java program to insert an character in an array

public class Answer implements QuestionInterface {
@Override
public String func(String str, int index, char ch)
{
   if(str==null)
   {
      
       return null;
   }
                if(index>str.length()+1)
                {
                    return str;
                }
              
                else
                {
StringBuilder st= new StringBuilder(str);
                st.insert(index-1, ch);
                String e=st.toString();
return e;
                }
}
}

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Java program to find duplicate element in an array

public class Answer implements QuestionInterface
 {
@Override
public char  func (String s){
char[] ch=s.toCharArray();
        int i,j;
        char b = 0;
        for(i=0;i<s.length();i++)
        {
            for(j=i+1;j<s.length();j++)
            {
                if(ch[j]==ch[i])
                {
                    b=ch[j];
                    break;
                }
            }
        }
        return b;
}
}