Li Kou brush question diary /day2/2022.6.24

Novice Village

Today, I'm still learning about the concept of array , character (String) String array , The creation method is similar to the general array , Just pay attention to the different types

One .List As an interface , Interface cannot be instantiated

List list=new List(); 
// Will cause compilation errors , This is because List It's an interface , Interface cannot be instantiated .

ArrayList list=new ArrayList();
// This instantiation method is correct . This is because ArrayList Is a class , Inherit and implement List Interface .

How to implement an interface ( With List Interface, for example )

Instantiate the object through the implementation class inherited from this interface List object , Such as

List list=new ArrayList();

This way of instantiation ： Created a ArrayList After implementing the object of the class, trace it up to List Interface , Now it's a List Object , There are some ArrayList Yes, but List No attributes and methods , It can't be used anymore .

 ArrayList list=new ArrayList();

The above method creates an object and retains ArrayList All properties and methods of .

Comparison of two creation methods

In most cases , What we use is to create a ArrayList The object that implements the class then rises to List Interface , Because the object created by this method is more convenient ,List Interface has multiple implementation classes , Now it's ArrayList, If you want to replace it with other implementation classes , Such as LinkedList perhaps Vector wait , At this time, we just need to change this line ： List list = new LinkedList(); Others use list The local code doesn't need to be changed at all . When writing code , More convenient .

But you can only call List Methods in interfaces , Cannot call ArrayList The method in .

List list=new ArrayList();

  Two .JAVA String.valueOf() Method instructions

From basic data type to String

int i = 10;
String str = String.valueOf(i);

At this time str It would be “10” 

String.valueOf(int i) and Integer.toString(int i) What's the difference? ？

String.valueOf() It can be JAVA Basic types (int,double,boolean etc. ) And the object (Object) convert to String type
toString() It's the method of the object , It can convert this object into String type , The conversion algorithm depends on the actual needs of the type , Basically JAVA Every object in it will have one toString Method .

Of the two The running results are the same , But the principle is different

String.valueOf() It can be applied to any data type , And there will be no abnormal report . 
Integer.toString() Say first int convert to Integer type , And then Integer convert to String type .

Add toString Method

//toString():  Return to indicate  Integer  It's worth it  String  object .
//toString(int i):  Return indicates that the specified  int  Of  String  object 

 

 

commonly String.valueOf() Most of them are used , There is no limit to the type of data applied , And no null pointer exception will be reported

3、 ... and . There are two ways to create an array of strings

​// Create an array of strings , This method is more flexible 
ArrayList<String> answer = new ArrayList<String>();
// Add string 
answer.add("BUFF");
//java in size（） Methods are for generic collections , If you want to know how many elements a generic type has , Call this method to see .
answer.size();

// I saw it on Baidu , A very rigid way of writing 
String [] answer = new String [20];
// Add an array 
answer[0] = "str1";
//java in length() Is for String Speaking of , If you want to see the length of the string , Then use length() Method ;
answer.length()

  Four . Example

  Their thinking : First create an array object (list), Re pass for Traversal adds a string to the array , If the above requirements are met , be add, If it doesn't fit , Just convert the integer String.valueOf Add to the array as a string

class Solution {
    public List<String> fizzBuzz(int n) {
        List<String> answer = new ArrayList<String>();
        for(int i=1;i<=n;i++){
            if(i%3==0&&i%5==0){
                answer.add("FizzBuzz");
            }
            else if(i%3==0){
                answer.add("Fizz");
            }
            else if(i%5==0){
                answer.add("Buzz");
            }
            else{
                answer.add(String.valueOf(i));
            }
        }
        return answer;
    }
}