/**
 * 数学题目类，表示一道数学题
 */
public class MathProblem {
    private String expression;  // 题目表达式
    private String answer;      // 答案
    private String difficulty;  // 难度级别
    
    public MathProblem(String expression, String answer, String difficulty) {
        this.expression = expression;
        this.answer = answer;
        this.difficulty = difficulty;
    }
    
    public String getExpression() {
        return expression;
    }
    
    public String getAnswer() {
        return answer;
    }
    
    public String getDifficulty() {
        return difficulty;
    }
    
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        MathProblem that = (MathProblem) obj;
        return expression.equals(that.expression);
    }
    
    @Override
    public int hashCode() {
        return expression.hashCode();
    }
    
    @Override
    public String toString() {
        return expression + " = ?";
    }
}