无参构造器

1、子类没有构造函数时，自动调用父类的无参构造函数

public class Animal {
    Animal(){
        System.out.println("Animal NoArgsConstructor");
    }

    Animal(int a){
        System.out.println("Animal args construction");
    }
}

public class Bird extends Animal {
    public static void main(String[] args) {
        Animal animal = new Bird();
    }
}

运行结果：Animal NoArgsConstructor

2、子类显示指定构造函数时，也会自动调用父类构造方法

public class Bird extends Animal {
    Bird() {
        System.out.println("Bird NoArgsConstructor");
    }

    public static void main(String[] args) {
        Animal animal = new Bird();
    }
}

class Animal {
    Animal(){
        System.out.println("Animal NoArgsConstructor");
    }

    Animal(int a){
        System.out.println("Animal args construction");
    }
}

运行结果：Animal NoArgsConstructor
Bird NoArgsConstructor

3、父类没有无参构造函数时，子类缺省无参构造函数或者指定无参构造函数都会报错

报错情况1：

public class Bird extends Animal {
    Bird() {
        System.out.println("Bird NoArgsConstructor");
    }

    public static void main(String[] args) {
        Animal animal = new Bird();
    }
}

class Animal {

    Animal(int a){
        System.out.println("Animal args construction");
    }
}

报错情况2：

public class Bird extends Animal {
    
    public static void main(String[] args) {
        Animal animal = new Bird();
    }
}

class Animal {

    Animal(int a){
        System.out.println("Animal args construction");
    }
}