determine if string has unique characters

The problem asks to "implement an algorithm to determine if a string has all unique character.

I saw the solution, but don't quite understand.

public boolean isUniqueChars(String str){
    if(str.length()>256) return false; 
        boolean[] char_set = new boolean[256];
        for(int i=0; i<str,length;i++){
        int val=str.charAt(i);
        if(char_set[val]) 
            return false;
        char_set[val]=true;
    } 
    return true;
}

Do we not use parseIntor(int)converter in front of the code? (str.charAt[i] will be automatically change to int?) What does boolean[] char set=new boolean[256] mean? Why do we need to set char_set[val]=true?