Problem_Generator/src/PrimaryProblem.java

import java.util.List;
import java.util.Random;

/**
 * 生成小学题目
 */
public class PrimaryProblem implements ProblemsGenerator {
  private static final Random RANDOM = new Random();
  private final BracketManager bracketManager = new BracketManager();
  private final ExpressionBuilder expressionBuilder = new ExpressionBuilder();
  
  @Override
  public String generate() {
    String expression;
    do {
      int numCount = 2 + RANDOM.nextInt(4);
      expression = expressionBuilder.generateBasicExpression(numCount);

      List<Integer> numPositions = expressionBuilder.getNumberPositions(expression);
      int pairs = bracketManager.decideParenthesesPairs(numCount);
      if (pairs > 0) {
        expression = bracketManager.addParentheses(expression, pairs, numPositions);
      }
      
    } while (expressionBuilder.hasDivideByZero(expression));
    
    return expression;
  }
}