解析器模式是一种行为型设计模式。其思想是:给定一个语言, 定义它的文法的一种表示,并定义一个解释器,该解释器使用该表示来解释语言中的句子。
//@author: v: jbossjf
public interface Expression {
double interpreter(HashMap<String, Double> var); // 声明一个解析操作
}
public class VarExp implements Expression {
private String value;
public VarExp(String value) {
this.value = value;
}
@Override
public double interpreter(HashMap<String, Double> var) {
return var.get(this.value); // 返回表达式所对应的值
}
}
public abstract class SymbolExp implements Expression {
protected Expression left; // 运算符左侧的表达式
protected Expression right; // 运算符右侧的表达式
public SymbolExp(Expression left, Expression right) {
this.left = left;
this.right = right;
}
}
// 加法
public class Add extends SymbolExp {
public Add(Expression left, Expression right) {
super(left, right);
}
@Override
public double interpreter(HashMap<String, Double> var) {
return left.interpreter(var) + right.interpreter(var); // 返回左右两个表达式之和
}
}
// 减法
public class Minus extends SymbolExp {
public Minus(Expression left, Expression right) {
super(left, right);
}
@Override
public double interpreter(HashMap<String, Double> var) {
return left.interpreter(var) - right.interpreter(var); // 返回左右两个表达式之差
}
}
public class Calculator {
private Expression expression;
public Calculator(String exp) {
Expression left;
Expression right;
Stack<Expression> stack = new Stack<>();
char[] chars = exp.toCharArray();
for (int i = 0, len = chars.length; i < len; i++) {
switch (chars[i]) {
case '+':
left = stack.pop(); // 从栈中取回上一个压入的元素作为左侧表达式
right = new VarExp(String.valueOf(chars[++i])); // 指针后移一位得到右侧表达式
stack.push(new Add(left, right)); // 将计算结果压回栈中
break;
case '-':
left = stack.pop();
right = new VarExp(String.valueOf(chars[++i]));
stack.push(new Minus(left, right));
break;
default:
// 如果是非运算符,则生成一个非终结表达式,然后压和栈中
stack.push(new VarExp(String.valueOf(chars[i])));
break;
}
}
this.expression = stack.pop(); // 遍历完成后取回栈中的元素
}
// 实际上,计算已经在构造器里完成了,这里只是取回结果的值
public double cal(HashMap<String, Double> var) {
return this.expression.interpreter(var);
}
}
public class Client {
// 运行程序
public void run() {
String exp = getExp();
HashMap<String, Double> var = getValues(exp);
Calculator calculator = new Calculator(exp);
System.out.printf("%s = %f", exp, calculator.cal(var));
}
// 要求用户输入表达式
private String getExp() {
Scanner sc = new Scanner(System.in);
System.out.println("please defined an expression:");
return sc.nextLine();
}
// 要求用户输入每个变量的值
private HashMap<String, Double> getValues(String exp) {
HashMap<String, Double> map = new HashMap<>();
Scanner sc = new Scanner(System.in);
char[] chars = exp.toCharArray();
for (char c : chars) {
String str = String.valueOf(c);
if (!"+-".contains(str) && !map.containsKey(str)) {
System.out.printf("%s = ", str);
map.put(str, sc.nextDouble());
}
}
return map;
}
}
// 测试
class Test {
public static void main(String[] args) {
Client client = new Client();
client.run();
}
}
please defined an expression:
a+b-c+b
a = 3
b = 2
c = 1
a+b-c+b = 6.000000