解析java中的继承

1 含义

在符合当代社会的约定俗成的逻辑的情况下,很多类具有相同的属性和方法,我们这些共有的属性和方法抽象成一个类后,通过extends关键字使得很多类与抽象成的类建立父子关系,其中很多类称为子类,抽象出来的类叫做父类。

2 为什么需要继承?

2.1 减少代码的冗余,提高复用性

例如Chinese类和Russian类都有name、sex、age这三个属性,

那么如果每个类都写的话,就是如下所示的代码

Chinese类

public class Chinese{private String name;private String sex;private int age;public Chinese() {}public Chinese(String name, String sex, int age) {this.name = name;this.sex = sex;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

Russian类

public class Russian {private String name;private String sex;private int age;public Russian() {}public Russian(String name, String sex, int age) {this.name = name;this.sex = sex;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

我们不难发现这两个类里面的一些属性其实是公共的,可以抽象成一个People类

改进后的代码如下所示

Chinese类

public class Chinese extends Peple{}

Russian类

public class Russian extends Peple{}

People类

public class Peple {private String name;private String sex;private int age;public Peple() {}public Peple(String name, String sex, int age) {this.name = name;this.sex = sex;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

我们可以发现此时的Chinese类和Russian类清爽多了,咱们也可以通过set和get方法去写入和读取它

2.2 便于功能的扩展

例如你想对某些类加上一个功能,没学继承之前你可能得这么干

Chinese类

public class Chinese {private String name;private String sex;private int age;//你想要增加weight体重这个属性,你得在Chinese里面增加weight这个属性private int weight;public Chinese() {}public Chinese(String name, String sex, int age, int weight) {this.name = name;this.sex = sex;this.age = age;this.weight = weight;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
//你得增加Chinese类中增加和修改体重的方法public int getWeight() {return weight;}public void setWeight(int weight) {this.weight = weight;}
}

Russian类

public class Russian{private String name;private String sex;private int age;//你想要增加weight体重这个属性,你得在Russsian里面增加weight这个属性private int weight;public Russian() {}public Russian(String name, String sex, int age, int weight) {this.name = name;this.sex = sex;this.age = age;this.weight = weight;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}//你得增加Russian类中增加和修改体重的方法public int getWeight() {return weight;}public void setWeight(int weight) {this.weight = weight;}
}

你会发现当需要增加一个属性时,你得将每个需要加上的类中加上这个属性以及它的set/get方法,非常的麻烦,当我们使用继承后,改进后的代码如下所示

Chinese类

public class Chinese extends Peple{}

Russian类

public class Russian extends Peple{}

People类

public class Peple {private String name;private String sex;private int age;//你想要增加weight体重这个属性,只需要在Chinese类和Russian类的父类去增加属性即可private int weight;public Peple() {}public Peple(String name, String sex, int age,int weight) {this.name = name;this.sex = sex;this.age = age;this.weight=weight;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}//你只需要在People里面写一次get/Set方法就行,这样Chinese类和Russian类都可以调用public int getWeight() {return weight;}public void setWeight(int weight) {this.weight = weight;}
}

这样去增加和修改共有的属性是很方便的

2.3 为多态性的使用,提供了前提

3 如何使用继承

3.1 判断两个类之间是否存在继承关系的方法

a.判断两个类是否存在逻辑层面上面的父子关系

例如Dog类和People类,显然两者之间不存在逻辑层面的上的父子关系,人不可能是狗的亲生父亲,狗也不可能是人的亲生父亲。

b.公式法: A类 is B类或者B类 is A类是否成立,其中后者为父类,前者为子类

例如Dog类和People类,既不满足Dog is People,也不满足People is Dog 而Animal类和Dog类,满足Dog is Animal,因此Animal类和Dog类存在继承关系,其中Animal是父类,Dog是子类

3.2 语法

class A extends B{//其中A类为子类,派生类//B类为父类,超类,基类
}

3.3 规则

3.3.1 子类继承父类,调用构造方法时,一定会需要先调上父类的其中的任意一个构造方法,再调用自己的构造方法,若子类构造方法里面没有调用父类的,那么就会报错。注意:子类无参会默认调用父类无参构造的(默认会使用super()