Java 无参数构造函数的应用

2024-07-14 08:40:20

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创建类时指定了有参数构造函数后，系统默认不会创建无参数构造函数，需要自己手动创建。

创建子类的对象实例时，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，则可以通过编译和运行。

子类声明super(id, city)显示调用父类有参构造函数

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);  }}

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

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("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