基于预测的文法分析器，一个明显的特点就是将非终结符定义为解析函数（方法），当非终结符号可以派生为其他非终结符号时，在解析函数中递归调用即可。这种方法的一个缺点，是难以处理需要回溯的情形，后面我们再详细分析。上次我们研究了诸如CR、LF、HTAB等单个字符的解析，这一篇来看看稍微复杂一点的多个符号连接的情形，包括CRLF和RULE两个符号。

/*    This file is one of the component a Context-free Grammar Parser Generator,    which accept a piece of text as the input, and generates a parser    for the inputted context-free grammar.    Copyright (C) 2013, Junbiao Pan (Email: panjunbiao@gmail.com)    This program is free software: you can redistribute it and/or modify    it under the terms of the GNU General Public License as published by    the Free Software Foundation, either version 3 of the License, or    any later version.    This program is distributed in the hope that it will be useful,    but WITHOUT ANY WARRANTY; without even the implied warranty of    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the    GNU General Public License for more details.    You should have received a copy of the GNU General Public License    along with this program.  If not, see 
    . *///  CRLF           =  CR LF	protected String CRLF() throws IOException, MatchException {      //      回车和换行符号，直接调用相应的CR、LF方法来进行解析就OK。		return CR() + LF();	}    //  CRLF           =  CR LF    @Test//  测试CRLF方法的单元测试方法    public void testCRLF() throws Exception {//  创建一个测试器，在测试器的test方法中调用被测方法CRLF()        Tester
    
      tester = new Tester
     
      () {            @Override            public String test(AbnfParser parser) throws MatchException, IOException {                return parser.CRLF();            }        };//      对于字符串0x0D + 0x0A，断言解析成功        Assert.assertEquals(String.valueOf((char)0x0D) + String.valueOf((char)0x0A), AbnfParserFactory.newInstance(new char[] {0x0D, 0x0A}).CRLF());//      对于字符串0x0D + 0x0A + 0x0C，断言解析成功，但最后面的0x0C不应该被解析。        Assert.assertEquals(String.valueOf((char)0x0D) + String.valueOf((char)0x0A), AbnfParserFactory.newInstance(new char[] {0x0D, 0x0A, 0x0C}).CRLF());//      对于字符串0x0D + 0x0A + 0x0D，断言解析成功，但最后面的0x0D不应该被解析。        Assert.assertEquals(String.valueOf((char)0x0D) + String.valueOf((char)0x0A), AbnfParserFactory.newInstance(new char[] {0x0D, 0x0A, 0x0D}).CRLF());//      对于字符串0x0D + 0x0A + 0x0A，断言解析成功，但最后面的0x0A不应该被解析。        Assert.assertEquals(String.valueOf((char)0x0D) + String.valueOf((char)0x0A), AbnfParserFactory.newInstance(new char[] {0x0D, 0x0A, 0x0A}).CRLF());//      对于空字符串0x0D + 0x0A + 0x0C，断言解析异常        Assertion.assertMatchException("", tester, 1, 1);//      对于字符串0x0D，断言解析异常        Assertion.assertMatchException("" + (char)0x0D, tester, 1, 1);//      对于字符串0x0A，断言解析异常        Assertion.assertMatchException("" + (char)0x0A, tester, 1, 1);    }//		        rule           =  rulename defined-as elements c-nl//      解析rule的方法	protected Rule rule() throws IOException, MatchException {//              rule的第一个元素是rulename，首先调用rulename()方法，并记录解析到的规则名		RuleName rulename = rulename();//              rulename后面紧接着defined-as元素，调用相应的方法		String definedAs = defined_as();//              defined-as后面接着elements元素，调用elements()		Elements elements = elements();//              elements后面接着c-nl元素，调用之。		c_nl();//              返回解析到的规则		return new Rule(rulename, definedAs, elements);	}    //		        rule           =  rulename defined-as elements c-nl    @Test    public void testRule() throws Exception {//      创建测试器        Tester
      
        tester = new Tester
       
        () {            @Override            public Rule test(AbnfParser parser) throws MatchException, IOException {                return parser.rule();            }        };        String input;        Elements elements;        elements = AbnfParserFactory.newInstance("b").elements();//      a=b是一条符合文法的规则        Assertion.assertMatch("a=b" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=", elements), 1, 2);//      a=/b是一条符合文法的规则        Assertion.assertMatch("a=/b" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=/", elements), 1, 2);        elements = AbnfParserFactory.newInstance("b/c").elements();        Assertion.assertMatch("a=b/c" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=", elements), 1, 2);        Assertion.assertMatch("a=/b/c" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=/", elements), 1, 2);        elements = AbnfParserFactory.newInstance("b c d").elements();//      a=b c d是一条符合文法的规则        Assertion.assertMatch("a=b c d" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=", elements), 1, 2);//      a=/b c d是一条符合文法的规则        Assertion.assertMatch("a=/b c d" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=/", elements), 1, 2);        elements = AbnfParserFactory.newInstance("[b]").elements();//      a=[b]是一条符合文法的规则        Assertion.assertMatch("a=[b]" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=", elements), 1, 2);//      a=/[b]是一条符合文法的规则        Assertion.assertMatch("a=/[b]" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=/", elements), 1, 2);        elements = AbnfParserFactory.newInstance("*b").elements();//      a=*b是一条符合文法的规则        Assertion.assertMatch("a=*b" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=", elements), 1, 2);//      a=/*b是一条符合文法的规则        Assertion.assertMatch("a=/*b" + (char)0x0D + (char)0x0A, tester, new Rule(new RuleName("a"), "=/", elements), 1, 2);    }
       
      
     
    

下一篇我们将分析更复杂的文法。