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--- zh:courses:java2026:ch03 [2026/03/09 21:15]
pzczxs 创建
+++ zh:courses:java2026:ch03 [2026/03/30 19:18] (当前版本)
pzczxs [上机作业]
@@ 行 1: / 行 1: @@
 ====== 第三章：类与对象 ======
 ===== 课件 =====
-下载：类与对象
+下载：{{ :zh:courses:java2026:ch03.pptx |类与对象}}
 ===== Point类 =====
@@ 行 334: / 行 334: @@
 ===== 上机作业 =====
   * 定义一个Date类，兼顾大月、小月和闰年2月等日期的计算，须包括方法tomorrow()和daysInMonth()。
+【参考答案】
+<file java Date.java>
+package cn.edu.bjut.chapter3;
+public class Date {
+	private int year, month, day;
+	public Date(int year, int month, int day) {
+		this.year = year;
+		this.month = month;
+		this.day = day;
+	}
+	public int getYear() {
+		return year;
+	}
+	public void setYear(int year) {
+		this.year = year;
+	}
+	public int getMonth() {
+		return month;
+	}
+	public void setMonth(int month) {
+		this.month = month;
+	}
+	public int getDay() {
+		return day;
+	}
+	public void setDay(int day) {
+		this.day = day;
+	}
+	@Override
+	public String toString() {
+		return year + "-" + month + "-" + day;
+	}
+	public int daysInMonth() {
+		switch (month) {
+		case 1:
+		case 3:
+		case 5:
+		case 7:
+		case 8:
+		case 10:
+		case 12:
+			return 31; //大月
+		case 4:
+		case 6:
+		case 9:
+		case 11:
+			return 30; //小月
+		default: // 2月
+			if (((year % 100 != 0) && (year % 4 == 0))
+					|| (year % 400 == 0)) {
+				return 29;
+			}
+			return 28;
+		}
+	}
+	public Date tomorrow() {
+		int tomorrowYear = year, tomorrowMonth = month, tomorrowDay = day;
+		if (day == daysInMonth()) {
+			tomorrowDay = 1;
+			tomorrowMonth++;
+			if (month == 12) {
+				tomorrowMonth = 1;
+				tomorrowYear++;
+			}
+		} else {
+			tomorrowDay++;
+		}
+		return (new Date(tomorrowYear, tomorrowMonth, tomorrowDay));
+	}
+	public static void main(String[] args) {
+		Date date = new Date(2017, 9, 30);
+		System.out.println("The number of days: " + date.daysInMonth());
+		System.out.println(date + "\n" + date.tomorrow());
+		Date date2 = new Date(2016, 2, 28);
+		System.out.println("The number of days: " + date2.daysInMonth());
+		System.out.println(date2 + "\n" + date2.tomorrow());
+	}
+}
+</file>
   * 国际专利分类号（IPC）的前15位由section（部），class（大类），subclass（小类），main group（大组）和sub group（小组）五部分组成，每位的要求如下表所示，具体例子如：“G06F   17/30”和“H01M   10/587”，定义一个Ipc类，通过toString()函数将这五部分拼接成IPC的实际表现形式；
@@ 行 343: / 行 439: @@
 | 9      | 分割符              | /          |
 | 10-14  | subgroup（左对齐）    | 0...99999  |
+【参考答案】
+<file java Ipc.java>
+package cn.edu.bjut.chapter3;
+public class Ipc {
+	private char section;
+	private int mainClass;
+	private char subclass;
+	private int mainGroup;
+	private int subgroup;
+	private final static char SEPARATOR = '/';
+	public Ipc(char section, int mainClass, char subclass, int mainGroup, int subgroup) {
+		this.section = section;
+		this.mainClass = mainClass;
+		this.subclass = subclass;
+		this.mainGroup = mainGroup;
+		this.subgroup = subgroup;
+	}
+	public char getSection() {
+		return section;
+	}
+	public void setSection(char section) {
+		this.section = section;
+	}
+	public int getMainClass() {
+		return mainClass;
+	}
+	public void setMainClass(int mainClass) {
+		this.mainClass = mainClass;
+	}
+	public char getSubclass() {
+		return subclass;
+	}
+	public void setSubclass(char subclass) {
+		this.subclass = subclass;
+	}
+	public int getMainGroup() {
+		return mainGroup;
+	}
+	public void setMainGroup(int mainGroup) {
+		this.mainGroup = mainGroup;
+	}
+	public int getSubgroup() {
+		return subgroup;
+	}
+	public void setSubgroup(int subgroup) {
+		this.subgroup = subgroup;
+	}
+	@Override
+	public String toString() {
+		StringBuilder sb = new StringBuilder();
+		sb.append(section);
+		String strMainClass = String.format("%2s", mainClass).replace(' ', '0');
+		sb.append(strMainClass);
+		sb.append(subclass);
+		String strMainGroup = String.format("%4s", mainGroup);
+		sb.append(strMainGroup);
+		sb.append(SEPARATOR);
+		sb.append(subgroup);
+		return sb.toString();
+	}
+	public static void main(String[] args) {
+		char section = 'A';
+		int mainClass = 4;
+		char subclass = 'B';
+		int mainGroup = 55;
+		int subgroup = 56;
+		Ipc ipc = new Ipc(section, mainClass, subclass, mainGroup, subgroup);
+		System.out.println(ipc);
+	}
+}
+</file>
   * 上题的Ipc类至少要包含两个构造函数，其中一个构造函数有五个参数（分别为部、大类、小类、大组和小组），另一个构造函数只有一个参数（以字符串形式表示的IPC分类号）。
+【参考答案】
