創(chuàng)建類時(shí)指定了有參數(shù)構(gòu)造函數(shù)后，系統(tǒng)默認(rèn)不會(huì)創(chuàng)建無(wú)參數(shù)構(gòu)造函數(shù)，需要自己手動(dòng)創(chuàng)建。
創(chuàng)建子類的對(duì)象實(shí)例時(shí)，默認(rèn)會(huì)先調(diào)用父類的無(wú)參數(shù)的構(gòu)造函數(shù)（默認(rèn)構(gòu)造函數(shù)）。
若父類未定義無(wú)參數(shù)構(gòu)造函數(shù)，則在編譯階段報(bào)錯(cuò)。
若子類指定了父類的有參構(gòu)造函數(shù)，則可以通過(guò)編譯和運(yùn)行。
子類聲明super(id, city)顯示調(diào)用父類有參構(gòu)造函數(shù)
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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(wú)參數(shù)構(gòu)造函數(shù)，子類不聲明super(id, city)
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public successor(string name, string sex) { this.name = name; this.sex = sex;} |
編譯報(bào)錯(cuò)
implicit super constructor predessor() is undefined. must explicitly invoke another constructor
父類顯示定義無(wú)參數(shù)構(gòu)造函數(shù)，方法調(diào)用的傳遞性
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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(diào)用了 predessor 無(wú)參的構(gòu)造函數(shù)"); } 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(diào)用了 successor 無(wú)參的構(gòu)造函數(shù)"); } 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(diào)用了 predessor 無(wú)參的構(gòu)造函數(shù)
調(diào)用了 successor 無(wú)參的構(gòu)造函數(shù)
successor [name=null, sex=null]
--------------
調(diào)用了 predessor 無(wú)參的構(gòu)造函數(shù)
successor [name=a, sex=male]
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原文鏈接：https://segmentfault.com/a/1190000014449811
