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Generic Java

Generic Java. Administrivia. ACM article from March, “Comments are More Important than Code” Check out http://acmqueue.com to find it Not required reading, but interesting Otherwise: keep hacking. The bane of casting. // Java 1.4 code littered with stuff like this: Map m=new HashMap();

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Generic Java

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  1. Generic Java

  2. Administrivia • ACM article from March, “Comments are More Important than Code” • Check out http://acmqueue.com to find it • Not required reading, but interesting • Otherwise: keep hacking

  3. The bane of casting • // Java 1.4 code littered with stuff like this: • Map m=new HashMap(); • m.put(”Hello”,new Integer(17)); • m.put(”world”,new Integer(-4)); • // later... • Integer i=(Integer)m.get(”world”);

  4. Bane of casting • Issue: Java 1.4 containers want to be generic -- hold any object type • Java has no way to express “container of any object type” • Punt by using Object as a proxy • Subtle conceptual problem: • Object doesn’t mean “any one specific type” • It means: anything that is a subclass of Object • Issue 1: Programmer has to track types by hand • Issue 2: Collection can have heterogenous types • Consequence: lots of casting and icky type checks

  5. Problem w/ Object way • Map m=new HashMap(); • m.put(”Hello”,new Integer(42)); • m.put(”Goodbye”,new Integer(-4)); • // later, someone evil/stupid comes in... • m.put(”Eris”,new LinkedList()); • // and you find out when... • for (Iterator i=m.keySet().iterator();i.hasNext();) { • Integer x=(Integer)m.get(i.next()); // BLAM! • }

  6. Problem w/ Object way • Map m=new HashMap(); • m.put(”Hello”,new Integer(42)); • m.put(”Goodbye”,new Integer(-4)); • // later, someone evil/stupid comes in... • m.put(”Eris”,new LinkedList()); • // and you find out when... • for (Iterator i=m.keySet().iterator();i.hasNext();) { • Object o=m.get(i.next()); • if (o instanceof Integer) { ... } // ick... • else { ... } // Duhhh... What now? • }

  7. Generics to the rescue (?) • New mechanism in Java 1.5: Generic types • List<String> l=new LinkedList<String>(); • Semantics: • “This list holds only Strings (possibly including subtypes -- i.e., subclasses -- of Strings)” • Makes life easier in a lot of ways

  8. The Generic way • Map<String,Integer> m=new • HashMap<String,Integer>(); • m.put(”Hello”,42); • m.put(”Goodbye”,-4); • // later, someone evil/stupid comes in... • m.put(”Eris”,new LinkedList()); • // Hah! Compile time error! Can’t do that!

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