CompSci 100E40.1Java 5 New Featuresÿ Genericsÿ Enhanced for loopÿ Autoboxing/unboxingÿ Typesafe enumsÿ Other Varargs Static Import Metadata New classes and methods VM EnhancementsCompSci 100E40.2Genericsÿ Allows classes to store objects whose type is irrelevant to storing class, while allowing type-safe retrievalÿ E.g., Collectionÿ SyntaxArrayList<String> list = new ArrayList<String>();list.put(“hello”); // put() takes a StringIterator<String> iter = list.iterator();String s = iter.next(); // next() returns a Stringÿ Compare with earlier JavaArrayList list = new ArrayList();list.put(“hello”); // put() takes an ObjectIterator iter = list.iterator();String s = (String)iter.next();// next() returns an Object which must be cast to StringCompSci 100E40.3Generics in API Docsÿ In API documentation, generics are given a type alias, e.g., “E”: Alias is arbitrary, but stands for the same type throughout class definition Can be on more than one type using different aliasesÿ Examples Class ArrayList<E>o add(E o)o E get(int index) Interface Map<K,V>o V put(K key, V value)o V get(Object key)o Collection<V> values()CompSci 100E40.4Enhanced for Loopÿ Replaced iterators, indexingÿ Iterators and indexing are prone to bounds errors// throws ArrayIndexOutOfBoundsExceptionfor (int i = 0; i <= arr.length; i++){ System.out.println(arr[i]); }// what does this do?Iterator iter = list.iterator();while (iter.hasNext()) {if (!“stop”.equals(iter.next())) {System.out.println(iter.next());}}CompSci 100E40.5Looping in Java 5ÿ Java 5 introduces new language syntax for looping over arrays and collections using for (aka “For-Each” loop)ÿ Syntax:for (type var: collection){// do something withvar}ÿ Examples:void processArray(String[] arr) {for (String s: arr)System.out.println(s.toUpperCase());}// generics work with new for loop to simplify syntax!void processList(List<String> list) {for (String s: list)System.out.println(s);}CompSci 100E40.6Autoboxing/Unboxingÿ Java primitive types provided for performance, but mix poorly with objects:// compilation error!ArrayList list = new ArrayList();list.add(42);int x = (int) list.get(0);// Kludgey fix provided by original Java: ugh!list.add(new Integer(42));int x = ((Integer)list.get(0)).intValue()ÿ Java 5 automatically “boxes” primitive types in Object types asneeeded:Integer objInt;objInt = 42; // equivalent to objInt = new Integer(42);CompSci 100E40.7Autoboxing with Generics and For-Eachÿ Note again how the new Java 5 features work together:// old syntaxInteger sumInteger(List list) {int sum = 0;Iterator iter = list.iterator();while (iter.hasNext()) {Integer iobj = (Integer) iter.next();sum += iobj.intValue();}}return new Integer(sum);// new syntaxInteger sumIntegers(List<Integer> list) {int sum = 0;for (int x: list) sum+= x;// auto-unboxingelementsreturn sum; // autobox return value}CompSci 100E40.8New Features: Limitationsÿ Generics are not everywhere, yet consider list.toArray() returning Object[]ÿ Enhanced for loop on non-parameterized collections is still annoying (obviously using generics helps, but what if you are forced to use legacy code?) for (Object o: list) { String s = (String)o; ... }ÿ For loop doesn't give you a good way to loop over multiple collections in parallel: still must do:int[] arr1, arr2;for (int i; i < arr1.length; i++) {int x = arr1[i] + arr2[i];}CompSci 100E40.9New Features: Limitations (con't)ÿ Autoboxing doesn't carry over to arrays, or to converting arrays to lists and vice versa: can't do the following:int[] arr = new int[100];Integer[] arrInts = arr;List<Integer> list = new ArrayList<Integer>();list.addAll(arr);CompSci 100E40.10Typesafe Enumsÿ Enums are a safer alternative to constants Old way:public static final int GO = 0;public static final int STOP = 1;public static final int YIELD = 2;.... Consider code taking these values as a parameter:void process(int status) {if (status == GO) ...if (status == STOP) ...if (status == YIELD) ...else ... // what does status == 10 mean?CompSci 100E40.11The Enum Alternativeÿ Enums define a type, just like a class or primitive typeÿ Enums are not interchangeable with ints, impossible to get undefined valuesÿ Enums can be enumerated using forÿ String representations of enums actually mean somethingÿ Examples:public enum TrafficLight { GO, STOP, YIELD }public TrafficLight myLight = STOP;for (TrafficLight t: TrafficLight.values()) {System.out.print(t);System.out.print(“ “);}// output: GO STOP YIELDCompSci 100E40.12Other New Featuresÿ Java 5 has many other new features, including: Varargs – variable-size argument lists for methods Static Import – import constants, e.g. Math.PI Metadata – attach extra information about code New classes and methods – Queue, Scanner, printf, etc. VM
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