什么是Java中的构造函数，它的作用和特点是什么？

public class Car {
    private String color;
    private int speed;
<pre class='brush:php;toolbar:false;'>// 构造函数
public Car(String color, int speed) {
    this.color = color;
    this.speed = speed;
}

// 其他方法...

public class Person {
    private String name;
    private int age;
<pre class='brush:php;toolbar:false;'>// 构造函数
public Person(String name, int age) {
    this.name = name;
    this.age = age;
}

public void introduce() {
    System.out.println("My name is " + name + " and I am " + age + " years old.");
}

public static void main(String[] args) {
    Person person = new Person("Alice", 30);
    person.introduce(); // 输出: My name is Alice and I am 30 years old.
}

public class BankAccount {
    private String accountNumber;
    private double balance;
    private String accountType;
<pre class='brush:php;toolbar:false;'>// 构造函数
public BankAccount(String accountNumber, double initialBalance, String accountType) {
    this.accountNumber = accountNumber;
    this.balance = initialBalance;
    this.accountType = accountType;

    if (accountType.equals("Savings")) {
        balance += 50; // 储蓄账户初始奖励
    } else if (accountType.equals("Checking")) {
        balance += 20; // 支票账户初始奖励
    }
}

public void displayBalance() {
    System.out.println("Account " + accountNumber + " has a balance of $" + balance);
}

public static void main(String[] args) {
    BankAccount savingsAccount = new BankAccount("12345", 1000, "Savings");
    BankAccount checkingAccount = new BankAccount("67890", 500, "Checking");

    savingsAccount.displayBalance(); // 输出: Account 12345 has a balance of $1050.0
    checkingAccount.displayBalance(); // 输出: Account 67890 has a balance of $520.0
}

public class User {
    private String username;
    private String email;
<pre class='brush:php;toolbar:false;'>// 无参构造函数
public User() {
    this.username = "default";
    this.email = "default@example.com";
}

// 有参构造函数
public User(String username, String email) {
    this.username = username;
    this.email = email;
}

public static void main(String[] args) {
    User user1 = new User();
    User user2 = new User("john", "john@example.com");

    System.out.println(user1.username + " - " + user1.email); // 输出: default - default@example.com
    System.out.println(user2.username + " - " + user2.email); // 输出: john - john@example.com
}