Week 2 Day 2 - Methods w/ Returns

Find code from class here.

Methods with Returns

The goal of today’s class is to extend our understanding of methods from something that simply prints information to something that can return, or save, a value.

By the end of today’s class, you should be able to answer the following questions:

How do you return a value from a method in Java?
When might you want to return a value instead of print it?
How do you set the type of the value to be returned from a method?
What happens if you try to return a value
Does the type of the value that is returned have to match the value(s) used as parameter(s)?

Questions from the Reading

Why do you ever need a method stub? What does it do?

public static double distance(double x1, double y1, double x2, double y2) {
    return 0.0; // stub
}

What is the difference between returning and printing? Doesn’t a void method return something with System.out.println()?

Lecture/Live-Code

The main difference between the material for this class and the material from last class is that our methods can now return values instead of just printing them. Consider the difference between these two methods:

Returns

public static void helloVoid() {
    System.out.println("Hello! (1)");
}
public static void helloReturn() {
    return "Hello! (2)";
}

They would both be called in simlar ways, but helloReturn() won’t actually print "Hello" by itself.

public static void main(String[] args) {
    helloVoid();
    helloReturn();
}

This makes some sense; remember that we are basically just running whatever code is in the method when we call it, so calling a method that doesn’t have a print statement won’t print anything. So what does it do?

You can think of it as the method name is directly replaced by whatever is in the return statement. In this case, helloReturn() will be a string.

public static void main(String[] args) {
    helloVoid();
    String b = helloReturn();
    System.out.println(b);
}

Calculations with Methods

It’s easier to see why we might want to do this if we are instead performing some kind of intermediate computation.

import java.util.Scanner;

public static double diameter(double radius) {
    double diameter = radius*2;
    return diameter;
}

public static double area(double d) {
    double area = d*Math.PI;
    return area;
}

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    double r = in.nextDouble();
    double d = diameter(r);
    double a = area(d);
    System.out.println("Circle area is: " + a);
}

You can see how this allows us to save intermediate steps.

Printing and Returning

Note that if we wanted to print something as well as return it, that is fine, and it will print at the point in the code where the method is called.

import java.util.Scanner;

public static double diameter(double radius) {
    double diameter = radius*2;
    return diameter;
}

public static double area(double d) {
    double area = d*Math.PI;
    System.out.println("Area calculated.")
    return area;
}

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    double r = in.nextDouble();
    System.out.println("radius: " + r)
    double d = diameter(r);
    System.out.println("diameter: " + d)
    double a = area(d);
    System.out.println("Circle area is: " + a);
}

Returns are Final

Once you return something, the method immediately stops executing. This is because it returns you back to the main method.

import java.util.Scanner;

public static double diameter(double radius) {
    double diameter = radius*2;
    return diameter;
}

public static double area(double d) {
    double area = d*Math.PI;
    return area;
    System.out.println("Hello!");
}

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    double r = in.nextDouble();
    double d = diameter(r);
    double a = area(d);
}

Composing Methods

You can call methods within other methods and use their returned values. You are already doing this in main, but you can do it in other methods, too.

import java.util.Scanner;

public static double diameter(double radius) {
    double diameter = radius*2;
    return diameter;
}

public static double area(double radius) {
    double area = diameter(radius)*Math.PI;
    return area;
    System.out.println("Hello!");
}

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    double r = in.nextDouble();
    double a = area(r);
}

In-Class Exercise

The quadratic formula is a simple way to find the roots of a polynomial function of the form

\[f(x) = ax^2 + bx + c\]

That is, if we set f(x) = 0 and solve the equation for x, we get:

\[x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}\]

The part of the quadratic formula under the radical (square root sign) is called the discriminant.

Write a program with two methods called discriminant() and quadratic().

The method discriminant() should take in three arguments, a, b, and c and return the discriminant (b^2 - 4ac).

The method quadratic() should take in three arguments, a, b, and c, call the discriminant() method somewhere in its body, and return the positive root of the polynomial with those coefficients.

As an example, the inputs a=1, b=-2, c=-8 should return 4.0.

Hints:

Recall that you can square a number by simply multiplying it by itself
Remember that to do a square root operation, you will have to import the Math module.
Right now, you don’t have to worry about equations with imaginary roots (where the discriminant is negative). Just make sure it works on the example input.