void parseMethodDeclaration() {

   ICompilationUnit unit = getCompilationUnitByName(unitName);
   ASTParser parser = ASTParser.newParser(AST.JLS4);
   parser.setKind(ASTParser.K_COMPILATION_UNIT);
   parser.setSource(unit);
   parser.setResolveBindings(true);

   CompilationUnit parse = (CompilationUnit) parser.createAST(null);

   parse.accept(new ASTVisitor() {

      @Override
      public boolean visit(MethodDeclaration node) {

          String unitName = node.resolveBinding().getDeclaringClass().getName();

          //if some conditions are satisfied
          addComment(unitName, node);

          return super.visit(node);
      }
   });
}

void addComment(String unitName, MethodDeclaration method){

    ICompilationUnit unit = getCompilationUnitByName(unitName);
    AST ast = method.getAST();
    ASTRewrite rewriter = ASTRewrite.create(ast);
    ListRewrite listRewrite = rewriter.getListRewrite(method.getBody(), 
        Block.STATEMENTS_PROPERTY);

    Statement placeHolder =(Statement)rewriter.createStringPlaceholder("//MyComment",
        ASTNode.EMPTY_STATEMENT);

    listRewrite.insertFirst(placeHolder, null);

    TextEdit edits = rewriter.rewriteAST();

    Document document = new Document(unit.getSource());
    edits.apply(document);
    unit.getBuffer().setContents(document.get());
}