我有一个用于解析标记的 StreamTokenizer。当我将以下内容传递给标准输入时:
a b_c d
解析的标记(在 stdout 上)是:
a
b
null
c
d
为什么会这样?如果下划线是单词字符,则应该有 3 个标记,第二个是“b_c”。如果下划线是分隔符,则应该有 4 个标记。我认为空 token 毫无意义。
Q1:为什么会出现null token?
Q2:为什么有人会设计一个StreamTokenizer来产生null token?
Ideone 脚本:http://ideone.com/e.js/RFbPpJ
import java.util.*;
import java.lang.*;
import java.io.*;
class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
StreamTokenizer st = new StreamTokenizer(br);
while (st.nextToken() != StreamTokenizer.TT_EOF) {
System.out.println(st.sval);
}
}
}
最佳答案
来自文档:
If the current token is a word token, this field contains a string giving the characters of the word token. When the current token is a quoted string token, this field contains the body of the string. The current token is a word when the value of the ttype field is TT_WORD. The current token is a quoted string token when the value of the ttype field is a quote character.
The initial value of this field is null.
这意味着不满足任何条件,输出null
。
换句话说,下划线的 ttype 既不被视为单词,也不被视为带引号的字符串。
ttype
的文档指定
After a call to the nextToken method, this field contains the type of the token just read. For a single character token, its value is the single character, converted to an integer. For a quoted string token, its value is the quote character. Otherwise, its value is one of the following: TT_WORD indicates that the token is a word. TT_NUMBER indicates that the token is a number. TT_EOL indicates that the end of line has been read. The field can only have this value if the eolIsSignificant method has been called with the argument true. TT_EOF indicates that the end of the input stream has been reached.
The initial value of this field is -4.
请注意 -4 值等于 TT_NOTHING。
要将下划线识别为单词,您可以使用 tokenizer.wordChars('_', '_');
wordChars is used to specify that all characters c in the range low <= c <= high are word constituents. A word token consists of a word constituent followed by zero or more word constituents or number constituents.
如果您希望下划线是普通字符而不是字符字符,那么还有一个 method为此。
请注意,将“_”作为 wordChars 的两个分隔符都将允许下划线作为单词字符,因此您可能需要设置符合您需要的边界。
编辑:为了回答您的评论,简而言之,下划线被视为标识符的一部分,这就是为什么它没有映射到任何东西,因此返回 null。
如果您查看 StreamTokenizer 类的未记录的私有(private)构造函数,您将更好地了解如何处理每个字符:
private StreamTokenizer() {
wordChars('a', 'z');
wordChars('A', 'Z');
wordChars(128 + 32, 255);
whitespaceChars(0, ' ');
commentChar('/');
quoteChar('"');
quoteChar('\'');
parseNumbers();
}
下划线是ASCII码95,不在边界内。
关于java.io.StreamTokenizer 在遇到下划线时生成空标记,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28749398/