Abstract Syntax Tree (AST)

What is AST?

It is a tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code.

Application in compilers

Abstract syntax trees are data structures widely used in compilers to represent the structure of program code. An AST is usually the result of the syntax analysis phase of a compiler. It often serves as an intermediate representation of the program through several stages that the compiler requires, and has a strong impact on the final output of the compiler.

For Example, This is a Source Code.

class GFG {
    public static void main (String[] args) {
        System.out.println("Hello World!");
    }
}

AST of above source code:

CLASS_DEF -> CLASS_DEF [1:0]
|--MODIFIERS -> MODIFIERS [1:0]
|   `--LITERAL_PUBLIC -> public [1:0]
|--LITERAL_CLASS -> class [1:7]
|--IDENT -> GFG [1:13]
`--OBJBLOCK -> OBJBLOCK [1:17]
    |--LCURLY -> { [1:17]
    |--METHOD_DEF -> METHOD_DEF [2:4]
    |   |--MODIFIERS -> MODIFIERS [2:4]
    |   |   |--LITERAL_PUBLIC -> public [2:4]
    |   |   `--LITERAL_STATIC -> static [2:11]
    |   |--TYPE -> TYPE [2:18]
    |   |   `--LITERAL_VOID -> void [2:18]
    |   |--IDENT -> main [2:23]
    |   |--LPAREN -> ( [2:27]
    |   |--PARAMETERS -> PARAMETERS [2:34]
    |   |   `--PARAMETER_DEF -> PARAMETER_DEF [2:34]
    |   |       |--MODIFIERS -> MODIFIERS [2:34]
    |   |       |--TYPE -> TYPE [2:34]
    |   |       |   `--ARRAY_DECLARATOR -> [ [2:34]
    |   |       |       |--IDENT -> String [2:28]
    |   |       |       `--RBRACK -> ] [2:35]
    |   |       `--IDENT -> args [2:37]
    |   |--RPAREN -> ) [2:41]
    |   `--SLIST -> { [2:43]
    |       |--EXPR -> EXPR [3:26]
    |       |   `--METHOD_CALL -> ( [3:26]
    |       |       |--DOT -> . [3:18]
    |       |       |   |--DOT -> . [3:14]
    |       |       |   |   |--IDENT -> System [3:8]
    |       |       |   |   `--IDENT -> out [3:15]
    |       |       |   `--IDENT -> println [3:19]
    |       |       |--ELIST -> ELIST [3:27]
    |       |       |   `--EXPR -> EXPR [3:27]
    |       |       |       `--STRING_LITERAL -> "Hello World!" [3:27]
    |       |       `--RPAREN -> ) [3:41]
    |       |--SEMI -> ; [3:42]
    |       `--RCURLY -> } [4:4]
    `--RCURLY -> } [5:0]