* 12.7在准备类Fraction时，创建一个类nul1denominatoreexception，它必须是一个检查异常，如果类Fraction的对象的分母为0，则抛出其实例。将Fraction类的各种方法完整呈现出来

好的，下面是一个完整的`Fraction`类，包括`NullDenominatorException`异常和其他方法：

public class Fraction {
    private int numerator;
    private int denominator;

    public Fraction(int numerator, int denominator) throws NullDenominatorException {
        this.numerator = numerator;
        setDenominator(denominator);
    }

    public void setDenominator(int denominator) throws NullDenominatorException {
        if (denominator == 0) {
            throw new NullDenominatorException();
        }
        this.denominator = denominator;
    }

    public int getNumerator() {
        return numerator;
    }

    public int getDenominator() {
        return denominator;
    }

    public Fraction add(Fraction other) throws NullDenominatorException {
        int newDenominator = denominator * other.getDenominator();
        int newNumerator = numerator * other.getDenominator() + other.getNumerator() * denominator;
        return new Fraction(newNumerator, newDenominator);
    }

    public Fraction subtract(Fraction other) throws NullDenominatorException {
        int newDenominator = denominator * other.getDenominator();
        int newNumerator = numerator * other.getDenominator() - other.getNumerator() * denominator;
        return new Fraction(newNumerator, newDenominator);
    }

    public Fraction multiply(Fraction other) throws NullDenominatorException {
        int newNumerator = numerator * other.getNumerator();
        int newDenominator = denominator * other.getDenominator();
        return new Fraction(newNumerator, newDenominator);
    }

    public Fraction divide(Fraction other) throws NullDenominatorException {
        int newNumerator = numerator * other.getDenominator();
        int newDenominator = denominator * other.getNumerator();
        return new Fraction(newNumerator, newDenominator);
    }

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

在这个类中，除了`NullDenominatorException`异常类之外，还包括了`getNumerator`、`getDenominator`、`add`、`subtract`、`multiply`、`divide`和`toString`方法。这些方法实现了分数的加减乘除和输出等功能。如果分母为0，则在构造函数和`setDenominator`方法中都会抛出`NullDenominatorException`异常。