Thinking in Java第四版Acess部分答案

Thinking in Java 4th Edition习题答案1.Access部分(NO.1) // access/AccessTest.java// TIJ4 Chapter Access, Exercise 5, page 227/* Create a class with public, private, protected and package-access fields and* method members. Create an object of this class and see what kind of compiler* messages you get when you try to access all the class members. Be aware that* classes in the same directory are part of the "default" package.*//* in same directory:* package access;** public class FourWays {* int a = 0;* public int b = 1;* protected int c = 2;* private int d = 3;* FourWays() { System.out.println("FourWays() constructor"); }* void showa() { System.out.println(a); }* public void showb() { System.out.println(b); }* protected void showc() { System.out.println(c); }* private void showd() { System.out.println(d); }* }*/package access; // run command java access.AccessTestpublic class AccessTest {public static void main(String[] args) {FourWays fw = new FourWays();fw.showa();fw.showb();fw.showc();fw.a = 10;fw.b = 20;fw.c = 30;fw.showa();fw.showb();fw.showc();//! fw.showd(); // private access, compiler can't touch}}(NO.2)// access/Collision.java// TIJ4 Chapter Access, Exercise 2, page 217/* Take the code fragments in this section and turn them into a program and* verify that collisions do occur.*/import net.mindview.simple.*;import java.util.*;public class Collision {public static void main(String[] args) {// Vector v = new Vector(); // ambiquous collisionnet.mindview.simple.Vector v1 = new net.mindview.simple.Vector();java.util.Vector v2 = new java.util.Vector();}}(NO.3)// access/ConnectionManager.java// TIJ4 Chapter Access, Exercise 8, page 233/* Following the form of the example Lunch.java, create a class called* ConnectionManager that manages a fixed array of Connection objects. The client * programmer must not be able to explicitly create Connection objects, but can* only get them via a static method in ConnectionManager. When ConnectionManager* runs out of objects, it returns a null reference. Test the classes in main(). */ package access;class Connection {private static int count = 0;private int i = 0;private Connection() { System.out.println("Connection()");}// Allow creation via static method:static Connection makeConnection() {count++;return new Connection();}public static int howMany() { return count; }public String toString() {return ("Connection " + count);}public class ConnectionManager {static int howManyLeft = 3;static Connection[] ca = new Connection[3];{for(Connection x : ca)x = Connection.makeConnection();}public static Connection getConnection() {if(howManyLeft > 0)return ca[--howManyLeft];else {System.out.println("No more connections");return null;}}public static void main(String[] args) {ConnectionManager cm = new ConnectionManager();System.out.println(cm.howManyLeft);cm.getConnection();System.out.println(howManyLeft);cm.getConnection();System.out.println(howManyLeft);cm.getConnection();System.out.println(cm.getConnection());System.out.println(howManyLeft);}}(NO.4)// access/cookie2/CookieMonster.java// TIJ4 Chapter Access, Exercise 4, page 227// Show that protected methods have package access but are not public. /* In directory Cookie2:* //access/cookie2/Cookie.java* //Creates a library* package access.cookie2;** public class Cookie {* public Cookie() {* System.out.println("Cookie contstructor");* }* protected void bite() { System.out.println("bite"); }*/package access.cookie2;public class CookieMonster {public static void main(String[] args) {Cookie x = new Cookie();x.bite(); // package access to protected method}}(NO.5)// access/debug/Debug.java// TIJ4 Chapter Access, Exercise 3, page 220/* Create two packages: debug and debugoff, containing an identical class with a * debug() method. The first version displays its String argument to the console, * the second does nothing. Use a static import line to import the class into a test * program, and demonstrate the conditional compilation effect.*//* In directory access/debugoff:* // access/debugoff/Debug.java* package access.debugoff;** public class Debug {* public static void debug(String s) { }* }*/package access.debug;public class Debug {public static void debug(String s) {System.out.println(s);}}(NO.6)// TIJ4 chapter Access, Exercise 9, page 233/* Create in access/local directory in your CLASSPATH:* // access/local/PackagedClass.java* package access.local;* class PackagedClass {* public PackagedClass() {* System.out.println("Creating a packaged class");* }* }* // Then, in another directrory create the file below and explain why compiler* generates error. Would making Foreign class part of access.local change anything? */// access/foreign/Foreign.javapackage access.foreign;import access.local.*;public class Foreign {public static void main(String[] args) {PackagedClass pc = new PackagedClass();}}/* Compiler error because: PackagedClass in not public, so no access outside of* package. Moving Foreign to local would allow package access to PackagedClass. */(NO.7)// MakeWidget.java// TIJ4 Chapter Access, Exercise 7, page 230/* Create the library according to the code fragments describing access and Widget. * Create a Widget in a class that is not part of the access package.*//* in access package:* // access/Widget.java* package access;** public class Widget {* public Widget() { System.out.println("Widget()"); }* }*/import access.