为语言创建解释器,通常由语言的语法和语法分析来定义。
以下是一个规则检验器实现,具有 and 和 or 规则,通过规则可以构建一颗解析树,用来检验一个文本是否满足解析树定义的规则。
例如一颗解析树为 D And (A Or (B C)),文本 “D A” 满足该解析树定义的规则。
这里的 Context 指的是 String。
public abstract class Expression {
public abstract boolean interpret(String str);
}
public class TerminalExpression extends Expression {
private String literal = null;
public TerminalExpression(String str) {
literal = str;
}
public boolean interpret(String str) {
StringTokenizer st = new StringTokenizer(str);
while (st.hasMoreTokens()) {
String test = st.nextToken();
if (test.equals(literal)) {
return true;
}
}
return false;
}
}
public class AndExpression extends Expression {
private Expression expression1 = null;
private Expression expression2 = null;
public AndExpression(Expression expression1, Expression expression2) {
this.expression1 = expression1;
this.expression2 = expression2;
}
public boolean interpret(String str) {
return expression1.interpret(str) && expression2.interpret(str);
}
}
public class OrExpression extends Expression {
private Expression expression1 = null;
private Expression expression2 = null;
public OrExpression(Expression expression1, Expression expression2) {
this.expression1 = expression1;
this.expression2 = expression2;
}
public boolean interpret(String str) {
return expression1.interpret(str) || expression2.interpret(str);
}
}
public class Client {
/**
* 构建解析树
*/
public static Expression buildInterpreterTree() {
// Literal
Expression terminal1 = new TerminalExpression("A");
Expression terminal2 = new TerminalExpression("B");
Expression terminal3 = new TerminalExpression("C");
Expression terminal4 = new TerminalExpression("D");
// B C
Expression alternation1 = new OrExpression(terminal2, terminal3);
// A Or (B C)
Expression alternation2 = new OrExpression(terminal1, alternation1);
// D And (A Or (B C))
return new AndExpression(terminal4, alternation2);
}
public static void main(String[] args) {
Expression define = buildInterpreterTree();
String context1 = "D A";
String context2 = "A B";
System.out.println(define.interpret(context1));
System.out.println(define.interpret(context2));
}
}
true
false