af-83fe-44c4-8e65-c375146414bb
35681f4f-e3a9-4a45-85f0-d7ecccf6c2df
a577f324-e33f-4396-be9b-7b80cfe821de
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Note: we must create new instance on every single iteration withing a loop, then only it could be, else single instance will get looped over the multiple times.
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public class GenerateUUID {
    public static void main(String args[]) {
        UUID uuid1 = UUID.randomUUID();
        for (int i = 0; i < 1000000; i++) {
            System.out.println(uuid1);  // 8-4-4-4-12 (36 char / 32 + 4 hyphen)
        }
    }
}
Output:
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
2bb222ac-23c0-488a-83c8-75c5c9bc5cdd
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