package gep.num;import gep.Expression;import java.util.ArrayList;import java.util.List;/** * 加法表達式 */public class Plus implements NExpression{	public static final char code = '+';	public static final int arity = 2;		private NExpression left;	private NExpression right;		public char getCode()	{		return code;	}	public int getArity()	{		return arity;	}	public void addChild(Expression child)	{		if (left==null)		{			left = (NExpression) child;			return; 		}				if (right==null)		{			right = (NExpression) child;			return;		}				throw new IllegalStateException("ADD_CHILD_ERROR");	}	public double evaluate()	{		return left.evaluate() + right.evaluate();	}		public String toString()	{		return "(" + left + ")+(" + right + ")";	}	public List getChildren()	{		List children = new ArrayList();		children.add(left);		children.add(right);				return children;	}}