CS453 Intro and PA1 1CS453 Lecture Exception Handling 1Plan for Today AST, CallGraph, and dynamic function stack– example– when do we know what? Control Flow– due to if stmts and while loops– due to returns and exceptions Exceptions in Java– try and catch blocks– finally blocks Exceptions in MiniJava compilerCS453 Lecture Exception Handling 2Example Programpublic static void main(String args[]) !{ foo(); int retval; retval = goo(); if (retval==ERROR) System.out.println(“goo ERROR in main”);}void foo() { int retval; retval = goo(); if (retval==ERROR) System.out.println(“goo ERROR in foo”);}int goo() { if (random()>42) return ERROR; return 0;}CS453 Lecture Exception Handling 3Example Program with Exceptionspublic static void main(String args[]) !{ try { foo(); goo(); } catch (Exception e) { System.out.println(“Caught in main”); }}void foo() { try { goo(); } catch (Exception e) { System.out.println(“Caught in foo”); }}void goo() throws SomeException { if (random()>42) throw new SomeException();}CS453 Lecture Exception Handling 4Exceptions in Javapublic static void main(String args[]) !{ PrintWriter out = null; try { out = new PrintWriter(new FileWriter(“b.txt”); } catch (IOException e) { System.err.println(“Caught IOException: “ + e.getMessage()); } finally { if (out != null) { System.out.println(“Closing PrintWriter”); out.close(); } } throw new MyException();}Picture from Sun “The Java Tutorials”CS453 Intro and PA1 2CS453 Lecture Exception Handling 5Exception usage in the MiniJava compiler try block– is it necessary?– applying the CheckTypes switch/visitor to the AST could result in aSemanticException being throwntry { // Create a lexer instance.! Lexer lexer = new Lexer(new PushbackReader( new BufferedReader(new FileReader(filename)), 1024)); ... // type checking! ast.apply(new CheckTypes(globalST, linesToNodes));! ! ! } catch(exceptions.SemanticException e) {! System.err.println(e.getMessage());! System.exit(1);! } catch (Exception e) {! e.printStackTrace();! System.exit(1);! } CS453 Lecture Exception Handling 6But why isn’t SemanticException a checked exception? Java rules (paraphrased)– any exception that is not an Error or RuntimeException subclass must becaught– if a method that doesn’t catch a checked exception it throws then it mustindicate with a “throws” clause that it could throw that exceptionPicture from Sun “The Java
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