Final

We can ensure a field or variable does not change by declaring it with the keyword final.

final Variables

We can use final before a local variable to make sure it doesn't get assigned a different value.

public void method() {
  final int constantVar = 15;
  final int declaredFinal;
  // ...
  declaredFinal = 42;
}

Once it has been assigned, attempting to reassign the final variable results in a compiler error.

public void notCompilingMethod() {
  final int constantVar = 15;
  constantVar = 99;  // COMPILER ERROR
}

We can mark method parameters as final (since they're just local variables).

public void paramsFinal(final int id, final String name) {
  // ...
}

final Fields

We can mark static and instance fields as final - again, this means they cannot be reassigned new values.

public class AccountFinal {
  public static final int ROUTING_NUMBER = 123456789;
  private double balance;
  private final String accountId;

  AccountFinal(String accountId){
    this.accountId = accountId;
  }

  //...
}

There are three key things to remember about final fields.

• final fields do not get default values, like null or 0.0: you must explicitly initialize them in the class.
• static fields must be initialized by the time the class is done initializing.

• Each final field must be given a value at its declaration, or in a static initializer block.

  public class ClassWithFinal {
    private static final String[] arrayOfNames;
  
    static {
      arrayOfNames = new String[100];
    }
  }
  

  * Instance fields must be initialized by the time the constructor has finished.

• This means the field must be given a value at declaration, in an initializer block, or in each constructor.

Drill

Look at the class below and decide which statements cause compiler errors. How would you fix them? See the solution for answers.

public class BrokenFinalClass {
 private String name;
 private final int uniqueId;
   
 private static final int MAX_NUMBER;
   
 MAX_NUMBER = 10000;
   
 public BrokenFinalClass(int uniqueId, String name){
   this.uniqueId = uniqueId;
   this.name = name;
 }
   
 public BrokenFinalClass(String name) {
   this.name = name;
 }

 public void setUniqueId(int id) {
   uniqueId = id;
 }
}

(Solution: ObjectInitialization/com.example.objs2.solutions.BrokenFinalClass)

Practice Exercise

final static fields are named using capital letters and snake case, like MAX_NUMBER. This is a convention developers use to denote constant values.

final and References

A final reference cannot be reassigned.

However, this does not mean that the data inside the referenced object cannot change, just that the reference cannot be changed to point to a different object.

public class ClassWithFinalReference {
  private final String[] finalArr;

  // initialize and set some elements
  {
    finalArr = new String[5];
    finalArr[0] = "Zero";
    finalArr[1] = "One";
  }

  public void changeData(int index, String data) {
    // reassign element
    finalArr[index] = data;    // COMPILES AND WORKS
    // reassign reference
//  finalArr = new String[10]; // WILL NOT COMPILE

  }
}

We can change the elements of finalArr, but we cannot make the field finalArr point to a different object.

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