Java编程练习解答汇总

# Java编程练习解答汇总 ## 1. Java编程基础 ### 1.1 可打印字符栈 `PrintableCharStack` 类的 `printStackElements()` 方法用于打印栈中的元素，且不会弹出元素。以下是相关代码： ```java //Filename: PrintableCharStack.java public class PrintableCharStack extends CharStack { // Instance method public void printStackElements() { for (int i = 0; i <= topOfStack; i++) System.out.print(stackArray[i]); // print each char on terminal System.out.println(); } // Constructor calls the constructor of the superclass explicitly. PrintableCharStack(int capacity) { super(capacity); } } //Filename: Client.java public class Client { public static void main(String[] args) { // Create a printable character stack. PrintableCharStack stack = new PrintableCharStack(40); // Create a string to push on the stack: String str = "!no tis ot nuf era skcatS"; int length = str.length(); // Push the string char by char onto the stack: for (int i = 0; i < length; i++) { stack.push(str.charAt(i)); } System.out.print("Stack contents: "); stack.printStackElements(); // Pop and print each char from the stack: while (!stack.isEmpty()) { System.out.print(stack.pop()); } System.out.println(); System.out.print("Stack contents: "); stack.printStackElements(); } } ``` ### 操作步骤 1. 创建 `PrintableCharStack` 对象。 2. 将字符串逐字符压入栈中。 3. 调用 `printStackElements()` 方法打印栈内容。 4. 弹出并打印栈中的每个字符。 5. 再次调用 `printStackElements()` 方法打印栈内容。 ## 2. 语言基础 ### 2.1 温度转换程序 以下程序可将华氏温度转换为摄氏温度，且编译和运行无错误： ```java //Filename: Temperature.java public class Temperature { public static void main(String[] args) { double fahrenheit = 62.5; double celsius = f2c(fahrenheit); System.out.println(fahrenheit + "F" + " = " + celsius + 'C'); } static double f2c(double fahr) { return (fahr - 32) * 5 / 9; } } ``` ### 操作步骤 1. 定义华氏温度变量。 2. 调用 `f2c()` 方法将华氏温度转换为摄氏温度。 3. 打印转换结果。 ## 3. 声明 ### 3.1 编辑上下文的JavaBean ```java public class EditContext { private Object selected; public void setSelected(Object newSelected) { selected = newSelected; } public Object getSelected() { return selected; } } ``` ### 3.2 测验评分程序 ```java public class QuizGrader { enum Result { CORRECT, WRONG, UNANSWERED } public static final int PASS_MARK = 5; public static void main(String[] args) { String[] correctAnswers = { "C", "A", "B", "D", "B", "C", "C", "A" }; System.out.println("Question Submitted Ans. Correct Ans. Result"); int numOfCorrectAnswers = 0; int numOfWrongAnswers = 0; int numOfUnanswered = 0; for (int i = 0; i < args.length; i++) { String submittedAnswer = args[i]; String correctAnswer = correctAnswers[i]; Result result = determineResult(submittedAnswer, correctAnswer); System.out.printf("%5d%10s%15s%15s%n", i + 1, submittedAnswer, correctAnswer, result); switch (result) { case CORRECT: numOfCorrectAnswers++; break; case WRONG: numOfWrongAnswers++; break; case UNANSWERED: numOfUnanswered++; break; } } System.out.println("No. of correct answers: " + numOfCorrectAnswers); System.out.println("No. of wrong answers: " + numOfWrongAnswers); System.out.println("No. of questions unanswered: " + numOfUnanswered); System.out.println("The candidate " + (numOfCorrectAnswers >= PASS_MARK ? "PASSED." : "FAILED.")); } public static Result determineResult(String submittedAnswer, String correctAnswer) { Result result = null; if (submittedAnswer.equals(correctAnswer)) result = Result.CORRECT; else if (submittedAnswer.equals("X")) result = Result.UNANSWERED; else result = Result.WRONG; return result; } } ``` ### 操作步骤 1. 定义正确答案数组。 2. 遍历提交的答案，调用 `determineResult()` 方法判断结果。 3. 统计正确、错误和未回答的问题数量。 4. 根据正确答案数量判断考生是否通过。 ## 4. 访问控制 ### 4.1 账户和数据库类 ```java //Filename: Account.java package com.megabankcorp.records; public class Account { } //Filename: Database.java package com.megabankcorp.system; import com.megabankcorp.records.Account; public abstract class Database { public abstract void deposit(Account acc, double amount); public abstract void withdraw(Account acc, double amount); protected abstract double amount(Account acc); final void transfer(Account from, Account to, double amount) { withdraw(from, amount); deposit(to, amount); } } ``` ### 操作说明 - `deposit()` 和 `withdraw()` 方法为抽象方法，可在任何地方访问。 - `amount()` 方法为抽象方法，仅在包内和子类中可访问。 - `transfer()` 方法为最终方法，仅在包内可访问。 ## 5. 运算符和表达式 ### 5.1 阳光传播时间计算 ```java //Filename: Sunlight.java public class Sunlight { public static void main(String[] args) { int kmFromSun = 150000000; int lightSpeed = 299792458; long mFromSun = kmFromSun * 1000L; int seconds = (int) (mFromSun / lightSpeed); System.out.print("Light will use "); printTime(seconds); System.out.println(" to travel from the sun to the earth."); } public static void printTime(int sec) { int min = sec / 60; sec = sec - (min * 60); System.out.print(min + " minute(s) and " + sec + " second(s)"); } } ``` ### 操作步骤 1. 定义地球到太阳的距离和光速。 2. 将距离转换为米，使用 `long` 类型避免溢出。 3. 计算光从太阳传播到地球所需的时间。 4. 调用 `printTime()` 方法格式化输出时间。 ## 6. 控制流 ### 6.1 寻找质数 #### 使用 `for` 循环 ```java //Filename: ForPrimes.java public class ForPrimes { final static int MAX = 100; public static void main(String[] args) { numbers: for (int num = 1; num < MAX; num++) { int divLim = (int) Math.sqrt(num); for (int div = 2; div <= divLim; div++) if ((num % div) == 0) continue numbers; System.out.println(num); } } } ``` #### 使用 `while` 循环 ```java //Filename: WhilePrimes.java public class WhilePrimes { final static int MAX = 100; public static void main(String[] args) { int num = 1; numbers: while (num < MAX) { int number = num++; int divLim = (int) Math.sqrt(number); int div = 2; while (div <= divLim) if ((number % div++) == 0) continue numbers; System.out.println(number); } } } ``` ### 操作步骤 1. 设定最大范围。 2. 遍历范围内的每个数。 3. 对于每个数，检查是否能被小于其平方根的数整除。 4. 若不能整除，则该数为质数，打印输出。 ### 6.2 发电厂控制系统 ```java package energy; public class PowerPlant { Reactor core; PowerPlant() { core = new Reactor(); } public void soundEvacuateAlarm() { } public int getOptimalThroughput() { return 0; } public static void main(String[] args) { PowerPlant plant = new PowerPlant(); Control ctrl = new Control(plant); ctrl.runSystem(); } } class Reactor { public int getThroughput() { return 0; } public boolean isCritical() { return false; } void increaseThroughput() throws ReactorCritical { } void decreaseThroughput() { } } class ReactorCritical extends Exception {} class Control { PowerPlant thePlant; final static int TOLERANCE = 10; public Control(PowerPlant p) { thePlant = p; } public void runSystem() { try { while (true) { int optimalThroughput = thePlant.getOptimalThroughput(); if (needAdjustment(optimalThroughput)) adjustThroughput(optimalThroughput); } } catch (ReactorCritical rc) { thePlant.soundEvacuateAlarm(); } finally { shutdown(); } } public boolean needAdjustment(int target) throws ReactorCritical { if (thePlant.core.isCritical()) throw new ReactorCritical(); return Math.abs(thePlant.core.getThroughput() - target) > TOLERANCE; } public void adjustThroughput(int target) throws ReactorCritical { while (needAdjustment(target)) { if ((thePlant.core.getThroughput() - target) > TOLERANCE) thePlant.core.increaseThroughput(); else thePlant.core.decreaseThroughput(); } } public void shutdown() { while (thePlant.core.getThroughput() > 0) { thePlant.core.decreaseThroughput(); } } } ``` ### 操作步骤 1. 创建 `PowerPlant` 对象和 `Control` 对象。 2. 调用 `Control` 对象的 `runSystem()` 方法。 3. 在 `runSystem()` 方法中，不断监测反应堆的最佳吞吐量和实际吞吐量。 4. 若两者差值超过容差，则调用 `adjustThroughput()` 方法调整吞吐量。 5. 若反应堆状态危急，触发疏散警报并关闭反应堆。 ## 7. 面向对象编程 ### 7.1 对数组应用函数 ```java //Filename: Exercise1.java package chap07_PE1; interface Function { public int evaluate(int arg); } class Half implements Function { public int evaluate(int arg) { return arg / 2; } } public class Exercise1 { public static int[] applyFunctionToArray(int[] arrIn) { int length = arrIn.length; int[] arrOut = new int[length]; Function func = new Half(); for (int i = 0; i < length; i++) arrOut[i] = func.evaluate(arrIn[i]); return arrOut; } public static void main(String[] args) { int length = 10; int[] myArr = new int[length]; for (int i = 0; i < length;) myArr[i] = ++i; for (int value : myArr) System.out.println(value); myArr = applyFunctionToArray(myArr); for (int value : myArr) System.out.print ```