父类写无参构造，子类不写构造，实例化子类，会同时调用父类构造方法

public class Father {private String name;private int age;public Father() {System.out.println("父类无参构造");}}
public class Son extends Father {private String name;private int age;
}
public static void main(String[] args) {Son son = new Son();}

父类写无参构造，子类无参构造，实例化子类，会先调用父类构造方法，再调用子类构造

public class Father {private String name;private int age;public Father() {System.out.println("父类无参构造");}}
public class Son extends Father {private String name;private int age;public Son() {System.out.println("子类无参构造");}
}
public static void main(String[] args) {Son son = new Son();}

父类没有无参构造，子类必须明确指定调用父类的哪个构造函数，并在子类的构造函数中使用 super 关键字调用。并且super必须在构造函数第一行，否则编译报错

public class Father {private String name;private int age;public Father(String name, int age) {this.name = name;this.age = age;System.out.println("父类有参构造");}}