用Java定义一个有理数类，实现有理数的基本运算：加、减、乘、除,约分。

好的，这是一个关于有理数类的编程问题。以下是用Java定义一个有理数类的基本代码：

public class RationalNumber {
    private int numerator;   // 分子
    private int denominator; // 分母

    public RationalNumber(int numerator, int denominator) {
        if (denominator == 0) {
            throw new IllegalArgumentException("分母不能为0");
        }
        // 化简分数
        int gcd = gcd(numerator, denominator);
        this.numerator = numerator / gcd;
        this.denominator = denominator / gcd;
        // 如果分母为负数，则将负号调整到分子上
        if (this.denominator < 0) {
            this.numerator = -this.numerator;
            this.denominator = -this.denominator;
        }
    }

    // 获取分子
    public int getNumerator() {
        return numerator;
    }

    // 获取分母
    public int getDenominator() {
        return denominator;
    }

    // 加法
    public RationalNumber add(RationalNumber other) {
        int top = this.numerator * other.denominator + this.denominator * other.numerator;
        int bottom = this.denominator * other.denominator;
        return new RationalNumber(top, bottom);
    }

    // 减法
    public RationalNumber subtract(RationalNumber other) {
        int top = this.numerator * other.denominator - this.denominator * other.numerator;
        int bottom = this.denominator * other.denominator;
        return new RationalNumber(top, bottom);
    }

    // 乘法
    public RationalNumber multiply(RationalNumber other) {
        int top = this.numerator * other.numerator;
        int bottom = this.denominator * other.denominator;
        return new RationalNumber(top, bottom);
    }

    // 除法
    public RationalNumber divide(RationalNumber other) {
        int top = this.numerator * other.denominator;
        int bottom = this.denominator * other.numerator;
        return new RationalNumber(top, bottom);
    }

    // 约分
    private int gcd(int a, int b) {
        if (b == 0) {
            return a;
        } else {
            return gcd(b, a % b);
        }
    }

    // 转化为字符串形式
    public String toString() {
        if (this.denominator == 1) {
            return String.valueOf(this.numerator);
        } else {
            return this.numerator + "/" + this.denominator;
        }
    }
}

您可以根据自己的需要对这个代码进行修改和补充。希望能对您有所帮助。