Chapter 11  Classes

Whenever you define a new class, you also create a new type with the same name. So way back in Section 1.4, when we defined the class Hello, we created a type named Hello. We didn’t declare any variables with type Hello, and we didn’t use new to create a Hello object. It wouldn’t have done much if we had – but we could have!

In this chapter, we will define classes that represent useful object types. We will also clarify the difference between classes and objects. Here are the most important ideas:

• Defining a class creates a new object type with the same name.

• Every object belongs to some object type; that is, it is an instance of some class.
• A class definition is like a template for objects: it specifies what attributes the objects have and what methods can operate on them.

• The new operator instantiates objects, that is, it creates new instances of a class.
• Think of a class like a blueprint for a house: you can use the same blueprint to build any number of houses.
• The methods that operate on an object type are defined in the class for that object.

11.1  The Time class

One common reason to define a new class is to encapsulate related data in an object that can be treated as a single unit. That way, we can use objects as parameters and return values, rather than passing and returning multiple values. This design principle is called data encapsulation.

We have already seen two types that encapsulate data in this way: Point and Rectangle. Another example, which we will implement ourselves, is Time, which represents a time of day. The data encapsulated in a Time object are an hour, a minute, and a number of seconds. Because every Time object contains these data, we define attributes to hold them.

Attributes are also called instance variables, because each instance has its own variables (as opposed to class variables, coming up in Section 12.3).

The first step is to decide what type each variable should be. It seems clear that hour and minute should be integers. Just to keep things interesting, let’s make second a double.

Instance variables are declared at the beginning of the class definition, outside of any method. By itself, this code fragment is a legal class definition:

The Time class is public, which means that it can be used in other classes. But the instance variables are private, which means they can only be accessed from inside the Time class. If you try to read or write them from another class, you will get a compiler error.

Private instance variables help keep classes isolated from each other so that changes in one class won’t require changes in other classes. It also simplifies what other programmers need to understand in order to use your classes. This kind of isolation is called information hiding.

11.2  Constructors

After declaring the instance variables, the next step is to define a constructor, which is a special method that initializes the instance variables. The syntax for constructors is similar to that of other methods, except:

• The name of the constructor is the same as the name of the class.
• Constructors have no return type (and no return value).
• The keyword static is omitted.

Here is an example constructor for the Time class:

This constructor does not take any arguments. Each line initializes an instance variable to zero (which in this example means midnight).

The name this is a keyword that refers to the object we are creating. You can use this the same way you use the name of any other object. For example, you can read and write the instance variables of this, and you can pass this as an argument to other methods. But you do not declare this, and you can’t make an assignment to it.

A common error when writing constructors is to put a return statement at the end. Like void methods, constructors do not return values.

To create a Time object, you must use the new operator:

When you invoke new, Java creates the object and calls your constructor to initialize the instance variables. When the constructor is done, new returns a reference to the new object. In this example, the reference gets assigned to the variable time, which has type Time. Figure 11.1 shows the result.

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Figure 11.1: State diagram of a Time object.

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Beginners sometimes make the mistake of invoking new inside the constructor. You don’t have to, and you shouldn’t. In this example, invoking new Time() in the constructor causes an infinite recursion:

11.3  More constructors

Like other methods, constructors can be overloaded, which means you can provide multiple constructors with different parameters. Java knows which constructor to invoke by matching the arguments you provide with the parameters of the constructors.

It is common to provide a constructor that takes no arguments, like the previous one, and a “value constructor”, like this one:

All this constructor does is copy values from the parameters to the instance variables. In this example, the names and types of the parameters are the same as the instance variables. As a result, the parameters shadow (or hide) the instance variables, so the keyword this is necessary to tell them apart. Parameters don’t have to use the same names, but that’s a common style.

To invoke this second constructor, you have to provide arguments after the new operator. This example creates a Time object that represents a fraction of a second before noon:

Overloading constructors provides the flexibility to create an object first and then fill in the attributes, or collect all the information before creating the object itself.

