花几千上万学习Java，真没必要！（十九）

1、StringBuilder：

测试代码1：

package stringbuilder.com;
import java.util.ArrayList;  
import java.util.List;  public class StringBuilderExample {  public static void main(String[] args) {  // 初始化StringBuilder  StringBuilder sb = new StringBuilder("This is a complex example using StringBuilder. ");  // 添加不同类型的数据  sb.append("It demonstrates ").append(42).append(" different operations. ");  // 使用循环构建字符串  List<String> items = new ArrayList<>();  items.add("Item 1");  items.add("Item 2");  items.add("Item 3");  sb.append("The list contains: ");  for (String item : items) {  sb.append(item).append(", ");  }  // 移除最后一个逗号和空格  if (sb.length() > 0 && sb.charAt(sb.length() - 1) == ',') {  sb.setLength(sb.length() - 2); // 移除逗号和空格  }  // 反转部分字符串  String partialString = sb.substring(0, sb.indexOf("list contains:") + "list contains:".length()).trim();  StringBuilder reversedPartial = new StringBuilder(partialString).reverse();  // 在StringBuilder中插入反转后的部分字符串  sb.insert(0, "Reversed partial: ").insert(0, reversedPartial).append("\n");  // 转换并打印结果  System.out.println(sb.toString());  // 字符串与另一个StringBuilder合并  StringBuilder anotherSb = new StringBuilder("Additional information: ");  anotherSb.append("This is more data to be appended.");  // 合并两个StringBuilder  sb.append(anotherSb);  // 打印最终结果  System.out.println(sb.toString());  }  
}

运行结果如下：

测试代码2：

package stringbuilder.com;
import java.time.LocalDateTime;  
import java.time.format.DateTimeFormatter;  
import java.util.HashMap;  
import java.util.HashSet;  
import java.util.Map;  
import java.util.Set;  public class StringBuilderTest {  private enum LogLevel {  DEBUG, INFO, WARN, ERROR  }  private static final Map<LogLevel, Set<String>> logMessages = new HashMap<>();  static {  // 初始化日志级别集合  for (LogLevel level : LogLevel.values()) {  logMessages.put(level, new HashSet<>());  }  }  public static void main(String[] args) {  // 日志记录  log(LogLevel.INFO, "System starting up...");  log(LogLevel.DEBUG, "Initializing configuration...");  log(LogLevel.WARN, "Old configuration file found, using defaults.");  log(LogLevel.ERROR, "Failed to connect to database!");  // WARN和ERROR级别的日志  printLogs(LogLevel.WARN, LogLevel.ERROR);  // 使用StringBuilder构建更复杂的日志消息  StringBuilder complexMessage = buildComplexLogMessage("UserActivity", "user123", LogLevel.INFO, "Logged in successfully.");  log(complexMessage);  // 打印所有WARN和ERROR级别的日志，包括新添加的  printLogs(LogLevel.WARN, LogLevel.ERROR);  }  public static void log(LogLevel level, String message) {  logMessages.get(level).add(formatLogMessage(level, message));  }  public static void log(StringBuilder message) {  // INFO级别的日志log(LogLevel.INFO, message.toString());  }  private static String formatLogMessage(LogLevel level, String message) {  DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");  return String.format("[%s] [%s] %s", formatter.format(LocalDateTime.now()), level.name(), message);  }  public static void printLogs(LogLevel... levels) {  for (LogLevel level : levels) {  System.out.println("--- " + level.name() + " LOGS ---");  for (String msg : logMessages.getOrDefault(level, Set.of())) {  System.out.println(msg);  }  if (logMessages.getOrDefault(level, Set.of()).isEmpty()) {  System.out.println("No " + level.name() + " logs found.");  }  }  }  public static StringBuilder buildComplexLogMessage(String category, String user, LogLevel level, String message) {  StringBuilder sb = new StringBuilder();  sb.append("[")  .append(category)  .append("]")  .append(" [")  .append(user)  .append("]")  .append(" [")  .append(level.name())  .append("] ")  .append(message);  // 额外的系统信息  if (level == LogLevel.ERROR) {  sb.append(" [System Info: Memory Usage High]");  }  return sb;  }  
}

运行结果如下：

测试代码3：

package stringbuilder.com;
public class ComplexStringBuilderTest {  @SuppressWarnings("unused")public static void main(String[] args) {  // 用户活动记录中的信息  String userId = "U12345";  String userName = "John Doe";  String activityType = "Login";  String ipAddress = "192.168.1.1";  boolean isSuccess = true;  String errorMessage = null; // 假设登录成功，没有错误信息  String additionalInfo = "User logged in from a trusted IP.";  // 初始化StringBuilder构建日志消息  StringBuilder logMessage = new StringBuilder();  // 添加基础信息  logMessage.append("Log Entry for User ").append(userId).append(" (").append(userName).append("):\n");  logMessage.append("Activity Type: ").append(activityType).append("\n");  logMessage.append("IP Address: ").append(ipAddress).append("\n");  // 根据是否成功添加不同的信息  if (isSuccess) {  logMessage.append("Activity Result: Success\n");  } else {  logMessage.append("Activity Result: Failed\n");  if (errorMessage != null && !errorMessage.isEmpty())logMessage.append("Error Message: ").append(errorMessage).append("\n");  }  // 添加附加信息  if (additionalInfo != null && !additionalInfo.isEmpty()) {  logMessage.append("Additional Information: ").append(additionalInfo).append("\n");  }  // 记录活动发生的时间  String timestamp = "2023-04-01T12:34:56Z";  logMessage.append("Timestamp: ").append(timestamp).append("\n");  // 输出构建的日志消息  System.out.println(logMessage.toString());  }  
}

