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程序结构设计

Context 编写测试代码
expression
Expression interface
四则运算Expression implement
NumberExpression implement
ExpressionParser 核心实现类

具体实现

1. 先上最重要的实现类：ExpressionParser（最重要）

package com.example.demo;import com.example.demo.expression.*;import java.util.regex.Matcher;
import java.util.regex.Pattern;public class ExpressionParser {private String input;private int index;public ExpressionParser(String input) {this.input = input.replaceAll("\\s", ""); // 移除空格this.index = 0;}public Expression parse() {return parseExpression();}private Expression parseExpression() {Expression left = parseTerm();while (index < input.length()) {char operator = input.charAt(index);if (operator == '+' || operator == '-') {index++;Expression right = parseTerm();if (operator == '+') {left = new AddExpression(left, right);} else {left = new SubtractExpression(left, right);}} else {break;}}return left;}private Expression parseTerm() {Expression left = parseFactor();while (index < input.length()) {char operator = input.charAt(index);if (operator == '*' || operator == '/') {index++;Expression right = parseFactor();if (operator == '*') {left = new MultiplyExpression(left, right);} else {left = new DivideExpression(left, right);}} else {break;}}return left;}private Expression parseFactor() {if (index < input.length()) {char currentChar = input.charAt(index);if (Character.isDigit(currentChar)) {return parseNumber();} else if (currentChar == '(') {index++;Expression expression = parseExpression();if (index < input.length() && input.charAt(index) == ')') {index++;return expression;} else {throw new IllegalArgumentException("Mismatched parentheses");}}}throw new IllegalArgumentException("Invalid expression");}private Expression parseNumber() {Pattern numberPattern = Pattern.compile("\\d+");Matcher matcher = numberPattern.matcher(input.substring(index));if (matcher.find()) {String numberStr = matcher.group();index += numberStr.length();return new NumberExpression(Integer.parseInt(numberStr));} else {throw new IllegalArgumentException("Invalid number");}}
}

2. 再上上下文测试代码：Context（程序入口，稍重要）

package com.example.demo;import com.example.demo.expression.Expression;public class Context {public static void main(String[] args) {String inputExpression = "(3 + 55) * 2 - 4 / 2";ExpressionParser parser = new ExpressionParser(inputExpression);Expression expression = parser.parse();int result = expression.interpret();System.out.println("Result: " + result);}
}

3. 使用到的接口和数据结构（不太重要的结构封装）

3.1 interface

package com.example.demo.expression;public interface Expression {int interpret();
}

3.1.1 NumberExpression

package com.example.demo;import com.example.demo.expression.Expression;public class NumberExpression implements Expression {private int number;public NumberExpression(int number) {this.number = number;}@Overridepublic int interpret() {return number;}
}

3.1.2 四则运算Expression实现类

3.1.2.1 AddExpression

package com.example.demo.expression;public class AddExpression implements Expression {private Expression left;private Expression right;public AddExpression(Expression left, Expression right) {this.left = left;this.right = right;}@Overridepublic int interpret() {return left.interpret() + right.interpret();}
}

3.1.2.2 SubtractExpression

package com.example.demo.expression;// 非终结符表达式：减法表达式
public class SubtractExpression implements Expression {private Expression left;private Expression right;public SubtractExpression(Expression left, Expression right) {this.left = left;this.right = right;}@Overridepublic int interpret() {return left.interpret() - right.interpret();}
}

3.1.2.3 MultiplyExpression

package com.example.demo.expression;// 非终结符表达式：乘法表达式
public class MultiplyExpression implements Expression {private Expression left;private Expression right;public MultiplyExpression(Expression left, Expression right) {this.left = left;this.right = right;}@Overridepublic int interpret() {return left.interpret() * right.interpret();}
}

3.1.2.4 DivideExpression

package com.example.demo.expression;// 非终结符表达式：除法表达式
public class DivideExpression implements Expression {private Expression left;private Expression right;public DivideExpression(Expression left, Expression right) {this.left = left;this.right = right;}@Overridepublic int interpret() {int divisor = right.interpret();if (divisor != 0) {return left.interpret() / divisor;} else {throw new ArithmeticException("Division by zero");}}
}