Java learning notes-inheritance and polymorphism (lower), java learning notes

Java learning notes-inheritance and polymorphism (lower), java learning notes

1. Develop superclass through inheritance)

2. Use the super keyword to evoke the constructor of superclasses

3. overwrite the method in the superclass

4. differentiate override and overload

5. Explore the toString () Class in the Object class

6. Discovery of polymorphism and dynamic binding

7. Explain why downward transformation is necessary

8. Explore the equals method in the Object class

9. storage, retrieval, and implementation of ArrayList objects

10. Use the ArrayList class to implement Stack

11. Use data and methods in the superclass, protected

12. Use the final module to disable coverage of classes and Methods

 

9. storage, retrieval, and implementation of ArrayList objects

ArrayList class, which is useful for storing objects. You can create an array to store objects. However, once an array is created, its size is fixed. Java provides the ArrayList class to store unlimited objects.

ArrayList Method Area

ArrayList ()	Create an empty class
Add (o: Object): void	Add an element to the end of the list
Add (index: int, o: Object): void	Add objects to a special position in the list
Clear (): void	Clear list
Contains (o: Object): boolean	Include an object?
Get (index: int): Object	Returns the element at a certain position.
IndexOf (o: Object): int	Returns the position of an element.
IsEmpty (): boolean	List of null values
LastIndexOf (o: Object): int	Location of the last object
Remove (o: Object): boolean	Match the element and remove it from the table
Size (): int	Table size
Set (index: int, o: Object): Object	Set elements at a certain position

TestArrayList. java

public class TestArrayList {    public static void main(String[] args){        java.util.ArrayList cityList = new java.util.ArrayList();                cityList.add("Beijing");        cityList.add("Shanghai");        cityList.add("Tianjin");        cityList.add("Xiamen");        cityList.add("Fuzhou");        System.out.println("List Size?"+cityList.size());        System.out.println("Is Guangzhou in the list ? The answer is "+cityList.contains("Guangzhou"));        System.out.println("The location of Shanghai in the list? The answer is "+cityList.indexOf("Shanghai"));        System.out.println("Is the list is empty? The answer is "+ cityList.isEmpty());                cityList.add(2, "Hangzhou");        cityList.remove("Tianjin");                cityList.remove(1);        System.out.println(cityList.toString());                for(int i = cityList.size()-1 ;i>=0;i--)            System.out.println(cityList.get(i)+" ");    }}

The result is as follows:

This object is used as a string array. if the object is stored?

Add the following code

list.add(new Circle(1, 2.5));list.add(new Circle(2, 15.5)); for (int i = 0 ; i<2;i++)   System.out.println("The area of the circle"+((Circle) list.get(i)).number() + " is + "+ (Circle)list.get(i).GetArea() );

The compiler reports an error when no type conversion is performed.

For more information about type conversion, see previous notes.

10. Use the ArrayList class to implement Stack

With regard to Stack usage, Stack is also a linear table, but this linear table is quite special and can only be inserted and popped at the end of the table, and the above example is changed, pushed and pop, you only need to call the corresponding method in the push method.

Data Region

Save object

Size

Method Area

Push: Find the end of the team and insert it.

Pop: locate the team end, get (index: int): Object, and delete the team end + remove (index: int): boolean

GetTop: Follow the method of the ArrayList class to modify it.

11. Use data and methods in the superclass, protected

If you want a subclass to obtain the data and methods of the parent class, non-Child classes cannot access the data and methods.

Public private protected

Public region blocks can be modified. The pirvate region block subclass cannot be accessed directly. The protected region blocks are visible but cannot be changed.

Visibility of data and methods

	Other classes	Same package	Subclass	Different packages
Public	True	True	True	True
Protected	True	True	True	--
Defaul	True	True	--	--
Private	True	--	--	--

 

 

 

 

12. Use the final module to disable coverage of classes and Methods

Final keywords

　　

public fina class c{.....}

public class Test{  public final  void main(){          .......    }}