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Java 构造方法

在 Java 中，构造方法（Constructor）是用于创建类的对象的特殊方法。

当使用 new 关键字创建对象时，构造方法会自动调用，用来初始化对象的属性。

构造方法具有以下几个特点：

• 与类名相同：构造方法的名称必须与类名完全一致，包括大小写。这是构造方法的一个基本要求。

• 没有返回类型：构造方法没有返回类型声明，即使是void也不写。这使得它与普通方法区分开来。

• 自动调用：每次使用new创建对象时，构造方法会自动调用，以初始化对象的属性和状态。

• 不能直接调用：构造方法只能通过new关键字在创建对象时调用，不能像普通方法那样直接调用。

• 支持重载：可以为一个类定义多个构造方法，只要它们的参数列表不同。通过重载，可以创建不同的构造方法以适应不同的初始化需求。

• 默认构造方法：如果没有定义任何构造方法，Java 会提供一个无参的默认构造方法。但一旦定义了任何其他构造方法，Java 不再提供默认构造方法。

• this关键字的使用：在构造方法中可以使用this来引用当前对象的属性、方法，或调用另一个构造方法（必须是构造方法的第一行），以避免重复代码。

• 不能被继承，但可以被调用：构造方法不能被子类继承，但子类可以使用super()来调用父类的构造方法，以便初始化继承的属性。

• 对象初始化保障：构造方法的主要作用是初始化对象的属性和状态，保证对象在创建时处于一个合法的初始状态。

构造方法的作用：

• 初始化对象属性：构造方法的主要作用是为对象的属性赋初值。
• 保证对象初始化的完整性：在构造方法中可以设置默认值或必要参数，从而避免对象未完全初始化的问题。

构造方法的类型

Java 中的构造方法分为两种类型：无参构造方法和有参构造方法。

1、无参构造方法（默认构造方法）

如果一个类中没有定义任何构造方法，Java 会默认提供一个无参构造方法。例如：

实例

public class Person {
public Person() {
System.out.println("Person对象已创建");
}
}

• 在没有显式定义构造方法时，Java 自动提供一个默认的无参构造方法。
• 一旦定义了其他构造方法，Java 将不再提供默认构造方法。

2、有参构造方法

可以定义带有参数的构造方法，用来在创建对象时为属性赋值：

实例

public class Person {
String name;
int age;
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public Person(String name, int age) {
this.name = name;
this.age = age;
}
}

调用有参构造方法时，可以为对象的属性进行初始化：

Person p = new Person("Alice", 25);

构造方法的重载

Java 支持构造方法的重载，即可以在同一个类中定义多个构造方法，只要参数列表不同即可。例如：

实例

public class Person {
String name;
int age;

public Person() {
this.name = "Unknown";
this.age = 0;
}

public Person(String name) {
this.name = name;
this.age = 0;
}

public Person(String name, int age) {
this.name = name;
this.age = age;
}
}

创建对象时，Java 会根据传入的参数数量和类型自动选择匹配的构造方法：

Person p1 = new Person(); // 调用无参构造方法 Person p2 = new Person("Alice"); // 调用单参数构造方法 Person p3 = new Person("Bob", 30); // 调用双参数构造方法

构造方法中的 this 关键字

在构造方法中，this 关键字通常用于两种情况：

1、引用当前对象的属性或方法：当构造方法的参数名与类属性名相同时，使用 this 来区分类属性和参数。例如：

实例

public Person(String name, int age) {
this.name = name; // this.name 表示类的属性
this.age = age;
}

2、调用另一个构造方法：可以使用 this() 调用当前类的其他构造方法，常用于避免重复代码，但必须放在构造方法的第一行。

实例

public Person(String name) {
this(name, 0); // 调用另一个双参数的构造方法
}

public Person(String name, int age) {
this.name = name;
this.age = age;
}