Java 無參數構造函數的應用

2024-07-14 08:40:20

字體：大中小

來源：轉載

供稿：網友

創建類時指定了有參數構造函數后，系統默認不會創建無參數構造函數，需要自己手動創建。

創建子類的對象實例時，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，則可以通過編譯和運行。

子類聲明super(id, city)顯示調用父類有參構造函數

package cn.lw.testpkg;/** * @author wanglei 2018年4月18日 */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

package cn.lw.testpkg;/** * @author wanglei 2018年4月18日 */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