Java 无参数构造函数

• 建立类时指定了有参数构造函数后，系统默认不会建立无参数构造函数，须要本身手动建立。
• 建立子类的对象实例时，默认会先调用父类的无参数的构造函数（默认构造函数）。
• 若父类未定义无参数构造函数，则在编译阶段报错。
• 若子类指定了父类的有参构造函数，则能够经过编译和运行。

子类声明super(id, city)显示调用父类有参构造函数

class Predessor {
    private int id;
    private String city;
    
    public Predessor(int id, String city) {
        this.id = id;
        this.city = city;
    }

    @Override
    public String toString() {
        return "Predessor [id=" + id + ", city=" + city + "]";
    }
}

class Successor extends Predessor {
    private String name;
    private String sex;
    
    public Successor(String name, String sex, int id, String city) {
        super(id, city);
        this.name = name;
        this.sex = sex;
    }

    @Override
    public String toString() {
        return "Successor [name=" + name + ", sex=" + sex + "]";
    }
}

public class CallConstructorTest {

    public static void main(String[] args) {
        Successor s2 = new Successor("A", "male", 1, "HZ");
        System.out.println(s2);
    }

}

输出

Successor [name=A, sex=male]

父类未定义无参数构造函数，子类不声明super(id, city)

public Successor(String name, String sex) {
    this.name = name;
    this.sex = sex;
}

编译报错

Implicit super constructor Predessor() is undefined. Must explicitly invoke another constructor

父类显示定义无参数构造函数，方法调用的传递性

class Predessor {
    private int id;
    private String city;
    
    public Predessor() {
        System.out.println("调用了 Predessor 无参的构造函数");
    }

    public Predessor(int id, String city) {
        this.id = id;
        this.city = city;
    }

    @Override
    public String toString() {
        return "Predessor [id=" + id + ", city=" + city + "]";
    }
}

class Successor extends Predessor {
    private String name;
    private String sex;
    
    public Successor() {
        System.out.println("调用了 Successor 无参的构造函数");
    }

    public Successor(String name, String sex) {
        this.name = name;
        this.sex = sex;
    }

    @Override
    public String toString() {
        return "Successor [name=" + name + ", sex=" + sex + "]";
    }
}

public class CallConstructorTest {

    public static void main(String[] args) {
        Successor s = new Successor();
        System.out.println(s);
        System.out.println("--------------");
        Successor s2 = new Successor("A", "male");
        System.out.println(s2);
    }

}

输出

调用了 Predessor 无参的构造函数
调用了 Successor 无参的构造函数
Successor [name=null, sex=null]
--------------
调用了 Predessor 无参的构造函数
Successor [name=A, sex=male]