Lecture 25

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Talking Points

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Getters and Setters

Write the following class:

What happens if we do the following?

Fraction f = new Fraction(2,3);
f.denominator = 0;
double d = f.asDouble();

To prevent this we make fields private and add getters (also called accessors or inspectors) and setters (also called mutators). The setters can check for bad behavior

public int getDenominator() {
    return this.denominator;
}
public void setDenominator(int denom) {
    if (denom != 0) {
        this.denominator = denom;
    } // else ignore the update request
}

What happens if we do the following?

Fraction f = new Fraction(2,0);
double d = f.asDouble();

To prevent this, we use the setters in the constructor too:

public Fraction(int num, int den) {
    // start it in a sane state
    this.numerator = 0;
    this.denominator = 1;
    // then use the setters
    this.setNumerator(num);
    this.setDenominator(den);
}

toString

Every class inherits a few methods from the Object class. This includes the method public String toString(). This is how a class is printed and added to Strings. But the default version of this gives nonsense.

It is always a good idea to write your own toString

public String toString() {
    return this.getNumerator() + "/" + this.getDenominator();
}

Then you can do things like

Fraction f = new Fraction(2, 3);
System.out.println(f + " = " + f.asDouble());

Extending Fraction class

We'll add some of the following.

Suggested practice: write the rest.

From lecture

Code from 001 (3pm): Fraction.java, TestFraction.java; slides

Code from 002 (1pm): Fraction.java, TestFraction.java; slides