In the same way that Java allows us to define inheritance relationships between classes, Java also provides us with a mechanism to prevent a class from being extended, to prevent a method from being overridden, or for a variable value from changing, by using the final keyword.
The effect of the final keyword depends on the context where it is being used, as follows:
final classes
We can use final in the class declaration to indicate that we don’t want to allow a class to have subclasses.
For example, let’s assume we’re building a library that holds the data of a dictionary and we want to model each entry we have, and we define a DictionaryEntry class for this.
Let’s also assume we don’t want users of our library to subclass it:
public final class DictionaryEntry {
private String word;
private String definition;
public DictionaryEntry(String word, String definition) {
this.word = word;
this.definition = definition;
}
public void printEntry() {
System.out.println(word + ": " + definition);
}
}
To clarify, a final class can still extend from another class, but no class can extend from it.
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final methods
In a similar way, if we want to allow our class to be extended but want to avoid certain methods from being overridden, you can mark those methods as final.
For example, maybe all we’re concerned about is that the behaviour of our printEntry method isn’t changed:
public class DictionaryEntry {
private String word;
private String definition;
public DictionaryEntry(String word, String definition) {
this.word = word;
this.definition = definition;
}
public final void printEntry() {
System.out.println(word + ": " + definition);
}
}In this case, users of our library can extend our DictionaryEntry class, but we are providing certain restrictions about the methods they can override.
final fields
A different behavior of the final keyword is when it is used with fields or variables in general. In this case, the final keyword indicates that the value of the variable can only be set once. If we try to change the value of a final variable, the compiler will show an error indicating that a value for that variable may have already been assigned.
In our DictionaryEntry class, it might not make sense to allow the word or definition to change after the object has been created, so we can mark those 2 fields as final:
private final String word;
private final String definition;
What happens if you don’t assign a value to word or definition in the constructor? Do you get a compilation error?
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final classes and fields are a starting point to Immutable Objects which we won’t cover in this course.
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Static final fields
A common pattern in Java is to declare constant values in an application as static final variables. These constants, by convention, are named in upper case.
For example:
public class DictionaryEntry {
private static final String WORD_SEPARATOR = ": ";
private final String word;
private final String definition;
public DictionaryEntry(String word, String definition) {
this.word = word;
this.definition = definition;
}
public final void printEntry() {
System.out.println(word + WORD_SEPARATOR + definition);
}
}