用 CRUD 操作进行 Java 文件处理
原文:https://www . geesforgeks . org/file-handling-in-Java-with-crud-operations/
到目前为止，使用 Java 程序的操作是在没有存储在任何地方的提示符/终端上完成的。但是在软件行业，大多数程序都是为了存储从程序中获取的信息而编写的。一种方法是将提取的信息存储在文件中。
什么是 Java 中的文件处理？
一个文件是一个用来存储各种类型信息的容器。通过创建一个具有唯一名称的文件，数据被永久存储在辅助内存中。文件可以由文本、图像或任何其他文档组成。
可以对文件进行的不同操作有:
- 创建新文件
- 打开现有文件
- 从文件中读取
- 写入文件
- 移动到文件中的特定位置
- 关闭文件
可在 Java 中用于文件处理的不同类:
- InputStream
- 输出流
- 滤波器 utputStream
- 档案输出流
- 字节数组输出流
- 字节阵列输入流
- fileinputstream
- 过滤输入流
- StringBufferInputStream
- SequenceInputStream
- 缓冲器 utputStream
- StringBufferInputStream
- DataOutputStream
- 打印流
- buffer edinpertstream
- datainputstream
- pushbakinputstream
在本文中，我们将学习如何使用类来执行 8 位(1 字节)的输入和输出 Java 字节流，该类由方法 writeBytes()和 readBytes() 组成，以字节的形式写入和读取数据。
执行文件操作的各种方法:
- 写入字节(字符串):将字符串作为字节序列写入文件。
- readLine(): 从此文件中读取下一行文本。
- getFilePointer(): 返回此文件中的当前偏移量。
- length(): 返回该文件的长度，返回类型为 long。
- close(): 关闭该随机访问文件流，并释放与该流相关联的任何系统资源。
- 设置长度(长新长度):设置该文件的长度。
- 寻道(长位置):设置文件指针偏移量，从该文件的开始位置开始测量，在该位置进行下一次读取或写入。
Java 文件打开方式:
| 价值 | 意义 | | --- | --- | | " r " | 仅供阅读。调用结果对象的任何写方法都将导致引发 IOException。 | | “rw” | 开放阅读和写作。如果文件不存在，将尝试创建它。 | | “rws” | 像“rw”一样，打开进行读写，还要求文件内容或元数据的每次更新都同步写入底层存储设备。 | | “rwd” | 像“rw”一样，打开进行读写，还要求文件内容的每次更新都同步写入底层存储设备。 |使用 RandomAccessFile 打开文件的语法:
File file = new File( filename )
RandomAccessFile raf = new RandomAccessFile(file, mode)
Java 中使用文件处理的 CRUD 操作
示例:考虑您希望在文件中保留朋友联系号码的记录。为了区分你朋友的名字和联系电话，你需要一个分隔符。为此，您需要选择一个分隔符，如“！”或“{ content }”；或者一些没有出现在你朋友名字里的特殊符号。然后我们将形成一个由名称、特殊符号和数字组成的字符串插入到文件中。
【friendsContact.txt 文件中联系人的语法:
Name!Number
如何用 Java 创建文件？
Java 语言(一种计算机语言，尤用于创建网站)
// Java program to create a file "friendsContact.txt"
// and add a new contact in the file
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class AddFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
// Get the number to be updated
// from the Command line argument
long newNumber = Long.parseLong(data[1]);
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name
// of contact already exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName
|| number == newNumber) {
found = true;
break;
}
}
if (found == false) {
// Enter the if block when a record
// is not already present in the file.
nameNumberString
= newName + "!"
+ String.valueOf(newNumber);
// writeBytes function to write a string
// as a sequence of bytes.
raf.writeBytes(nameNumberString);
// To insert the next record in new line.
raf.writeBytes(System.lineSeparator());
// Print the message
System.out.println(" Friend added. ");
// Closing the resources.
raf.close();
}
// The contact to be updated
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
catch (NumberFormatException nef) {
System.out.println(nef);
}
}
}
输出:
在新创建的文件中编译并添加联系人:
javac AddFriend.java
java AddFriend abc 1111111111
Friend added
java AddFriend pqr 1111111111
Input name or number already exist
文件:
如何用 Java 读取文件？
Java 语言(一种计算机语言，尤用于创建网站)
// Java program to read from file "friendsContact.txt"
// and display the contacts
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class DisplayFriends {
public static void main(String data[])
{
try {
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Traversing the file
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// Print the contact data
System.out.println(
"Friend Name: " + name + "\n"
+ "Contact Number: " + number + "\n");
}
catch (IOException ioe)
{
System.out.println(ioe);
}
catch (NumberFormatException nef)
{
System.out.println(nef);
}
}
}
输出:
从文件中编译和读取联系人:
javac DisplayFriends.java
java DisplayFriends
Friend Name: abc
Contact Number: 1234567890
Friend Name: lmn
Contact Number: 3333333333
Friend Name: xyz
Contact Number: 4444444444
文件:
如何用 Java 更新文件？
Java 语言(一种计算机语言，尤用于创建网站)
// Java program to update in the file "friendsContact.txt"
// and change the number of an old contact
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class UpdateFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
// Get the number to be updated
// from the Command line argument
long newNumber = Long.parseLong(data[1]);
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name
// of contact already exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName
|| number == newNumber) {
found = true;
break;
}
}
// Update the contact if record exists.
if (found == true) {
// Creating a temporary file
// with file pointer as tmpFile.
