symbolstack.java

基于算符优先关心的

package parser;
import java.util.ArrayList;
/**
 * the stack which will be used to contain the expression and $
 * @author Yuanhang Yang
 *
 */
public class SymbolStack{
	ArrayList<Symbol> prefix = new ArrayList<Symbol>();
	
	public SymbolStack() {

	}
	/**function: add symbol s into the ArrayList :prefix
	 * 
	 * @param s  symbol
	 */
	public void push(Symbol s){
		prefix.add(s);
	}
	/**function: remove the most top token from prefix
	 * 
	 * @return	 the  symbol have be removed
	 */
	public Symbol pop(){		
		return prefix.remove(prefix.size()-1);
	}
	/**function:get the top token in prefix 
	 * 
	 * @return  the top token 
	 */
	public Symbol peek(){
		return prefix.get(prefix.size()-1);
	}
	/**function: get the frist Terminal from prefix
	 * 
	 * @return the first Terminal in the prefix
	 */
	public Symbol getFirstTerminal(){
		for(int i = prefix.size()-1; i>=0; --i){
			if(prefix.get(i).tokenType <= TokenType.dollor )
				return prefix.get(i);
		}
		return null;
	}
}