Java parser to generate custom AST

[1001mei]

Background

The java parser library currently being used returns a concrete syntax tree (CST). The process of converting it into an abstract syntax tree (AST) takes much effort and is complicated by the fact that it does not following Java Language Specification (JLS) faithfully. For example, the precedence of * over + in the expression 1 + 2 * 3 is not reflected in the CST output.

In an attempt to get rid of above hassles, this PR uses a parser generator pegjs to generate a Java parser that can return a custom AST. The references used are as follows:

• Java grammar
• pegjs library documentation
• Sample Java PEG grammar (it is kind of outdated but useful as a reference
• Sample PEG grammar from pegjs github

Work Done

• Support parsing for simple class and method declaration
• Support parsing for arithmetic expressions
• Write more tests to check for correctness

[1001mei]

Seems like we will be continue using java-parser library, so will close this PR and parser branch will be used for my own exploration.