Once you get the hang of it, writing constructors gets boring. You can write them quickly just by looking at the list of instance variables. In fact, some IDEs can generate them for you.

Pulling it all together, here is the complete class definition so far:

11.4  Getters and setters

Recall that the instance variables of Time are private. We can access them from within the Time class, but if we try to access them from another class, the compiler generates an error.

For example, here’s a new class called TimeClient, because a class that uses objects defined in another class is called a client:

If you try to compile this code, you will get a message like hour has private access in Time. There are three ways to solve this problem:

• We could make the instance variables public.
• We could provide methods to access the instance variables.
• We could decide that it’s not a problem, and refuse to let other classes access the instance variables.

The first choice is appealing because it’s simple. But the problem is that when Class A accesses the instance variables of Class B directly, A becomes “dependent” on B. If anything in B changes later, it is likely that A will have to change, too.

But if A only uses methods to interact with B, A and B are “independent”, which means that we can make changes in B without affecting A (as long as we don’t change the method signatures).

So if we decide that TimeClient should be able to read the instance variables of Time, we can provide methods to do it:

Methods like these are formally called “accessors”, but more commonly referred to as getters. By convention, the method that gets a variable named something is called getSomething.

If we decide that TimeClient should also be able to modify the instance variables of Time, we can provide methods to do that, too:

These methods are formally called “mutators”, but more commonly known as setters. The naming convention is similar; the method that sets something is usually called setSomething.

Writing getters and setters can get boring, but many IDEs can generate them for you based on the instance variables.

11.5  Displaying objects

If you create a Time object and display it with println:

The output will look something like:

When Java displays the value of an object type, it displays the name of the type and the address of the object (in hexadecimal). This address can be useful for debugging, if you want to keep track of individual objects.

To display Time objects in a way that is more meaningful to users, you could write a method to display the hour, minute, and second. Using printTime in Section 4.6 as a starting point, we could write:

The output of this method, given the time object from the previous section, would be 11:59:59.9. We can use printf to write it more concisely:

As a reminder, you need to use \%d with integers and \%f with floating-point numbers. The 02 option means “total width 2, with leading zeros if necessary”, and the 04.1 option means “total width 4, one digit after the decimal point, leading zeros if necessary”.

11.6  The toString method

Every object type has a method called toString that returns a string representation of the object. When you display an object using print or println, Java invokes the object’s toString method.

By default it simply displays the type of the object and its address, but you can override this behavior by providing your own toString method. For example, here is a toString method for Time:

The definition does not have the keyword static, because it is not a static method. It is an instance method, so called because when you invoke it, you invoke it on an instance of the class (Time in this case). Instance methods are sometimes called “non-static”; you might see this term in an error message.

The body of the method is similar to printTime in the previous section, with two changes:

• Inside the method, we use this to refer to the current instance; that is, the object the method is invoked on.
• Instead of printf, it uses String.format, which returns a formatted String rather than displaying it.

Now you can call toString directly:

Or you can invoke it indirectly through println:

In this example, this in toString refers to the same object as time. The output is 11:59:59.9.

11.7  The equals method

We have seen two ways to check whether values are equal: the == operator and the equals method. With objects you can use either one, but they are not the same.

• The == operator checks whether objects are identical; that is, whether they are the same object.

• The equals method checks whether they are equivalent; that is, whether they have the same value.

The definition of identity is always the same, so the == operator always does the same thing. But the definition of equivalence is different for different objects, so objects can define their own equals methods.

Consider the following variables:

Figure 11.2 is a state diagram that shows these variables and their values.

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Figure 11.2: State diagram of three Time variables.

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The assignment operator copies references, so time1 and time2 refer to the same object. Because they are identical, time1 == time2 is true.

But time1 and time3 refer to different objects. Because they are not identical, time1 == time3 is false.

By default, the equals method does the same thing as ==. For Time objects, that’s probably not what we want. For example, time1 and time3 represent the same time of day, so we should consider them equivalent.

