Make non-nullable type in Java

Question

Is it possible to make non-nullable type in Java? Objects of this type shouldn't can be null. How?

Answer 1

It is a reasonably common practice to use an @NotNull annotation which is supported by some IDEs and maven plugins. In Java 8 you can write

@NotNull String text;
@NotNull List<@NotNull String> strings = ...;

This is not a language feature, but if you need this, it is available.

Note: there isn't a standard @NotNull annotation :( So the tools which support this allow you to configure which one(s) you want. I use the one which comes with IntelliJ. It gives you warnings in the editor with auto-fixes and add runtime checks for null arguments, return values and variables.

Note: IntelliJ is able to work out if a field is not nullable by usage as well.

Answer 2

I have recently come across the Checker Framework which has a checker for null.

@Nullable indicates a type that includes the null value. For example, the type Boolean is nullable: a variable of type Boolean always has one of the values TRUE, FALSE, or null.

@NonNull indicates a type that does not include the null value. The type boolean is non-null; a variable of type boolean always has one of the values true or false. The type @NonNull Boolean is also non-null: a variable of type @NonNull Boolean always has one of the values TRUE or FALSE — never null. Dereferencing an expression of non-null type can never cause a null pointer exception.

@PolyNull indicates qualifier polymorphism. For a description of @PolyNull, see Section 19.2.

@MonotonicNonNull indicates a reference that may be null, but if it ever becomes non-null, then it never becomes null again. This is appropriate for lazily-initialized fields, among other uses. When the variable is read, its type is treated as @Nullable, but when the variable is assigned, its type is treated as @NonNull.

I have not had time yet to try it out.

Answer 3

You can't do this in pure Java. A null in Java is a literal value of a reference to a particular object.

Since all objects in Java are reference types, not allowing a null does not make grammatical sense.

Answer 4

You cannot do that. It is a language feature. At most what you can do is put null checks.

Answer 5

You can't do it in pure java, but the annotation @nonnull will tell the compiler to test for possible paths in which the object is assigned a null value. It then throws a build error requiring you to fix the object having a null value.