java继承
1.什么是继承 是一种java的机制,子类可以继承父类的所有属性和方法;当子类继承父类时。可以重复使用父类中的属性和方法,也可以添加新的属性和方法, 示例
class B{
……
}
}
class A extends B{
……
}
}
extends关键字用于继承,中文意思延伸,可以理解为,class A extends B;表示A 是B的延伸 ,含义是添加新的功能,在java中被继承的类称为父类或超类,新类叫做子类 上例代码中,A是父类,B是子类。
2.继承的优点 可以重用代码,提高代码的重用性 3.分类, 单继承,多级继承,分层继承 ①单一继承
class Animals{
void output(){
System.out.println("this is a animal");
}
}
class Dog extends Animals {
void output1(){
System.out.println("this is a dog");
}
}
public class ex {
public static void main(String[] args) {
Dog s1 = new Dog();
s1.output();
s1.output1();
}
}
结果
this is a animal
this is a dog
Process finished with exit code 0
②多级继承
class Animals{
void output(){
System.out.println("this is a animal");
}
}
class Dog extends Animals {
void output1(){
System.out.println("this is a dog");
}
}
class Dogbaby extends Dog{
void output2(){
System.out.println("this is a Dogbaby");
}
}
public class ex {
public static void main(String[] args) {
Dogbaby s1 = new Dogbaby();
s1.output();
s1.output1();
s1.output2();
}
}
结果
this is a animal
this is a dog
this is a Dogbaby
Process finished with exit code 0
分层继承
class Animals{
void output(){
System.out.println("this is a animal");
}
}
class Dog extends Animals {
void output1(){
System.out.println("this is a dog");
}
}
class Cat extends Animals{
void output2(){
System.out.println("this is a cat");
}
}
public class ex {
public static void main(String[] args) {
Dog s1 = new Dog ();
Cat s2 = new Cat();
s1.output();
s1.output1();
s2.output();
s2.output2();
}
}
结果
this is a animal
this is a dog
this is a animal
this is a cat
Process finished with exit code 0
4,java中的类不支持多继承 多继承:指一个类继承多个类的特性,可以理解为一个父类可以产生许多子类(分层继承),但是一个子类不能有许多个父类产生,(这里我的理解就好比高中数学里面的映射,可以一对一,一对多,但是不能多对一) java中不能对继承的原因 为了编程语言简单化,少出错,假设有三个类分别A,B,C,B是A父类,C也是A的父类,如果C,B类中有同名的属性或者方法,当A类的对象调用这个同名方法时会存在歧义,又因为编译错误总比运行时错误要好的多如若继承两个类,编译器就会报错。 错误代码:
class Animals{
void output(){
System.out.println("this is a animal");
}
}
class Dog {
void output1(){
System.out.println("this is a dog");
}
}
class Dogbaby extends Animals{
void output2(){
System.out.println("this is a Dogbaby1");
}
}
class Dogbaby extends Dog{
void output3(){
System.out.println("this is a Dogbaby2");
}
}
public class ex {
public static void main(String[] args) {
Dogbaby s1 = new Dogbaby ();
s1.output();
s1.output2();
}
}
编译错误
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