CSV形式の１行分の文字列からList<String>への変換をinterface method にする。

先日書いた、CSV形式の１行分の文字列からList<String>への変換 - Oboe吹きプログラマの黙示録
を単純に、interface のメソッドにする。

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.function.Function;

public interface Csvtolist extends Serializable{

    public static Function<String, List<String>> of(){
        return Csvtolist::create;
    }
    
    public static List<String> create(String str){
        char DELIMITER = ',';
        char CARRIAGE_RETURN = '\r';
        char NEWLINE = '\n';
        char DOUBLE_QUOTE = '"';
        if (str==null || str.length()==0){
            return Collections.emptyList();
        }
        List<String> tokens = new ArrayList<String>();
        StringBuilder tokenBuf = new StringBuilder();
        boolean insideDoubleQuote = false;
        boolean isDoubleQuoteEscapeActive = false;
        StringBuilder wspBuf = new StringBuilder();
        for(int ii=0; ii < str.length(); ii++){
            final char ch = str.charAt(ii);
            if (ch==CARRIAGE_RETURN || ch==NEWLINE){
                if (insideDoubleQuote){
                    tokenBuf.append(ch);
                }else{
                    throw new RuntimeException("unquoted "
                     + (ch=='\n' ? "newline" : "carriage return")
                     + " found at position #" + (ii+1));
                }
            }else if(ch==DOUBLE_QUOTE){
                if (insideDoubleQuote){
                    if (isDoubleQuoteEscapeActive){
                        tokenBuf.append(ch);
                        isDoubleQuoteEscapeActive = false;
                    }else if(((ii+1) < str.length()) && str.charAt(ii+1)==DOUBLE_QUOTE){
                        isDoubleQuoteEscapeActive = true;
                    }else{
                        insideDoubleQuote = false;
                    }
                }else{
                    insideDoubleQuote = true;
                    if (wspBuf.length() != 0){
                        if (tokenBuf.length() != 0){
                            tokenBuf.append(wspBuf);
                        }
                        wspBuf.delete(0, wspBuf.length());
                    }
                }
            }else{
                if (insideDoubleQuote){
                    tokenBuf.append(ch);
                }else{
                    if (ch==DELIMITER){
                        tokens.add(tokenBuf.toString());
                        tokenBuf.delete(0, tokenBuf.length());
                        wspBuf.delete(0, wspBuf.length());
                    }else if(Character.isWhitespace(ch)){
                         wspBuf.append(ch);
                    }else{
                        if (wspBuf.length() != 0){
                            if (tokenBuf.length() != 0){
                                tokenBuf.append(wspBuf);
                            }
                            wspBuf.delete(0, wspBuf.length());
                        }
                        tokenBuf.append(ch);
                    }
                }
            }
        }
        if (insideDoubleQuote){
            throw new RuntimeException("terminating double quote not found");
        }
        tokens.add(tokenBuf.toString());
        return tokens;
    }
}

of() で、Function<String, List<String>> を返すようにしているので、
Optional の map で使用できる。

List<String> list = Optional.ofNullable(str).map(Csvtolist.of()).orElse(List.of());

結局、以下のように書いても同じではあるけど。

List<String> list = Optional.ofNullable(str).map(Csvtolist::create).orElse(List.of());