设计模式(22)：解释器模式

解释器

是一种不常用的设计模式
用于描述如何构成一个简单的语言解释器，主要用于使用面向对象语言开发的解释器和解释器设计
当我们需要开发一种新的语言时，可以考虑使用解释器模式
尽量不要使用解释器模式，后期维护会有很大麻烦。在项目中，可以使用jruby、groovy、java的js引擎来替代解释器的作用，弥补java语言的不足。

开发中常见的场景

EL表达式的处理
正则表达式解释器
SQL语法的解释器
数学表达式解释器

举例代码实现

解析和执行数学表达式
输入"5+4*5-8/4",输出“23”
抽象解释器接口

/*** 抽象解释器接口*/
public interface Expression {int interpret(Context context);
}

终结符表达式

/*** 数值表达式---终结符表达式*/
public class NumberExpression implements Expression{private Integer number;public NumberExpression(Integer number) {super();this.number = number;}@Overridepublic int interpret(Context context) {return number;}
}
/*** 运算符号表达式---终结符表达式* 	symbol：* 		1：加 * 		2：减 * 		3：乘 * 		4：除 */
public class SymbolExpression implements Expression {private int symbol;	public SymbolExpression(int symbol) {super();this.symbol = symbol;}@Overridepublic int interpret(Context context) {return symbol;}
}

非终结符表达式

/*** 加法表达式——加法表达式也是数值表达式的一种*/
public class AdditionExpression extends NumberExpression{private NumberExpression left;private NumberExpression right;public AdditionExpression(NumberExpression left, NumberExpression right) {super(1);this.left = left;this.right = right;}@Overridepublic int interpret(Context context) {return left.interpret(context) + right.interpret(context);}
}
/*** 减法表达式---减法表达式也是数值表达式的一种*/
public class SubtractExpression extends NumberExpression{private NumberExpression left;private NumberExpression right;public SubtractExpression(NumberExpression left, NumberExpression right) {super(2);this.left = left;this.right = right;}@Overridepublic int interpret(Context context) {return left.interpret(context) - right.interpret(context);}
}
/*** 乘法表达式——乘法表达式也是数值表达式的一种*/
public class MultiplicationExpression extends NumberExpression{private NumberExpression left;private NumberExpression right;public MultiplicationExpression(NumberExpression left, NumberExpression right) {super(3);this.left = left;this.right = right;}@Overridepublic int interpret(Context context) {return left.interpret(context) * right.interpret(context);}
}
/*** 除法表达式——除法表达式也是数值表达式的一种*/
public class DivisionExpression extends NumberExpression{private NumberExpression left;private NumberExpression right;public DivisionExpression(NumberExpression left, NumberExpression right) {super(4);this.left = left;this.right = right;}@Overridepublic int interpret(Context context) {return left.interpret(context) / right.interpret(context);}
}

上下文类(context)

public class Context {private Expression expression;public Context(String expression) {LinkedList<Expression> linkedList = new LinkedList<Expression>();int num = 0;for(int i=0;i<expression.length();i++) {char ati;if((ati=expression.charAt(i))>='0' && ati<='9') {num = num*10 + (ati-'0');}else {Expression peek;SymbolExpression symbol;NumberExpression left,right;if(!linkedList.isEmpty() && ((peek=linkedList.peekLast()) instanceof SymbolExpression) && (peek.interpret(this)==3 || peek.interpret(this)==4)){symbol = (SymbolExpression)linkedList.pollLast();left = (NumberExpression)linkedList.pollLast();right = new NumberExpression(num);if(symbol.interpret(this)==3){linkedList.addLast(new MultiplicationExpression(left, right));}else{linkedList.addLast(new DivisionExpression(left, right));}}else{linkedList.addLast(new NumberExpression(num));}SymbolExpression symbolExpression = null;switch (ati) {case '+':symbolExpression = new SymbolExpression(1);break;case '-':symbolExpression = new SymbolExpression(2);break;case '*':symbolExpression = new SymbolExpression(3);break;case '/':symbolExpression = new SymbolExpression(4);break;default:break;}num = 0;linkedList.addLast(symbolExpression);}}Expression peek;if(!linkedList.isEmpty() &&(	(peek=linkedList.peekLast()) instanceof SymbolExpression) && (peek.interpret(this)==3 || peek.interpret(this)==4)){Expression symbol = linkedList.pollLast();NumberExpression left = (NumberExpression)linkedList.pollLast();NumberExpression right = new NumberExpression(num);if(symbol.interpret(this)==3){linkedList.addLast(new MultiplicationExpression(left, right));}else{linkedList.addLast(new DivisionExpression(left, right));}}else{linkedList.add(new NumberExpression(num));}init(linkedList);}private void init(LinkedList<Expression> linkedList){SymbolExpression symbolExpression = null;NumberExpression left = null;while(!linkedList.isEmpty()){Expression tempExpression = linkedList.pollFirst();if(tempExpression instanceof SymbolExpression){symbolExpression = (SymbolExpression)tempExpression;}else{if(left==null){left = (NumberExpression)tempExpression;}else{NumberExpression right = (NumberExpression)tempExpression;switch (symbolExpression.interpret(this)) {case 1:left = new AdditionExpression(left, right);break;case 2:left = new SubtractExpression(left, right);break;case 3:left = new MultiplicationExpression(left, right);break;case 4:left = new DivisionExpression(left, right);break;default:break;}}}}this.expression = left;}public int calculate(){return this.expression.interpret(this);}
}

客户端调用

public static void main(String[] args) {Context context = new Context("33+12*9+42/2+6/3");int calculate = context.calculate();System.out.println(calculate);
}