File tmpFile = new File("temp.txt");
// Opening this temporary file
// in ReadWrite Mode
RandomAccessFile tmpraf
= new RandomAccessFile(tmpFile, "rw");
// Set file pointer to start
raf.seek(0);
// Traversing the friendsContact.txt file
while (raf.getFilePointer()
< raf.length()) {
// Reading the contact from the file
nameNumberString = raf.readLine();
index = nameNumberString.indexOf('!');
name = nameNumberString.substring(
0, index);
// Check if the fetched contact
// is the one to be updated
if (name.equals(inputName)) {
// Update the number of this contact
nameNumberString
= name + "!"
+ String.valueOf(newNumber);
}
// Add this contact in the temporary
// file
tmpraf.writeBytes(nameNumberString);
// Add the line separator in the
// temporary file
tmpraf.writeBytes(
System.lineSeparator());
}
// The contact has been updated now
// So copy the updated content from
// the temporary file to original file.
// Set both files pointers to start
raf.seek(0);
tmpraf.seek(0);
// Copy the contents from
// the temporary file to original file.
while (tmpraf.getFilePointer()
< tmpraf.length()) {
raf.writeBytes(tmpraf.readLine());
raf.writeBytes(System.lineSeparator());
}
// Set the length of the original file
// to that of temporary.
raf.setLength(tmpraf.length());
// Closing the resources.
tmpraf.close();
raf.close();
// Deleting the temporary file
tmpFile.delete();
System.out.println(" Friend updated. ");
}
// The contact to be updated
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
catch (NumberFormatException nef) {
System.out.println(nef);
}
}
}
输出:
编译并更新文件中的联系人:
javac UpdateFriend.java
java UpdateFriend abc 1234567890
Friend updated.
java UpdateFriend tqr
Input name does not exists.
文件:
如何在 Java 中删除文件？
Java 语言(一种计算机语言，尤用于创建网站)
// Java program to delete a contact
// from the file "friendsContact.txt"
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class DeleteFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name of contact exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName) {
found = true;
break;
}
}
// Delete the contact if record exists.
if (found == true) {
// Creating a temporary file
// with file pointer as tmpFile.
File tmpFile = new File("temp.txt");
// Opening this temporary file
// in ReadWrite Mode
RandomAccessFile tmpraf
= new RandomAccessFile(tmpFile, "rw");
// Set file pointer to start
raf.seek(0);
// Traversing the friendsContact.txt file
while (raf.getFilePointer()
< raf.length()) {
// Reading the contact from the file
nameNumberString = raf.readLine();
index = nameNumberString.indexOf('!');
name = nameNumberString.substring(
0, index);
// Check if the fetched contact
// is the one to be deleted
if (name.equals(inputName)) {
// Skip inserting this contact
// into the temporary file
continue;
}
// Add this contact in the temporary
// file
tmpraf.writeBytes(nameNumberString);
// Add the line separator in the
// temporary file
tmpraf.writeBytes(
System.lineSeparator());
}
// The contact has been deleted now
// So copy the updated content from
// the temporary file to original file.
// Set both files pointers to start
raf.seek(0);
tmpraf.seek(0);
// Copy the contents from
// the temporary file to original file.
while (tmpraf.getFilePointer()
< tmpraf.length()) {
raf.writeBytes(tmpraf.readLine());
raf.writeBytes(System.lineSeparator());
}
// Set the length of the original file
// to that of temporary.
raf.setLength(tmpraf.length());
// Closing the resources.
tmpraf.close();
raf.close();
// Deleting the temporary file
tmpFile.delete();
System.out.println(" Friend deleted. ");
}
// The contact to be deleted
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
}
}
输出:
编辑和删除文件中的联系人:
javac DeleteFriend.java
java DeleteFriend pqr
Friend deleted.
java DeleteFriend tqr
Input name does not exists.
文件:
创造
// Java program to create a file "friendsContact.txt"
// and add a new contact in the file
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class AddFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
// Get the number to be updated
// from the Command line argument
long newNumber = Long.parseLong(data[1]);
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name
// of contact already exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName
|| number == newNumber) {
found = true;
break;
}
}
if (found == false) {
// Enter the if block when a record
// is not already present in the file.
nameNumberString
= newName + "!"