We can provide an equals method that implements this notion of equivalence:

equals is an instance method, so it uses this to refer to the current object and it doesn’t have the keyword static. We can invoke equals as follows:

Inside the equals method, this refers to the same object as time1, and that refers to the same object as time3. Since their instance variables are equal, the result is true.

Many objects use a similar notion of equivalence; that is, two objects are equivalent if their instance variables are equal. But other definitions are possible.

11.8  Adding times

Suppose you are going to a movie that starts at 18:50 (or 6:50 PM), and the running time is 2 hours 16 minutes. What time does the movie end?

We’ll use Time objects to figure it out. Here are two ways we could “add” Time objects:

• We could write a static method that takes the two Time objects as parameters.
• We could write an instance method that gets invoked on one object and takes the other as a parameter.

To demonstrate the difference, we’ll do both. Here is a rough draft that uses the static approach:

And here’s how we would invoke the static method:

On the other hand, here’s what it looks like as an instance method:

The changes are:

• We removed the keyword static.
• We removed the first parameter.
• We replaced t1 with this.

Optionally, you could replace t2 with that. Unlike this, that is not a keyword; it’s just a slightly clever variable name.

And here’s how we would invoke the instance method:

That’s all there is to it. Static methods and instance methods do the same thing, and you can convert from one to the other with just a few changes.

There’s only one problem: the addition code itself is not correct. For this example, it returns 20:66, which is not a valid time. If second exceeds 59, we have to “carry” into the minutes column, and if minute exceeds 59, we have to carry into hour.

Here is a better version of add:

It’s still possible that hour may exceed 23, but there’s no days attribute to carry into. In that case, sum.hour -= 24 would yield the correct result.

11.9  Pure methods and modifiers

This implementation of add does not modify either of the parameters. Instead, it creates and returns a new Time object. As an alternative, we could have written a method like this:

The increment method modifies an existing Time object. It doesn’t create a new one, and it doesn’t return anything.

In contrast, methods like add are called pure because:

• They don’t modify the parameters.
• They don’t have any other “side effects”, like printing.
• The return value only depends on the parameters, not on any other state.

Methods like increment, which breaks the first rule, are sometimes called modifiers. They are usually void methods, but sometimes they return a reference to the object they modify.

Modifiers can be more efficient because they don’t create new objects. But they can also be error-prone. When objects are aliased, the effects of modifiers can be confusing.

To make a class immutable, like String, you can provide getters but no setters and pure methods but no modifiers. Immutable objects can be more difficult to work with, at first, but they can save you from long hours of debugging.

11.10  Vocabulary

class:

Previously, we defined a class as a collection of related methods. Now you know that a class is also a template for a new type of object.

instance:

A member of a class. Every object is an instance of some class.

instantiate:

Create a new instance of a class in the computer’s memory.

data encapsulation:

A technique for bundling multiple named variables into a single object.

instance variable:

An attribute of an object; a non-static variable defined at the class level.

information hiding:

The practice of making instance variables private to limit dependencies between classes.

constructor:

A special method that initializes the instance variables of a newly-constructed object.

shadowing:

Defining a local variable or parameter with the same name and type as an instance variable.

client:

A class that uses objects defined in another class.

getter:

A method that returns the value of an instance variable.

setter:

A method that assigns a value to an instance variable.

override:

Replacing a default implementation of a method, such as toString.

instance method:

A non-static method that has access to this and the instance variables.

identical:

Two values that are the same; in the case of objects, two variables that refer to the same object.

equivalent:

Two objects that are “equal” but not necessarily identical, as defined by the equals method.

pure method:

A static method that depends only on its parameters and no other data.

modifier method:

A method that changes the state (instance variables) of an object.

11.11  Exercises

The code for this chapter is in the ch11 directory of ThinkJavaCode. See page ?? for instructions on how to download the repository. Before you start the exercises, we recommend that you compile and run the examples.

At this point you know enough to read Appendix B, which is about simple 2D graphics and animations. During the next few chapters, you should take a detour to read this appendix and work through the exercises.