用java定义类Fraction，在其中定义属性:numerator和denominator，分别表示分子和分母。 然定再定义两个分数的加、减、乘、除运算的方法。 在主类中输入2个分数，计算它们的四则运算结果。

在Java中，我们可以创建一个名为`Fraction`的类来定义分数，并为其提供属性和操作方法。首先，我们需要定义类的基本结构：

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

    public Fraction(int numerator, int denominator) {
        if (denominator != 0) {
            this.numerator = numerator;
            this.denominator = denominator;
        } else {
            throw new IllegalArgumentException("Denominator cannot be zero.");
        }
    }

    // 加法
    public Fraction add(Fraction other) {
        int newNumerator = this.numerator * other.denominator + other.numerator * this.denominator;
        int newDenominator = this.denominator * other.denominator;
        return new Fraction(newNumerator, newDenominator);
    }

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

    // 乘法
    public Fraction multiply(Fraction other) {
        return new Fraction(this.numerator * other.numerator, this.denominator * other.denominator);
    }

    // 除法，需要检查除数是否为0
    public Fraction divide(Fraction other) {
        if (other.denominator != 0) {
            return new Fraction(this.numerator * other.denominator, this.denominator * other.numerator);
        } else {
            throw new ArithmeticException("Cannot divide by zero.");
        }
    }

    @Override
    public String toString() {
        return "(" + numerator + "/" + denominator + ")";
    }
}

接下来，在主类中你可以这样使用这个`Fraction`类来进行四则运算：

public class Main {
    public static void main(String[] args) {
        Fraction f1 = new Fraction(4, 5);
        Fraction f2 = new Fraction(2, 7);

        System.out.println("f1: " + f1); // 输出第一个分数
        System.out.println("f2: " + f2); // 输出第二个分数

        // 计算并打印结果
        Fraction sum = f1.add(f2);
        Fraction difference = f1.subtract(f2);
        Fraction product = f1.multiply(f2);
        Fraction quotient = f1.divide(f2); // 这里假设其他不是零

        System.out.println("Addition: " + sum);
        System.out.println("Subtraction: " + difference);
        System.out.println("Multiplication: " + product);
        System.out.println("Division: " + quotient);
    }
}