+ String.valueOf(newNumber);
// writeBytes function to write a string
// as a sequence of bytes.
raf.writeBytes(nameNumberString);
// To insert the next record in new line.
raf.writeBytes(System.lineSeparator());
// Print the message
System.out.println(" Friend added. ");
// Closing the resources.
raf.close();
}
// The contact to be updated
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
catch (NumberFormatException nef) {
System.out.println(nef);
}
}
}
阅读
// Java program to read from file "friendsContact.txt"
// and display the contacts
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class DisplayFriends {
public static void main(String data[])
{
try {
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Traversing the file
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// Print the contact data
System.out.println(
"Friend Name: " + name + "\n"
+ "Contact Number: " + number + "\n");
}
catch (IOException ioe)
{
System.out.println(ioe);
}
catch (NumberFormatException nef)
{
System.out.println(nef);
}
}
}
更新
// Java program to update in the file "friendsContact.txt"
// and change the number of an old contact
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class UpdateFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
// Get the number to be updated
// from the Command line argument
long newNumber = Long.parseLong(data[1]);
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name
// of contact already exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName
|| number == newNumber) {
found = true;
break;
}
}
// Update the contact if record exists.
if (found == true) {
// Creating a temporary file
// with file pointer as tmpFile.
File tmpFile = new File("temp.txt");
// Opening this temporary file
// in ReadWrite Mode
RandomAccessFile tmpraf
= new RandomAccessFile(tmpFile, "rw");
// Set file pointer to start
raf.seek(0);
// Traversing the friendsContact.txt file
while (raf.getFilePointer()
< raf.length()) {
// Reading the contact from the file
nameNumberString = raf.readLine();
index = nameNumberString.indexOf('!');
name = nameNumberString.substring(
0, index);
// Check if the fetched contact
// is the one to be updated
if (name.equals(inputName)) {
// Update the number of this contact
nameNumberString
= name + "!"
+ String.valueOf(newNumber);
}
// Add this contact in the temporary
// file
tmpraf.writeBytes(nameNumberString);
// Add the line separator in the
// temporary file
tmpraf.writeBytes(
System.lineSeparator());
}
// The contact has been updated now
// So copy the updated content from
// the temporary file to original file.
// Set both files pointers to start
raf.seek(0);
tmpraf.seek(0);
// Copy the contents from
// the temporary file to original file.
while (tmpraf.getFilePointer()
< tmpraf.length()) {
raf.writeBytes(tmpraf.readLine());
raf.writeBytes(System.lineSeparator());
}
// Set the length of the original file
// to that of temporary.
raf.setLength(tmpraf.length());
// Closing the resources.
tmpraf.close();
raf.close();
// Deleting the temporary file
tmpFile.delete();
System.out.println(" Friend updated. ");
}
// The contact to be updated
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
catch (NumberFormatException nef) {
System.out.println(nef);
}
}
}
删除
// Java program to delete a contact
// from the file "friendsContact.txt"
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.NumberFormatException;
class DeleteFriend {
public static void main(String data[])
{
try {
// Get the name of the contact to be updated
// from the Command line argument
String newName = data[0];
String nameNumberString;
String name;
long number;
int index;
// Using file pointer creating the file.
File file = new File("friendsContact.txt");
if (!file.exists()) {
// Create a new file if not exists.
file.createNewFile();
}
// Opening file in reading and write mode.
RandomAccessFile raf
= new RandomAccessFile(file, "rw");
boolean found = false;
// Checking whether the name of contact exists.
// getFilePointer() give the current offset
// value from start of the file.
while (raf.getFilePointer() < raf.length()) {
// reading line from the file.
nameNumberString = raf.readLine();
// splitting the string to get name and
// number
String[] lineSplit
= nameNumberString.split("!");
// separating name and number.
name = lineSplit[0];
number = Long.parseLong(lineSplit[1]);
// if condition to find existence of record.
if (name == newName) {
found = true;
break;
}
}
// Delete the contact if record exists.
if (found == true) {
// Creating a temporary file
// with file pointer as tmpFile.
File tmpFile = new File("temp.txt");
// Opening this temporary file
// in ReadWrite Mode
RandomAccessFile tmpraf
= new RandomAccessFile(tmpFile, "rw");
// Set file pointer to start
raf.seek(0);
// Traversing the friendsContact.txt file
while (raf.getFilePointer()
< raf.length()) {
// Reading the contact from the file
nameNumberString = raf.readLine();
index = nameNumberString.indexOf('!');
name = nameNumberString.substring(
0, index);
// Check if the fetched contact
// is the one to be deleted
if (name.equals(inputName)) {
// Skip inserting this contact
// into the temporary file
continue;
}
// Add this contact in the temporary
// file
tmpraf.writeBytes(nameNumberString);
// Add the line separator in the
// temporary file
tmpraf.writeBytes(
System.lineSeparator());
}
// The contact has been deleted now
// So copy the updated content from
// the temporary file to original file.
// Set both files pointers to start
raf.seek(0);
tmpraf.seek(0);
// Copy the contents from
// the temporary file to original file.
while (tmpraf.getFilePointer()
< tmpraf.length()) {
raf.writeBytes(tmpraf.readLine());
raf.writeBytes(System.lineSeparator());
}
// Set the length of the original file
// to that of temporary.
raf.setLength(tmpraf.length());
// Closing the resources.
tmpraf.close();
raf.close();
// Deleting the temporary file
tmpFile.delete();
System.out.println(" Friend deleted. ");
}
// The contact to be deleted
// could not be found
else {
// Closing the resources.
raf.close();
// Print the message
System.out.println(" Input name"
+ " does not exists. ");
}
}
catch (IOException ioe) {
System.out.println(ioe);
}
}
}
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