t_addminus.java

中山大学编译原理课程的一个实验

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package symbols;

import exceptions.MissingOperandException;
import exceptions.TypeMismatchedException;
import java.util.ArrayList;

/**
 *
 * @author Beeven
 */
public class T_ADDMINUS extends Terminal implements TerminalReduce{

    public Expr reduce(ArrayList<Expr> stack) throws TypeMismatchedException, MissingOperandException {
        int post=stack.size()-1;
        Expr a=stack.get(post-2);
        Expr o=stack.get(post-1);
        Expr b=stack.get(post);
        if((b.type==Type.addminus)) throw new MissingOperandException();
        if(a instanceof BoolExpr)
            throw new TypeMismatchedException();
        else if(!(a instanceof ArithExpr))
            throw new MissingOperandException();
        ArithExpr ae;
        if(o.tag==Tag.ADD)
            ae=new ArithExpr(((ArithExpr)a).value+((ArithExpr)b).value);
        else
            ae=new ArithExpr(((ArithExpr)a).value-((ArithExpr)b).value);
        stack.remove(post);
        stack.remove(post-1);
        stack.remove(post-2);
        stack.add(ae);
        return ae;
    }

}