设计原则之里氏替换原则(实例加UML完整例子)

里氏替换原则

1.定义

  子类可以实现父类的抽象方法&＃xff0c;但不能覆盖父类的非抽象方法。
  子类中可以增加自己特有的方法

1.2 UML类图

1.3 设计背景

  有时候父类有多个子类&＃xff0c;但在这些子类中有一个特例&＃xff0c;如果不遵循里氏替换原则&＃xff0c;重写了父类的方法&＃xff0c;虽然实现了功能&＃xff0c;会整个继承体系的可复用性会比较差&＃xff0c;比如B继承了A&＃xff0c;B重写了A的fun1()方法&＃xff0c;C又继承了B&＃xff0c;这个时候C使用fun1()方法&＃xff0c;但使用的确是B的fun1()&＃xff0c;而不是A的fun1()方法。按照继承设计的作用&＃xff0c;子类是可以继承父类的方法和成员&＃xff0c;减小代码冗余&＃xff0c;增加复用。但是因为重写导致了复用性降低&＃xff0c;C本来是想使用到A的fun1()方法&＃xff0c;但是确没有使用到。

public class A
{void fun1(int a, int b) {int result &＃61; a-b;System.out.println(a &＃43; " - " &＃43; b &＃43; " &＃61; " &＃43; result);}
}

public class B extends A
{/*** 子类重写了父类的fun1方法*/void fun1(int a, int b){int result &＃61; a&＃43;b;System.out.println(a &＃43; " &＃43; " &＃43; b &＃43; " &＃61; " &＃43; result);}void fun2(int a, int b){int result &＃61; a*b;System.out.println(a &＃43; " * " &＃43; b &＃43; " &＃61; " &＃43; result);}
}

public class C extends B
{void fun3(int a, int b){int result &＃61; a/b;System.out.println(a &＃43; " / " &＃43; b &＃43; " &＃61; " &＃43; result);}
}

public class Test
{public static void main(String[] args) {A a &＃61; new A();a.fun1(12, 4); //12 - 4 &＃61; 8B b &＃61; new B();b.fun1(12, 4);//12 &＃43; 4 &＃61; 16C c &＃61; new C();c.fun1(12, 4);//12 &＃43; 4 &＃61; 16A ab &＃61; new B();ab.fun1(12, 5);//12 &＃43; 5 &＃61; 17 运行时多态}
}

1.4 实现思路

  如果既想满足里氏替换原则&＃xff0c;又想满足这个子类的功能时&＃xff0c;可以在子类中可以增加自己特有的方法fun2,当然也可以不走继承&＃xff0c;使用关联&＃xff0c;聚合&＃xff0c;组合来解决问题。

1.5 实现场景

public class A
{public void fun1(int a, int b) {int result &＃61; a-b;System.out.println(a &＃43; " - " &＃43; b &＃43; " &＃61; " &＃43; result);}
}

public class A1
{public void fun1(int a, int b){A aObject &＃61; new A();//使用依赖关系 让A1可以使用A的方法aObject.fun1(a,b);}public void fun2(int a, int b){int result &＃61; a&＃43;b;System.out.println(a &＃43; " &＃43; " &＃43; b &＃43; " &＃61; " &＃43; result);}
}


public class B extends A
{/*** &＃64;param a* &＃64;param b* 增加加自己特有的方法fun2*/public void fun2(int a, int b){int result &＃61; a&＃43;b;System.out.println(a &＃43; " &＃43; " &＃43; b &＃43; " &＃61; " &＃43; result);}}

public class Test
{public static void main(String[] args) {A a &＃61; new A();a.fun1(12, 4); //12 - 4 &＃61; 8B b &＃61; new B();b.fun1(12, 4);//12 - 4 &＃61; 8b.fun2(12, 4);//12 &＃43; 4 &＃61; 16A1 a1 &＃61; new A1();a1.fun1(12, 4);//12 - 4 &＃61; 8a1.fun2(12, 4);//12 &＃43; 4 &＃61; 16}
}