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DS15_Expression.java
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108 lines (98 loc) · 2.55 KB
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//这个例子不错，需要再好好看看
import java.util.*;
//上下文（环境）角色，使用HashMap 来存储变量对应的数值
class Context{
private Map valueMap = new HashMap();
public void addValue(Variable x , int y){
Integer yi = new Integer(y);
valueMap.put(x , yi);
}
public int LookupValue(Variable x){
int i = ((Integer)valueMap.get(x)).intValue();
return i ;
}
}
//抽象表达式角色，也可以用接口来实现
abstract class Expression{
public abstract int interpret(Context con);
}
//终结符表达式角色
class Constant extends Expression{
private int i ;
public Constant(int i){
this.i = i;
}
public int interpret(Context con){
return i ;
}
}
class Variable extends Expression{
public int interpret(Context con) {
//this 为调用interpret 方法的Variable 对象
return con.LookupValue(this);
}
}
//非终结符表达式角色
class Add extends Expression{
private Expression left ,right ;
public Add(Expression left , Expression right) {
this.left = left ;
this.right= right ;
}
public int interpret(Context con){
return left.interpret(con) + right.interpret(con);
}
}
class Subtract extends Expression{
private Expression left , right ;
public Subtract(Expression left , Expression right) {
this.left = left ;
this.right= right ;
}
public int interpret(Context con){
return left.interpret(con) - right.interpret(con);
}
}
class Multiply extends Expression{
private Expression left , right ;
public Multiply(Expression left , Expression right){
this.left = left ;
this.right= right ;
}
public int interpret(Context con){
return left.interpret(con) * right.interpret(con);
}
}
class Division extends Expression{
private Expression left , right ;
public Division(Expression left , Expression right){
this.left = left ;
this.right= right ;
}
public int interpret(Context con){
try{
return left.interpret(con) / right.interpret(con);
}catch(ArithmeticException ae) {
System.out.println("被除数为0！");
return -11111;
}
}
}
public class DS15_Expression
{
private static Expression ex ;
private static Context con ;
public static void main(String[] args){
con = new Context();
//设置变量、常量
Variable a = new Variable();
Variable b = new Variable();
Constant c = new Constant(2);
//为变量赋值
con.addValue(a , 5);
con.addValue(b , 3);
//运算，对句子的结构由我们自己来分析，构造
ex = new Division(new Multiply(a , b), new Add(new Subtract(a , b) , c));
System.out.println("运算结果为："+ex.interpret(con));
}
}