Introduction

Final modifier can be applied to variables, methods and classes.
Marking a:

1. Variable final means the value cannot be changed after it is assigned.
2. Method or a class means it cannot be overridden (for methods) or extended (for classes).

Declaring final local variables

For final variables there are several aspects to consider.

We do not need to assign a value to the final variable when we declare it. What we have to assure is the a value has been assigned to it before this final variable is used. We will get a compilation error in case we don’t follow this rule. Example which illustrates this:

private void printZooInfo(boolean isWeekend) {
    final int giraffe = 5;
    final long lemur;
    if (isWeekend) lemur = 5;
    giraffe = 3; // DOES NOT COMPILE   
    System.out.println(giraffe+" "+lemur); // DOES NOT COMPILE
}

Here we have two compilation errors:

1. The giraffe variable has an assigned value so we can’t assign a new value because it has been declared as final. We will get a compilation error.
2. When attempting to use lemur variable we will get a compilation error. If condition isWeekend is false we can’t assign the value to lemur so we will the error the error compilation because a local variable to has to be declared and assigned before using it (despite the fact of being declared as final or not).

When we mark a variable as final it does not mean that the object associated with it cannot be modified. Example to illustrate this:

final StringBuilder cobra = new StringBuilder();
cobra.append("Hssssss");
cobra.append("Hssssss!!!");

We have declared the variable as constant but the content of the class can be modified.

Adding final to Instance and static variables

Instance and static class variables can be marked as final too.

When we mark as final a:

1. Instance variable which it means that it must be assigned a value when it is declared or when the object is instantiated (Remember: We can only assign once, like Local Variables). Example to illustrate this:

   public class PolarBear {
   final int age = 10;
   final int fishEaten;
   final String name;
   
   { fishEaten = 10; }
   
   public PolarBear() {
     name = "Robert";
   }
   public PolarBear(int height) {
     this();
   }
   }

   Does this code compile? Yes. Everything. Exercise: Explain why.

2. Static variable which it means they have to use static initializers instead of instance initializers. Example to illustrate this:

   public class Panda {
   // We assign a value when we declare the final variable
   final static String name = "Ronda";
   static final int bamboo;
   static final double height; // DOES NOT COMPILE - Why? Because we do not have assign any value to height variable  
   // It will work because we are initializing a final static variable through an static initializer
   static { bamboo = 5;}}

Writing final methods

Methods marked as final cannot be overriden by a subclass. This avoids polymorphic behavior and always ensures that it is always called the same version method. Be aware because a method can have abstract or final modifier but not both at the same time.

When we combine inheritance with final methods we always get an error compilation.

We cannot declare a method final and abstract at the same time. It is not allowed by the compiler and of course we will get a compilation error.
Example to illustrate this:

abstract class ZooKeeper {   
    public abstract final void openZoo(); // DOES NOT COMPILE
}

Marking Classes final

A final class is one class that cannot be extended. In fact we will get a compilation error if we tried. Example to illustrate this:

public final class Reptile {}
public class Snake extends Reptile {} // DOES NOT COMPILE

We cannot use abstract and final modifiers at the same time.

public abstract final class Eagle {} // DOES NOT COMPILE

It also happens the same for interfaces.

public final interface Hawk {} // DOES NOT COMPILE

We will get a compilation error in both cases.