+<file java Ipc.java>
+package cn.edu.bjut.chapter3;
+public class Ipc {
+	private char section;
+	private int mainClass;
+	private char subclass;
+	private int mainGroup;
+	private int subgroup;
+	private final char SEPARATOR = '/';
+	public Ipc(char section, int mainClass, char subclass, int mainGroup, int subgroup) {
+		this.section = section;
+		this.mainClass = mainClass;
+		this.subclass = subclass;
+		this.mainGroup = mainGroup;
+		this.subgroup = subgroup;
+	}
+	// example: G09F 17/30
+	public Ipc(String symbol) {
+		this.section = symbol.charAt(0);
+		this.mainClass = Integer.parseInt(symbol.substring(1, 3));
+		this.subclass = symbol.charAt(3);
+		int pos = symbol.indexOf(SEPARATOR);
+		this.mainGroup = Integer.parseInt(symbol.substring(4, pos).trim());
+		this.subgroup = Integer.parseInt(symbol.substring(pos + 1));
+	}
+	public char getSection() {
+		return section;
+	}
+	public void setSection(char section) {
+		this.section = section;
+	}
+	public int getMainClass() {
+		return mainClass;
+	}
+	public void setMainClass(int mainClass) {
+		this.mainClass = mainClass;
+	}
+	public char getSubclass() {
+		return subclass;
+	}
+	public void setSubclass(char subclass) {
+		this.subclass = subclass;
+	}
+	public int getMainGroup() {
+		return mainGroup;
+	}
+	public void setMainGroup(int mainGroup) {
+		this.mainGroup = mainGroup;
+	}
+	public int getSubgroup() {
+		return subgroup;
+	}
+	public void setSubgroup(int subgroup) {
+		this.subgroup = subgroup;
+	}
+	@Override
+	public String toString() {
+		StringBuilder sb = new StringBuilder();
+		sb.append(section);
+		String strMainClass = String.format("%2s", mainClass).replace(' ', '0');
+		sb.append(strMainClass);
+		sb.append(subclass);
+		String strMainGroup = String.format("%4s", mainGroup);
+		sb.append(strMainGroup);
+		sb.append(SEPARATOR);
+		sb.append(subgroup);
+		return sb.toString();
+	}
+	public static void main(String[] args) {
+		char section = 'A';
+		int mainClass = 4;
+		char subclass = 'B';
+		int mainGroup = 55;
+		int subgroup = 56;
+		Ipc ipc = new Ipc(section, mainClass, subclass, mainGroup, subgroup);
+		System.out.println(ipc);
+		String symbol = "G09F 17/30";
+		System.out.println(new Ipc(symbol));
+	}
+}
+</file>
+  * 定义一个StringUtils类，至少包括方法isIpc()和capitalize()等。
+【参考答案】
+<file java StringUtils.java>
+package cn.edu.bjut.utils;
+public class StringUtils {
+	public final static char SEPARATOR = '/';
+	public static String capitalize(String str) {
+		if (str == null) {
+			return null;
+		}
+		if (str.length() == 1) {
+			return str.toUpperCase();
+		}
+		return str.substring(0, 1).toUpperCase() + str.substring(1).toLowerCase();
+	}
+	public static boolean isIpc(String str) {
+		char section = str.charAt(0);
+		if (section < 'A' || section > 'H') {
+			return false;
+		}
+		int mainClass = Integer.parseInt(str.substring(1, 3));
+		if (mainClass < 1 || mainClass > 99) {
+			return false;
+		}
+		char subclass = str.charAt(3);
+		if (subclass < 'A' || subclass > 'Z') {
+			return false;
+		}
+		int pos = str.indexOf(SEPARATOR);
+		int mainGroup = Integer.parseInt(str.substring(4, pos).trim());
+		if (mainGroup < 1 || mainGroup > 9999) {
+			return false;
+		}
+		int subgroup = Integer.parseInt(str.substring(pos + 1));
+		if (subgroup < 0 || subgroup > 99999) {
+			return false;
+		}
+		return true;
+	}
+	public static void main(String[] args) {
+		String str = "heLLO";
+		System.out.println(StringUtils.capitalize(str));
+		String symbol = "G06F 17/30";
+		System.out.println(StringUtils.isIpc(symbol));
+	}
+}
+</file>
+  * 定义一个MathUtils类，至少包括方法sum()和LogSumExp()等。
+注意：LogSumExp的计算公式为$y = \log \sum_i {e^{x_i}}$。
+该公式有以下两个问题：（1）当$x_i$有一个偏大时，$e^{x_i}$计算值会溢出（overflow），致使数值计算出现问题；（2）当$x_i$都偏小时，$\sum_i {e^{x_i}}$接近于0，对其计算log也会出现数值计算问题。
+【参考答案】
+<file java MathUtils.java>
+package cn.edu.bjut.utils;
+public class MathUtils {
+	public static double logsum(double loga, double logb) {
+		double sum = (loga > logb)?
+				(loga + Math.log(1.0 + Math.exp(logb - loga))): (logb + Math.log(1.0 + Math.exp(loga - logb)));
+		return sum;
+	}
+	public static void main(String[] args) {
+		double loga = 10;
+		double logb = 1000;
+		double sum = MathUtils.logsum(loga, logb);
+		System.out.println(sum);
+		System.out.println(Math.exp(1000));
+	}
+}
+</file>
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