1. Object class

2. Date class

3. DateFormat class

4. Calendar class

1.Object class (in java.lang package)

The parent class of all classes, inheriting Object classes by default if one class does not specify a parent class

• ToString() method: direct output statement output object calls toString() method

  package com.day7;
  
  /**
   * @author SFJ
   * @date 2019/11/13
   * @time 21:47
   **/
  public class Test1 {
  }
  class Student extends Object{
      private int id;
      private String name;
      private int age;
      public Student(){
      }
      public String toString()
      {
          return "StudentIs::"+id+" name:"+name+" age"+age;
      }
  }
  

   

• boolean equals() method: inherits the equals() method of the Object class
  Parameters: 1.Object obj: Any object can be passed
  2. Basic Data Type: Compare Values to Equality
  3. Reference data type: Compare the address values of two objects (not the comparison, but can be overridden)

  package com.day7;
  
  /**
   * @author SFJ
   * @date 2019/11/13
   * @time 22:12
   **/
  public class Test2 {
      public static void main(String[] args) {
          Student student1 = new Student();
          Student student2 = student1;
          System.out.println(student1.equals(student2));
      }}
  class Student extends Object{
      private int id;
      private String name;
      private int age;
      public Student(){
      }
      public String toString()
      {
          return "StudentIs::"+id+" name:"+name+" age"+age;
      }
      @Override
      public boolean equals(Object obj) {
          if (this == obj) return true;//Returns true if it is the same object;
          if (obj == null || getClass() != obj.getClass()) return false;//If the parameter is empty, the type information is different
          Student student = (Student) obj;//Base types are equal and reference types are given to the equals static method of the Objects class for identical rule judgment
          return student.id==id &&student.name==name && student.age==age;
      }
  }
  

  2. DateTime class (in java.util package) --Date class

* for date and time, Date for a specific instant, accurate to milliseconds

package com.day7;

import javax.jws.soap.SOAPBinding;
import java.util.Date;

/**
 * @author SFJ
 * @date 2019/11/13
 * @time 22:28
 **/
public class Test3 {
    public static void main(String[] args) {
        Date date1 = new Date();//Assign a Date object and initialize it
        System.out.println(date1);
        System.out.println("Get the current millisecond value:"+date1.getTime());

        Date date2 = new Date(19980920);//Assign a Date object and initialize it
        System.out.println(date1.getYear());
        System.out.println(date2.getTime());
    }
}

3.DateFormat class (abstract class) ->subclass: SimpleDateFormat

java.text.DateFormat is an abstract class of date and time formatting subclasses that helps us convert date text, that is, back and forth between Date and String objects.

1. Construction method: public SimpleDateFormat (String pattern): Symbols in given mode and default language date format
2. Common methods:
   1.public String format(Date date): Format the Date object as a string
   2.public Date parse(String souce): parses a string into a Date object

package com.day7;

import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * @author SFJ
 * @date 2019/11/13
 * @time 22:47
 **/
public class Test4 {
    public static void main(String[] args) {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dddd");
        Date date = new Date();
        System.out.println(simpleDateFormat.format(date));;
        String string = "2019-11-13 22:52";
        Date date1 = simpleDateFormat.parse(string);
        System.out.println(date1);
    }
}