創建類時指定了有參數構造函數後，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
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]
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