运行结果如下：

测试代码4:

package stringbuilder.com;
public class OrderInfoBuilder {  public static void main(String[] args) {  // 订单数据  String orderId = "O00123456";  String customerName = "John Doe";  String customerEmail = "john.doe@example.com";  double totalAmount = 199.99;  // 订单包含多个商品  Product product1 = new Product("Product A", 1, 99.99);  Product product2 = new Product("Product B", 2, 50.00);  Product[] products = {product1, product2};  // 使用StringBuilder构建订单信息  StringBuilder orderInfo = buildOrderInfo(orderId, customerName, customerEmail, totalAmount, products);  // 输出订单信息  System.out.println(orderInfo.toString());  }  public static StringBuilder buildOrderInfo(String orderId, String customerName, String customerEmail, double totalAmount, Product[] products) {  StringBuilder sb = new StringBuilder();  // 添加订单基本信息  sb.append("Order Information:\n");  sb.append("Order ID: ").append(orderId).append("\n");  sb.append("Customer Name: ").append(customerName).append("\n");  sb.append("Customer Email: ").append(customerEmail).append("\n");  sb.append("Total Amount: $").append(String.format("%.2f", totalAmount)).append("\n");  // 添加商品列表  sb.append("\nItems in Order:\n");  for (Product product : products) {  sb.append(String.format(" - %s (Quantity: %d, Price: $%.2f)\n",   product.getName(), product.getQuantity(), product.getPrice()));  }  // 订单处理的额外信息  String processingStatus = "Processing";  String shippingAddress = "1234 Main St, Anytown, USA";  // 订单处理状态  sb.append("\nOrder Processing Status: ").append(processingStatus).append("\n");  // 配送地址  if (shippingAddress != null && !shippingAddress.isEmpty()) {  sb.append("Shipping Address: ").append(shippingAddress).append("\n");  }  // 订单备注信息String remarks = ""; // 来自用户输入或数据库  // 订单备注if (remarks != null && !remarks.isEmpty()) {  sb.append("\nOrder Remarks: ").append(remarks).append("\n");  }  // 时间戳  String timestamp = "2023-04-01T12:34:56Z";  sb.append("\nOrder Timestamp: ").append(timestamp).append("\n");  return sb;  }  // 订单中的商品类  static class Product {  private String name;  private int quantity;  private double price;  public Product(String name, int quantity, double price) {  this.name = name;  this.quantity = quantity;  this.price = price;  }  public String getName() {  return name;  }  public int getQuantity() {  return quantity;  }  public double getPrice() {  return price;  }  }  
}

运行结果如下：

2、 StringBuilder与StringBuffer的区别：

测试代码：

package stringbuilder.com;
public class ThreadSafetyDemo {public static void main(String[] args) {//使用StringBuilder（非线程安全）//创建两个线程（thread1和thread2），它们同时尝试向同一个StringBuilder实例追加内容。//由于StringBuilder不是线程安全的，这两个线程可能会相互干扰，导致最终的字符串内容不可预测。//StringBuilder的输出是乱序的。StringBuilder sbNonThreadSafe = new StringBuilder();Thread thread1 = new Thread(() -> {for (int i = 0; i < 1000; i++) {sbNonThreadSafe.append("Thread 1: ");sbNonThreadSafe.append(i);sbNonThreadSafe.append("\n");}});Thread thread2 = new Thread(() -> {for (int i = 0; i < 1000; i++) {sbNonThreadSafe.append("Thread 2: ");sbNonThreadSafe.append(i);sbNonThreadSafe.append("\n");}});thread1.start();thread2.start();try {thread1.join();thread2.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("StringBuilder result (may be unpredictable):");System.out.println(sbNonThreadSafe.toString());// 使用StringBuffer（线程安全）//创建了两个新的线程（thread3和thread4），使用StringBuffer实例。//StringBuffer是线程安全的，即使两个线程同时尝试修改它，最终的字符串内容也将是可预测的。//每个线程将按顺序追加其内容，而不会出现内容错乱的情况。//StringBuffer的输出是有序。StringBuffer sbThreadSafe = new StringBuffer();Thread thread3 = new Thread(() -> {for (int i = 0; i < 1000; i++) {sbThreadSafe.append("Thread 3: ");sbThreadSafe.append(i);sbThreadSafe.append("\n");}});Thread thread4 = new Thread(() -> {for (int i = 0; i < 1000; i++) {sbThreadSafe.append("Thread 4: ");sbThreadSafe.append(i);sbThreadSafe.append("\n");}});thread3.start();thread4.start();try {thread3.join();thread4.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("\nStringBuffer result (predictable):");System.out.println(sbThreadSafe.toString());}
}

运行结果如下：