Inner class and API, exception

Inner class

• Access characteristics of internal classes
  Internal classes can directly access members of external classes, including private classes
  An object must be created for an external class to access members of an internal class
• The definition location of the class inside the member

In a class, a method is in the same place as a member variable

• External member internal class format

Format: external class name Internal class name object name = external class object Internal class objects;

Example: outer Inner oi = new Outer(). new Inner();

• Recommended usage scheme of inner class of member

For the purpose of designing a class as an internal class, most of them do not want to be accessed by the outside world, so the definition of the internal class should be privatized. After privatization, a method that can be called by the outside world is provided. The internal class object is created and called inside the method.

The biggest difference between c + + internal class and java internal class is that the internal class object of c + + has no pointer to the external class object and cannot access the non static members of the external class object; java's non static internal class object has a pointer to the external class object, which can access the non static members of the external class object.
There are multiple inner classes in java, as well as anonymous inner classes.

Local class (less used)

• **Local internal class definition location**

A local inner class is a class defined in a method

• Local internal class mode

Local internal classes cannot be used directly by the outside world. You need to create objects inside the method and use them

This class can directly access members of external classes or local variables in methods

Anonymous Inner Class

• Premise of anonymous inner class

There is a class or interface, where the class can be concrete or abstract

• Format of anonymous inner class

Format: new class name () {rewrite method} new interface name () {rewrite method}

new Inter()
{ 
@Override 
public void method(){} 
}

• The nature of anonymous inner classes

Essence: it is an anonymous object that inherits the class or implements the subclass of the interface

You can call the parent class member method through the anonymous object, and you can also receive the object in a polymorphic way

Anonymous inner classes can be accepted in the form of polymorphism

Inter i = new Inter(){ @Override public void method(){ } }

Anonymous inner classes call methods directly

interface Inter
{ void method(); }

class Test
{ 
public static void main(String[] args){
 new Inter(){ 
 @Override 
 public void method()
 { System.out.println("I am an anonymous inner class"); }
  }.method(); // Direct call method}
 }

Use of anonymous inner classes in development

When a method needs a subclass object of an interface or abstract class, we can pass an anonymous inner class to simplify the traditional code

System class

public static void exit​(int status)
Terminate the current java virtual machine. Non zero indicates abnormal termination

public static long currentTimeMillis() returns the current time in milliseconds

Basic type packing class

Integer to string:Also available String.valueof

String str = Integer.toString(i);

String to integer:It can also be used parseInt method

int i = Integer.valueOf(str).intValue();

double Type, please see Double method

toLowerCase，toUpperCase Convert case

Date class

Date​()
Allocate a Date object and initialize it so that it represents the allocated time, in the nearest millisecond.

boolean after​(Date when) Tests whether this date is after the specified date.  
boolean before​(Date when) Test whether this date is before the specified date.  
Object clone​() Returns a copy of this object.  
int compareTo​(Date anotherDate) Compare two dates to order.  
boolean equals​(Object obj) Compare the two dates to be equal.  
static Date from​(Instant instant) from Instant Object acquisition Date An example. 

case

 SimpleDateFormat sdf = new SimpleDateFormat();
        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy year M month d day");
        Date date = new Date();
        String format = sdf1.format(date);
        System.out.println(format);
2022 January 14

Implementation of date tool class

public class data {
    private data() { };
    public static String ToString(Date d, String format) {
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        return sdf.format(d);
    }
    public static Date todata(String S, String format) throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        return sdf.parse(S);
    }
}

Calendar Class

static Calendar getInstance​() Gets the calendar using the default time zone and locale.  
static Calendar getInstance​(Locale aLocale) Gets the calendar using the default time zone and the specified locale.  
static Calendar getInstance​(TimeZone zone) Gets the calendar using the specified time zone and default locale.  
static Calendar getInstance​(TimeZone zone, Locale aLocale) Gets the calendar with the specified time zone and locale 


abstract void add​(int field, int amount) Add or subtract the specified amount of time from the given calendar field according to the rules of the calendar.  
boolean after​(Object when) return Calendar Indicates whether the specified Object Represents the time after the time of.  
boolean before​(Object when) Return to this Calendar Indicates whether the specified Object Represents the time before the time of. 

February day case

public class hello {
    public static void main(String[] args)  {
        Calendar calendar = Calendar.getInstance();
        System.out.println("Please enter the year to query:");
        Scanner Sc = new Scanner(System.in);
        int number=Sc.nextInt();
        calendar.set(number,2,1);
        calendar.add(Calendar.DAY_OF_MONTH,-1);
        System.out.println(number+"In February of"+calendar.get(Calendar.DAY_OF_MONTH)+"day");
    }
}

Please enter the year to query:
2018
2018 There are 28 days in February