KotlinCS 124 LogoJava

Static

public class Math {
public static final double PI = 3.141597;
public static int pow(int base, int exponent) {
assert exponent >= 0 : "No support for negative exponents";
int result = 1;
for (int i = 0; i < exponent; i++) {
result *= base;
}
return result;
}
}
System.out.println(Math.PI);
System.out.println(Math.pow(2, 3));

As we continue, we begin by describing a new design keyword: static. We’ll also introduce introduce one new piece of object syntax.

Warm Up Debugging Challenge
Warm Up Debugging Challenge

But first, let’s warm up with another graded debugging challenge!

Create a public class CountLetters providing a single static method countLetters. countLetters accepts an array of Strings and returns a Map from Strings to Integers. (You can reject null arguments using assert.)

The map should contain counts of the passed Strings based on their first letter. For example, provided the array {"test", "me", "testing"} your Map should be {"t": 2, "m": 1}. You should ignore empty Strings and not include any zero counts.

As a reminder, you can retrieve the first character of a String as a char using charAt. You may find substring more helpful. You may also want to examine the Map getOrDefault method. You can use any Map implementation in java.util.

Instructions

this
this

this is a keyword in Java that you can use in your instance methods. It always refers to the current instance that is executing the method.

So this:

public class Course {
private String number;
Course(String setNumber) {
number = setNumber;
}
}

is equivalent to this:

public class Course {
private String number;
// We are using this to distinguish between number, the instance variable, and number, the method parameter
Course(String number) {
this.number = number;
}
}

The example above is one use of this. However, we’ll usually just go the first route, and choose parameter names that don’t conflict with our instance variable names. This helps avoid mistakes. checkstyle will help you with this, complaining (as it does above) that your parameter “hides a field”.

However, there is one place where we will use this. Let’s go through it together:

public class Course {
private String number;
Course(String setNumber) {
number = setNumber;
}
}

static Methods
static Methods

The object methods we have been writing so far are known as instance methods:

public class Person {
private String name;
public Person(String setName) {
name = setName;
}
public String getName() {
return name;
}
}
Person prof = new Person("Geoff");
Person student = new Person("You");
System.out.println(prof.getName());
System.out.println(student.getName());

Even though they share the same implementation, you can think of each Person as having its own getName method that can access its instance variables, in this case name.

However, Java also allows us to create methods that are attached to the class, not to the instance. To do that, we use a new keyword: static. Let’s see how that works:

public class Person {
}

static v. Instance
static v. Instance

static class methods cannot access instance variables. Let’s look at why, and the differences between class and instance methods:

public class Person {
private String name;
public Person(String setName) {
name = setName;
}
// Can't access name without an instance!
public static String getName() {
return name;
}
}
Person student = new Person("You");
System.out.println(student.getName());

Uses for Static Methods
Uses for Static Methods

static class methods are not uncommon. Particularly when you have methods that don’t need an instance to function properly. Let’s look at one example of a class that consists almost entirely of static methods, Math:

Practice: Solve Static Adder

Created By: Geoffrey Challen
/ Version: 2020.9.0

Create a class named Math. Math should have one public static method named add. add should accept two int arguments and return their sum.

static Fields
static Fields

You can also use the static keyword on fields. One common use for this is to establish constants to make our code more readable and less error-prone.

To do this, we combine static with a keyword that we haven’t seen before, final. final creates a variable that cannot be changed after it has been set. We can combine static and final to create a symbolic constant. Let’s look at how:

public class Math {
}

Non-Final static Fields
Non-Final static Fields

Can you create modifiable—that is, non-final—static fields? Yes. However. This is extremely rare, and very easy to get wrong. We won’t test you on it, but view this walkthrough to see how this works:

public class Course {
}

Practice: Solve Toggler Object

Created By: Geoffrey Challen
/ Version: 2021.9.0

Define a public class named Toggler with a single public instance method named toggle that takes no parameters and returns a boolean. Toggler should also provide a single public constructor that accepts a boolean argument and sets the initial state of the Toggler instance.

Toggler maintains one piece of private state: the boolean. Calling toggle changes the boolean from true to false or false to true and returns the new (not the old) state of the boolean. So, for example:

Note that the internal state should be private.

Homework: Solve Stepper Object

Created By: Geoffrey Challen
/ Version: 2022.9.0

Define a class named Stepper with a single public instance method named next that takes no parameters and returns an int. Called multiple times, next returns a sequence of values separated by a step amount provided to the constructor. Stepper should also provide a single public constructor that accepts a int argument and sets the step amount.

Stepper maintains two pieces of private state: the step amount, and the current value, which always starts at 0. Calling next increments the current value by the step amount, but returns the previous value. So, for example:

Note that the internal state should be private.

More Practice

Need more practice? Head over to the practice page.