Java 無參數構造函數

• 建立類時指定了有參數構造函數後，系統默認不會建立無參數構造函數，須要本身手動建立。
• 建立子類的對象實例時，默認會先調用父類的無參數的構造函數（默認構造函數）。
• 若父類未定義無參數構造函數，則在編譯階段報錯。
• 若子類指定了父類的有參構造函數，則能夠經過編譯和運行。

子類聲明super(id, city)顯示調用父類有參構造函數

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);
    }

}

輸出

Successor [name=A, sex=male]

父類未定義無參數構造函數，子類不聲明super(id, city)

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

編譯報錯

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

父類顯示定義無參數構造函數，方法調用的傳遞性

class Predessor {
    private int id;
    private String city;
    
    public Predessor() {
        System.out.println("調用了 Predessor 無參的構造函數");
    }

    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("調用了 Successor 無參的構造函數");
    }

    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);
    }

}

輸出

調用了 Predessor 無參的構造函數
調用了 Successor 無參的構造函數
Successor [name=null, sex=null]
--------------
調用了 Predessor 無參的構造函數
Successor [name=A, sex=male]