无法访问Java中相同类型的私有成员

你应该在你的 Fraction

public class Fraction{
   private int numerator;
   private int denominator;
   public Fraction(final int numerator, final int denominator){
     this.numerator = numerator;
     this.denominator = denominator;
   }
   public int getNumerator(){
     return this.numerator;
   }
   public int getDenominator(){
     return this.denominator;
   }
   public void setNumerator(final int numerator){
    this.numerator = numerator;
   }
   public void setDenominator(final int denominator){
    this.denominator = denominator;
   }
}

public Fraction add(Fraction f) {
    int add1 = (this.numerator*f.getDenominator())+(this.denominator*f.getNumerator());
    int add2 = this.denominator*f.getDenominator();
    return createFraction(add1,add2); //createFraction can be simply new Fraction(add1, add2);
}

您可以使用反射来获取字段的值,方法是使用 Field.setAccessible

用法示例:

Field field = f.getClass().getDeclaredField("numerator"); 
f.setAccessible(true);
int numerator = (int) field.get(f);

public Fraction add(Fraction f) throws NoSuchFieldException, IllegalAccessException{
    Field fieldNumerator = f.getClass().getDeclaredField("numerator");
    fieldNumerator.setAccessible(true);
    Field fieldDenominator = f.getClass().getDeclaredField("denominator");
    fieldDenominator.setAccessible(true);
    final int numerator = (int) fieldNumerator.get(f);
    final int denominator = (int) fieldDenominator.get(f);
    int add1 = (this.numerator*denominator)+(this.denominator*numerator);
    int add2 = this.denominator*denominator;
    return createFraction(add1,add2); 
}