thinking in java 第四版答案

// generics/Ex29.java // TIJ4 Chapter Generics, Exercise 29, page 694 /* Create a generic method that takes as an argument a Holder<List<?>>. * Determine what methods you can and can't call for the Holder and for * the List. Repeat for an argument of List<Holder<?>>. */ import java.util.*; import static org.greggordon.tools.Print.*; public class Ex29 { static void f1(Holder<List<?>> holder) { println("Calling methods for the Holder: "); println("holder: " + holder); println("holder.get(): " + holder.get()); println("Calling holder.set(Arrays.asList(1,2,3)"); holder.set(Arrays.asList(1,2,3)); println("holder.get(): " + holder.get()); int[] ia = {1,2,3}; println("int[] ia = {1,2,3}"); println("holder.equals(ia): " + holder.equals(ia)); List iaList = Arrays.asList(ia); println("List iaList = Arrays.asList(ia)"); println("holder.equals(iaList): " + holder.equals(iaList)); List<Integer> l = new ArrayList<Integer>(); for(int i = 1; i < 4; i++) l.add(i); println("l = List<Integer>() and add 1,2,3"); println("holder.equals(l): " + holder.equals(l)); println(); println("Calling methods for the List: "); println("holder.get(): " + holder.get()); println("holder.get().getClass(): " + holder.get().getClass()); // Incompatible types: // List<? extends Integer> list = holder.get(); // OK, but can't add anything: List<?> list = holder.get(); println("list: " + list); // list.add(new Object()); List list1 = holder.get(); // OK, but: // list1.add(new Object()); // Warning: unchecked call to add(E) and // UnsupportedOperationException at runtime println("list1.getClass(): " + list1.getClass()); println("list.equals(list1): " + list.equals(list1)); // list.clear(); // runtime UnsupportedOperationException println("list.contains(1): " + list.contains(1)); Collection<Integer> c = new HashSet<Integer>(); for(int i = 1; i < 4; i++) c.add(i); println("list.containsAll(c): " + list.containsAll(c)); println("list.equals(c): " + list.equals(c)); println("list = " + list); println("c = " + c); println("c.getClass(): " + c.getClass()); // c is not a List println("list.get(0) = " + list.get(0)); println("list.hashCode() = " + list.hashCode()); println("list.indexOf(2) = " + list.indexOf(2)); ListIterator li = list.listIterator(); println("After ListIterator li = list.listIterator():"); println("li.next() = " + li.next()); // runtime errors: UnsupportedOperationException: // list.remove(0); // list.removeAll(c); // list.retainAll(c1); // list.set(1,4); // compile error: set(int, ?) in list<?> cannot // be applied to (int,int) println("list.size() = " + list.size()); println("list.subList(1,2) = " + list.subList(1,2)); Object[] oa = list.toArray(); println("After Object[] oa = list.toArray():"); print("oa = "); for(Object o : oa) print(o + " "); println(); // Error: runtime error ArrayStoreException: // Double[] da = list.toArray(new Double[3]); Number[] na = list.toArray(new Number[3]); println("After Number[] na = list.toArray(new Number[3]):"); print("na = "); for(Number n : na) print(n + " "); println(); } static void f2(List<Holder<?>> list) { println("Calling methods for the List: "); println("list = " + list); println("Adding Integer, String, Double"); list.add(new Holder<Integer>(1)); list.add(new Holder<String>("two")); list.add(new Holder<Double>(3.0)); println("list = " + list); println("Elements of list: "); for(Holder h : list) println("h.get() = " + h.get()); println("Adding Float"); list.add(3, new Holder<Float>(4.0f)); println("list = " + list); println("Elements of list: "); for(Holder h : list) println("h.get() = " + h.get()); println("list.clear()"); list.clear(); println("list = " + list); Collection<Holder<?>> c = new ArrayList<Holder<?>>(); c.add(new Holder<String>("one")); c.add(new Holder<Float>(2.0f)); c.add(new Holder<Double>(3.0)); list.addAll(c); println("Adding String, Float, Double"); println("list = " + list); println("Elements of list: "); for(Holder h : list) println("h.get() = " + h.get().getClass().getSimpleName() + " " + h.get()); list.add(3, new Holder<String>("four")); println("Elements of list: "); for(Holder h : list) println("h.get() = " + h.get().getClass().getSimpleName() + " " + h.get()); Object listClone = ((ArrayList)list).clone(); println("((ArrayList)list).clone() = " + listClone); println("((ArrayList)list).clone().getClass() = " + ((ArrayList)list).clone().getClass()); // Appears to be an ArrayList, but cannot assign: // ArrayList alistClone = ((ArrayList)list).clone(); // found Object println("Replacing element with h4, new Holder<Integer>(4)"); Holder<Integer> h4 = new Holder<Integer>(4); list.set(3, h4); println("Elements of list Holders: "); for(Holder h : list) println(h.get().getClass().getSimpleName() + " " + h.get()); println("list.contains(list.set(3, h4): " + list.contains(list.set(3, h4))); println("list.contains(4): " + list.contains(4)); println("list.contains(h4): " + list.contains(h4)); println("Adding null member to list"); list.add(null); println("list.contains(null): " + list.contains(null)); println("list.get(0).get(): " + list.get(0).get()); println("list.indexOf(h4) = " + list.indexOf(h4)); println("list.indexOf(null) = " + list.indexOf(null)); println("list.isEmpty(): " + list.isEmpty()); println("list.lastIndexOf(null) = " + list.lastIndexOf(null)); println("Removing index 0"); list.remove(0); println("Elements of list Holders: "); for(Holder h : list) { if(h == null) println("null"); else println(h.get().getClass().getSimpleName() + " " + h.get()); } println("Removing null"); println("Elements of list Holders: "); for(Holder h : list) { if(h == null) println("null"); else println(h.get().getClass().getSimpleName() + " " + h.get()); } // Error: removeRange has protected access: // ((ArrayList)list).removeRange(0,2); println("list.size() = " + list.size()); Object[] oa = list.toArray(); print("list.toArray() = "); for(int i = 0; i < oa.length; i++) print(oa[i] + " "); println(); Holder[] ha = ((ArrayList<Holder<?>>)list).toArray(new Holder[4]); println("(ArrayList<Holder<?>>list).toArray(new Holder[4]) = "); for(int i = 0; i < ha.length; i++) print(ha[i] + " "); println(); println("Holder[4] Holders are holding: "); for(Holder h : ha) { if(h == null) println("null"); else println(h.get()); } println(); println("Calling methods for the Holder: "); println("list = " + list); print("Three Holders (one null) in list:"); for(int i = 0; i < list.size(); i++) { if(list.get(i) == null) println("null"); else println(list.get(i).getClass().getSimpleName()); } list.remove(3); println("Holders are holding:"); for(Holder h : list) println(h.get()); Holder<?> h1 = list.get(0); Holder<?> h2 = list.get(1); Holder<?> h3 = list.get(2); println(list.get(0).getClass() + ": " + list.get(0).get().getClass().getSimpleName() + ", " + list.get(0).get()); println(list.get(1).getClass() + ": " + list.get(1).get().getClass().getSimpleName() + ", " + list.get(1).get()); println(list.get(2).getClass() + ": " + list.get(2).get().getClass().getSimpleName() + ", " + list.get(2).get()); // Error: cannot set <?> to int or Object: // h3.set(5); // h2.set(new Object()); } public static void main(String[] args) { println("new Holder<List<?>> holder"); f1(new Holder<List<?>>()); println(); println("List<Holder<?>> list = new ArrayList<Holder<?>>();"); List<Holder<?>>