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thead { display: table-header-group; }
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异常处理（Throwable）和继承

Exception类是Throwable类的子类

Throwable类的主要方法

自定义异常类继承于Exception类，通过throw new Exception手动抛出异常

Java不支持多继承

继承的特性：

  • 子类拥有父类非 private 的属性、方法。
  • 子类可以拥有自己的属性和方法，即子类可以对父类进行扩展。
  • 子类可以用自己的方式实现父类的方法。
  • Java 的继承是单继承，但是可以多重继承，单继承就是一个子类只能继承一个父类，多重继承就是，例如 A 类继承 B 类，B 类继承 C 类，所以按照关系就是 C 类是 B 类的父类，B 类是 A 类的父类，这是 Java 继承区别于 C++ 继承的一个特性。
  • 提高了类之间的耦合性（继承的缺点，耦合度高就会造成代码之间的联系越紧密，代码独立性越差）。

继承调用父子类构造函数顺序问题

子类是不继承父类的构造器（构造方法或者构造函数）的，它只是调用（隐式或显式）。如果父类的构造器带有参数，则必须在子类的构造器中显式地通过 super 关键字调用父类的构造器并配以适当的参数列表。

如果父类构造器没有参数，则在子类的构造器中不需要使用 super 关键字调用父类构造器，系统会自动调用父类的无参构造器。

  • 子类构造器自动调用父类构造器，先运行父类构造器的内容再运行子类构造器的内容。
  • 父类带有参数的构造器通过super才能调用，否则自动调用父类无参构造器。
    • class SuperClass {
      
  private int n;
      
  SuperClass(){
      
    System.out.println("SuperClass()");
      
  }
      
  SuperClass(int n) {
      
    System.out.println("SuperClass(int n)");
      
    this.n = n;
      
  }
      
}
      
// SubClass 类继承
      
class SubClass extends SuperClass{
      
  private int n;

  SubClass(){ // 自动调用父类的无参数构造器
      
    System.out.println("SubClass");
      
  }  

  public SubClass(int n){
      
    super(300);  // 调用父类中带有参数的构造器
      
    System.out.println("SubClass(int n):"+n);
      
    this.n = n;
      
  }
      
}
      
// SubClass2 类继承
      
class SubClass2 extends SuperClass{
      
  private int n;

  SubClass2(){
      
    super(300);  // 调用父类中带有参数的构造器
      
    System.out.println("SubClass2");
      
  }  

  public SubClass2(int n){ // 自动调用父类的无参数构造器
      
    System.out.println("SubClass2(int n):"+n);
      
    this.n = n;
      
  }
      
}
      
public class TestSuperSub{
      
  public static void main (String args[]){
      
    System.out.println("------SubClass 类继承------");
      
    SubClass sc1 = new SubClass();
      
    SubClass sc2 = new SubClass(100);
      
    System.out.println("------SubClass2 类继承------");
      
    SubClass2 sc3 = new SubClass2();
      
    SubClass2 sc4 = new SubClass2(200);
      
  }
      
}

  • 输出结果：

------SubClass 类继承------

SuperClass()

SubClass

SuperClass(int n)

SubClass(int n):100

------SubClass2 类继承------

SuperClass(int n)

SubClass2

SuperClass()

SubClass2(int n):200

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