Kotlin Language Features Related to Null Handling

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Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,...
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A Code Analyzer for a Subset of Java Language (J-) Using ANTLR - PART 2

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As we know, semantic analysis of a compiler includes checking the types of expressions.
To do that, we have to maintain a Symbol Table which maps variable names to their types and locations. There maybe many variables with the same name which are in different scopes. This is legal in most languages and we have to handle this situation.

We have to know when following scopes are entered in:
- class scope
- method scope
- no inner block scopes (my language does not support this currently)

We will do type-checking in two phases. First we'll populate the Symbol Table and it will contain the symbols with their scopes. Then, we'll check the types of symbols that are subject to addition operation by querying the symbol table.

ANTLR allows us to inject code in different places of the generated parser. We need to know when a class/method scope is entered and the end of any scope is reached. Below is some grammar code from the project:


classDecl
@init {visitor.beforeClass(); System.out.println("class classScope started");}
@after {visitor.afterClass(); System.out.println("class classScope ended");}
: ^(CLASS ID varDecl* methodDecl*) {visitor.onClassName($ID.text); System.out.println("class is: " + $ID.text);}
;

methodDecl
@init {visitor.beforeMethod(); System.out.println("method classScope started");}
@after {visitor.afterMethod(); System.out.println("method classScope ended");}
: ^(METHOD type ID formalParameter* block) {visitor.onMethodName($ID.text);}
;

We inject a visitor to the parser because we don't want to clutter the tree grammar code with type-checking details.

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