import java.io.*; import java.time.LocalDate;public class Test {public static void main(String[] args){RandomAccessTest.test();} }/*2.3 讀寫二進制數據*//*2.3.2 隨機訪問文件寫了大半天，突然發現這個實驗好像不是太嚴謹：1.RandomAccessFile算長度時，應該是根據字節數算出來的2.寫字符串時，我們只是指定了碼元數量，我們寫的是固定碼元數量的字符串3.這樣的化，我們記錄的Employee.RECORD_SIZE根本就代表不了一條記錄的長度4.但是我們最后又通過RandomAccessFile的長度和Employee.RECORD_SIZE來計算記錄數量5.我覺得這個實驗有問題，以后研究吧*/class Employee {private String name;private double salary;private LocalDate hireDay;public static final int NAME_SIZE = 30;public static final int RECORD_SIZE = 50;public Employee(String name, double salary, LocalDate hireDay) {this.name = name;this.salary = salary;this.hireDay = hireDay;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public LocalDate getHireDay() {return hireDay;}public void setHireDay(LocalDate hireDay) {this.hireDay = hireDay;}@Overridepublic String toString() {return "Employee{" +"name='" + name + '\'' +", salary=" + salary +", hireDay=" + hireDay +'}';} }class DataIO {//Java核心技術 卷二 第十版 2.3.2節//寫出從字符串開頭開始的指定數量的碼元，如果碼元過少，該方法會用‘0’來補齊字符串public static void writeFixedString(String s, int size, DataOutput output) throws IOException {for (int i = 0; i < size; i++) {char ch =0;if(i<s.length())ch = s.charAt(i);output.write(ch);}}//Java核心技術 卷二 第十版 2.3.2節//從輸入流中讀入字符，直至讀入size個碼元，或者直至遇到具有0值的字符串，然后跳出輸入字段中剩余的0值。public static String readFixedString1(int size, DataInput in) throws IOException {StringBuilder sb = new StringBuilder();for (int i = 0; i < size; i++) {char c;if ((c = in.readChar()) != 0) {sb.append(c);}}return sb.toString();}//功能和上一個方法是一樣的，但是這個效率會高那么一點點public static String readFixedString2(int size, DataInput in) throws IOException {StringBuilder sb = new StringBuilder();/*int i;for (i = 0; i < size; i++) {char c;if ((c = in.readChar()) == 0) {break;}sb.append(c);}in.skipBytes(2*(size-i)); //這個地方不是太嚴謹*///用書中代碼測試一下int i =0;boolean more = true;while (more && i < size) {char ch = in.readChar();i++;if (ch == 0) {more = false;} else {sb.append(ch);}}in.skipBytes(2 * (size - i));return sb.toString();} }class RandomAccessTest {public static void test() {Employee[] staff = new Employee[]{new Employee("A", 10, LocalDate.now()),new Employee("B", 20, LocalDate.now()),new Employee("C", 30, LocalDate.now())};//寫入try(DataOutputStream out = new DataOutputStream(new FileOutputStream("employee1.dat"))) {for (Employee e : staff) {writeData(out, e);}} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}//讀取try(RandomAccessFile in = new RandomAccessFile("employee1.dat","r")) {int n = (int) (in.length() / Employee.RECORD_SIZE);Employee[] newStaff = new Employee[n];for (int i = n - 1; i >= 0; i--) {in.seek(i*Employee.RECORD_SIZE);newStaff[i] = readDate(in);}for (Employee e : newStaff) {System.out.println(e);}} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}}private static void writeData(DataOutput out, Employee employee) throws IOException {DataIO.writeFixedString(employee.getName(), Employee.NAME_SIZE, out);out.writeDouble(employee.getSalary());LocalDate hireDay = employee.getHireDay();out.writeInt(hireDay.getYear());out.writeInt(hireDay.getMonthValue());out.writeInt(hireDay.getDayOfMonth());}private static Employee readDate(DataInput input) throws IOException {String name = DataIO.readFixedString2(Employee.NAME_SIZE, input);double salary = input.readDouble();inty= input.readInt(),m= input.readInt(),d= input.readInt();return new Employee(name, salary, LocalDate.of(y, m, d));} }/*2.3.3 ZIP文檔ZipFile API：ZipFile(String name)ZipFile(File file)Enumeration entries()ZipEntry getEntry(String name)InputStream getInputStream(ZipEntry ze)String getName()從這個類的API中可以看出來，還有一種使用Zip的方案。先通過ZipFile對象，得到這個壓縮包中的每一條記錄，然后再指定某條具體的記錄，得到其中的數據。*/

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《Java核心技術卷二》筆記

轉載于:https://www.cnblogs.com/junjie2019/p/10597230.html

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