*;public class MakeWidget {public static void main(String[] args) {Widget w = new Widget();}}(NO.8)// access/ProtectedData.java// TIJ4 Chapter Access, Exercise 6, page 228/* Create a class with protected data. Create a second class in the same file with* a method that manipulates the protected data in the first class.*/class SomeData {protected int a = 13;}class DataChanger {static void change(SomeData sd, int i) { sd.a = i; }}public class ProtectedData {public static void main(String[] args) {SomeData x = new SomeData();System.out.println(x.a);DataChanger.change(x, 99);System.out.println(x.a);}}(NO.9)// access/UnpackagedMyClass.java// TIJ4 Chapter Access, Exercise 1, page 217// Create a class in a package. Create an instance of your class outside of that package./* In another directory:* // access/mypackage/MyPackagedClass.java** package access.mypackage;** public class MyPackagedClass {* public MyPackagedClass() {System.out.println("MyPackagedClass()");}* }*/public class UnpackagedMyClass {public static void main(String[] args) {access.mypackage.MyPackagedClass m = new access.mypackage.MyPackagedClass();}}// arrays/ArrayOfGenerics10.java// TIJ4 Chapter Arrays, Exercise 10, page 762// Modify ArrayOfGeneric.java to use containers instead of arrays. // Show that you can eliminate the compile-time warnings. import java.util.*;public class ArrayOfGenerics10 {@SuppressWarnings("unchecked")public static void main(String[] args) {List<List<String>> lls = new ArrayList<List<String>>();List<List> l = new ArrayList<List>();// lls = (List<List<String>>)l; // error: inconvertible typeslls.add(new ArrayList<String>());List<Object> lo = new ArrayList<Object>();// lo = lls; // error: incompatible types// Compile-time warning eliminated:List<List<BerylliumSphere>> llb =new ArrayList<List<BerylliumSphere>>();for(int i = 0; i < 10; i++) {llb.add(new ArrayList<BerylliumSphere>());for(int j = 0; j < 2; j++)llb.get(i).add(new BerylliumSphere());}System.out.println(llb);}}// arrays/ContainerComparison15.java// TIJ4 Chapter Arrays, Exercise 15, page 775// Modify ContainerComparison.java by creating a Generator for // BerylliumSphere, and change main() to use that Generator// with Generated.array().import net.mindview.util.*;import java.util.*;import static net.mindview.util.Print.*;class BerylliumSphereGenerator implements Generator<BerylliumSphere> { public BerylliumSphere next() {return new BerylliumSphere();}}public class ContainerComparison15 {public static void main(String[] args) {BerylliumSphere[] spheres = new BerylliumSphere[5];BerylliumSphereGenerator bsg = new BerylliumSphereGenerator();Generated.array(spheres, bsg);print(Arrays.toString(spheres));print(spheres[4]);List<BerylliumSphere> sphereList =new ArrayList<BerylliumSphere>();for(int i = 0; i < 5; i++)sphereList.add(bsg.next());print(sphereList);print(sphereList.get(4));}}// arrays/Ex1.java// TIJ4 Chapter Arrays, Exercise 1, page 752/* Create a method that takes an array of BerylliumSphere as an argument. * Call the method, creating the argument dynamically. Demonstrate that* ordinary aggregate array intitialization doesn't work in this case.* Discover the only situations where ordinary aggregate array initialization * works, and where dynamic aggregate array initialization is redundant.*/import java.util.*;import static net.mindview.util.Print.*;class A { public String toString() { return "A Object"; } }public class Ex1 {// Method that takes BerylliumSphere[] as argument// and prints the argument as array elements and as List or String:public static void test(BerylliumSphere[] a) {print(Arrays.asList(a));}// Generic version:public static <T> void test(T[] t) {print(Arrays.asList(t));}// int version:public static void test(int[] ia) {print(Arrays.toString(ia));}public static void main(String[] args) {print("For objects, e.g., BerylliumSpheres:");// Array is created and initialized (aggregate initialization)// but all elements are null, not BerylliumSpheres:test(new BerylliumSphere[3]);// Dynamic aggregate initialization works;// elements are now BerylliumSpheres:test(new BerylliumSphere[]{new BerylliumSphere(), new BerylliumSphere() });// Aggregate initialization this way works:BerylliumSphere[] a = { new BerylliumSphere(),new BerylliumSphere(), new BerylliumSphere() };test(a);// Elements initialized to null:BerylliumSphere[] bsa = new BerylliumSphere[2];test(bsa);bsa = a;test(bsa);print("-------------");print("For primitives, e.g., int:");// Dynamic aggregate initialization works:test(new int[]{ new Integer(0), new Integer(0) });// But may be considered redundant, since// aggregate initialization works:// elements initialized to zero (not null):test(new int[2]);// Ordinary aggregate initialization this way also works:int[] ia = { 1, 2, 3, };test(ia);}}// arrays/Ex11.java// TIJ4 Chapter Arrays, Exercise 11, page 774// Show that autoboxing doesn't work with arrays.import static org.greggordon.tools.Print.*;public class Ex11 {public static void main(String[] args) {int[] ia = new int[2];ia[0] = new Integer(1); // will be autoboxed to intia[0] = new Integer(2); // autoboxing does work here// an array of primitive int:println(ia.getClass().getName());Integer[] Ia = new Integer[2];Ia[0] = (int)1; // will be autoboxed to IntegerIa[1] = (int)2; // autoboxing works here// an array of Integer:println(Ia.getClass().getName());println(Ia[0] instanceof Integer);for(int i = 0; i < Ia.length; i++)System.out.println(Ia[i].getClass().getName());// Integer[] cannot be assigned to int[]:// ia = Ia; // error: incompatible typesIa[0] = (int)ia[0]; // more autoboxingprintln(Ia[0].getClass().getName());}}。