How to create a new map using the constructor?

Question

I want to do two things

• Create a private instance variable that is a map

• To create an empty instance in my constructor, which embeds the map and assigns it to the previous private instance variable.

The private instance I have is

private final Map<Character, SortedSet<String>> thesaurus = new HashMap <Character, SortedSet<String>>(); 

but how to create an instance variable in a constructor that will refer to the thesaurus of a private variable when creating constructors.

for example

 public class Book{ private final Map<Character, SortedSet<String>> thesaurus = new HashMap <Character, SortedSet<String>>(); public Book(){ super(); /* What do i put here as an empty instance * variable that implements a map and how * do i assign it to thesaurus? */ } 

Answer 1

It is not clear what you are asking, but here are a few points:

• You cannot declare an instance variable in a constructor; you need to declare it as a type member (i.e. as a field).
• You can assign values ​​to already declared instance variables in the constructor.
• You do not need to assign values ​​to instance variables in the constructor; You can do it in ads.

---

When you write something like this:

 public class Book{ private final Map<Character, SortedSet<String>> thesaurus = new HashMap <Character, SortedSet<String>>(); //... } 

You then declared thesaurus as an instance variable of class Book , and you also initialized its value as new HashMap . Since this field is final , you can no longer set its value as something else (disallowing reflection-based attacks).

You can, if you want to choose this, move the initialization to the constructor. You can do this even if the field is final (depending on specific assignment rules).

 public class Book{ private final Map<Character, SortedSet<String>> thesaurus; public class Book { thesaurus = new HashMap <Character, SortedSet<String>>(); } //... } 

Something similar happens sometimes when, for example, creating an initial value can raise a checked exception and therefore should be placed in a try-catch .

Another option is to initialize the fields in the instance initializer block:

  private final Map<Character, SortedSet<String>> thesaurus; { thesaurus = new HashMap <Character, SortedSet<String>>(); } 

And another option is to reorganize the mentioned instance initializer block into a helper method:

  private final Map<Character, SortedSet<String>> thesaurus = emptyMap(); private static Map<Character, Sorted<String>> emptyMap() { return new HashMap <Character, SortedSet<String>>(); } 

References

• JLS 8.3 Field Declarations
  • 8.3.1.2 final Fields
  • 8.3.2 Initializing Fields
• JLS 8.6 Instance Initializers
• JLS 16 Specific Purpose

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Answer 2

You are already initializing your thesaurus variable with a map. You can move it to the constructor, for example:

 public class Book { private final Map<Character, SortedSet<String>> thesaurus; public Book(){ this.thesaurus = new HashMap <Character, SortedSet<String>>(); } } 

No need to change. In either case, the instance field will be initialized each time an instance is created. Also note that you don't need super() , as it is implicit.