Reverse a String

public static void reverse(String str){
if(str==null||str.size==1)
return;
int i=0;
int j=str.size-1;
char temp;
char[] ch=str.toCharArray();
while(i<j){
temp=ch[i];
ch[i]=ch[j];
ch[j]=temp;
i++; j--;
}
return;
}