解释器模式

• 定义：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，这个解释器使用该表示来解释语言中的句子
• 简单来说：为了解释一种语言而为语言创建的解释器
• 类型：行为型
• 适用场景：某个特定类型问题发生频率足够高
• 优点：语法由很多类表示，容易改变及扩展此“语言”
• 缺点：当语法规则数目太多时，增加了系统复杂度
• 相关设计模式：
  • 解释器模式和适配器模式：有点类似，但是适配器模式不需要预先知道要适配的规则，而解释器要把规则写好，根据规则去执行解释

Coding

• 场景：实现6 100 11 + *按顺序数字入栈，遇到符号出栈计算结果

/**
 * 解释器接口以及它的三个实现类
 */
public interface Interpreter {
    int interpret();
}
public class AddInterpreter implements Interpreter {
    private Interpreter firstExpression,secondExpression;

    public AddInterpreter(Interpreter firstExpression, Interpreter secondExpression) {
        this.firstExpression = firstExpression;
        this.secondExpression = secondExpression;
    }

    @Override
    public int interpret() {
        return this.firstExpression.interpret() + this.secondExpression.interpret();
    }

    @Override
    public String toString() {
        return "+";
    }
}
public class MultiInterpreter implements Interpreter {

    private Interpreter firstExpression, secondExpression;

    public MultiInterpreter(Interpreter firstExpression, Interpreter secondExpression) {
        this.firstExpression = firstExpression;
        this.secondExpression = secondExpression;
    }

    @Override
    public int interpret() {
        return this.firstExpression.interpret() * this.secondExpression.interpret();
    }

    @Override
    public String toString() {
        return "*";
    }
}
public class NumberInterpreter implements Interpreter {

    private int number;

    public NumberInterpreter(int number) {
        this.number = number;
    }

    public NumberInterpreter(String number) {
        this.number = Integer.parseInt(number);
    }

    @Override
    public int interpret() {
        return this.number;
    }
}
public class ExpressionParser {
    private Stack<Interpreter> stack = new Stack<Interpreter>();

    public int parse(String str) {
        String[] strItemArray = str.split(" ");
        for (String symbol : strItemArray) {
            if (!OperatorUtil.isOperator(symbol)) {
                Interpreter numberExpression = new NumberInterpreter(symbol);
                stack.push(numberExpression);
                System.out.println(String.format("入栈: %d", numberExpression.interpret()));
            } else {
                //是运算符号，可以计算
                Interpreter firstExpression = stack.pop();
                Interpreter secondExpression = stack.pop();
                System.out.println(String.format("出栈: %d 和 %d",
                        firstExpression.interpret(), secondExpression.interpret()));
                Interpreter operator = OperatorUtil.getExpressionObject(firstExpression, secondExpression, symbol);
                System.out.println(String.format("应用运算符: %s", operator));
                int result = operator.interpret();
                NumberInterpreter resultExpression = new NumberInterpreter(result);
                stack.push(resultExpression);
                System.out.println(String.format("阶段结果入栈: %d", resultExpression.interpret()));
            }
        }
        int result = stack.pop().interpret();
        return result;
    }
}
public class OperatorUtil {
    public static boolean isOperator(String symbol) {
        return (symbol.equals("+") || symbol.equals("*"));
    }

    public static Interpreter getExpressionObject(Interpreter firstExpression, Interpreter secondExpression, String symbol) {
        if (symbol.equals("+")) {
            return new AddInterpreter(firstExpression, secondExpression);
        } else if (symbol.equals("*")) {
            return new MultiInterpreter(firstExpression, secondExpression);
        }
        return null;
    }
}

测试

public class Test {
    public static void main(String[] args) {
        String inputStr = "6 100 11 + *";
        ExpressionParser expressionParser = new ExpressionParser();
        int result = expressionParser.parse(inputStr);
        System.out.println("解释器计算结果: " + result);
    }
}
============输出================
入栈: 6
入栈: 100
入栈: 11
出栈: 11 和 100
应用运算符: +
阶段结果入栈: 111
出栈: 111 和 6
应用运算符: *
阶段结果入栈: 666
解释器计算结果: 666

UML

源码中的应用

java.util.regex.Pattern：正则表达式就是一种语法，通过JDK中的正则解释器把它解释出来

public final class Pattern
    implements java.io.Serializable
{
    private int parsePastWhitespace(int ch) {
        while (ASCII.isSpace(ch) || ch == '#') {
            while (ASCII.isSpace(ch))
                ch = temp[cursor++];
            if (ch == '#')
                ch = parsePastLine();
        }
        return ch;
    }
    /**
     * xmode parse past comment to end of line.
     */
    private int parsePastLine() {
        int ch = temp[cursor++];
        while (ch != 0 && !isLineSeparator(ch))
            ch = temp[cursor++];
        return ch;
    }