1. Packaging Classes

1.1 Basic Type Packaging Class (Memory)

• Role of basic types of wrapper classes

  The benefit of encapsulating basic data types as objects is that you can define more functional methods within the object to manipulate the data

  One of the common operations: for conversion between basic data types and strings

• Packaging Class for Base Type

  Basic data types	Packaging Class
  byte	Byte
  short	Short
  int	Integer
  long	Long
  float	Float
  double	Double
  char	Character
  boolean	Boolean

1.2 Integer class (application)

• Overview of Integer classes

  Wrap the value of the original type int in an object

• Integer class construction method

  Method Name	Explain
  public Integer(int value)	Create Integer object from int value (obsolete)
  public Integer(String s)	Create Integer objects from String values (obsolete)
  public static Integer valueOf(int i)	Returns an Integer instance representing the specified int value
  public static Integer valueOf(String s)	Returns an Integer object String that holds the specified value

• Sample Code

  public class IntegerDemo {
      public static void main(String[] args) {
          //public Integer(int value): Create an Integer object based on an int value (obsolete)
          Integer i1 = new Integer(100);
          System.out.println(i1);
  
          //public Integer(String s): Create Integer objects based on String values (obsolete)
          Integer i2 = new Integer("100");
  //        Integer i2 = new Integer("abc"); //NumberFormatException
          System.out.println(i2);
          System.out.println("--------");
  
          //public static Integer valueOf(int i): Returns an Integer instance representing the specified int value
          Integer i3 = Integer.valueOf(100);
          System.out.println(i3);
  
          //public static Integer valueOf(String s): Returns an Integer object String that holds the specified value
          Integer i4 = Integer.valueOf("100");
          System.out.println(i4);
      }
  }
  

1.3int and String type conversion (memory)

• int to String

  • Conversion mode

    • Method 1: Add an empty string directly after the number
    • Mode 2: valueOf() via String class static method

  • Sample Code

    public class IntegerDemo {
        public static void main(String[] args) {
            //int --- String
            int number = 100;
            //Mode 1
            String s1 = number + "";
            System.out.println(s1);
            //Mode 2
            //public static String valueOf(int i)
            String s2 = String.valueOf(number);
            System.out.println(s2);
            System.out.println("--------");
        }
    }
    

• String to int

  • Conversion mode

    • Mode 1: Convert a string number to Integer before calling the valueOf() method
    • Mode 2: Convert via Integer static method parseInt()

  • Sample Code

    public class IntegerDemo {
        public static void main(String[] args) {
            //String --- int
            String s = "100";
            //Mode 1: String --- Integer --- int
            Integer i = Integer.valueOf(s);
            //public int intValue()
            int x = i.intValue();
            System.out.println(x);
            //Mode 2
            //public static int parseInt(String s)
            int y = Integer.parseInt(s);
            System.out.println(y);
        }
    }
    

1.4 String Data Sorting Case (Application)

• Case Requirements

  There is a string:'91 27 46 38 50', please write a program to achieve the final output:'27 38 46 50 91'

• code implementation

  public class IntegerTest {
      public static void main(String[] args) {
          //Define a string
          String s = "91 27 46 38 50";
  
          //Store numeric data in a string in an array of type int
          String[] strArray = s.split(" ");
  //        for(int i=0; i<strArray.length; i++) {
  //            System.out.println(strArray[i]);
  //        }
  
          //Define an int array to store each element of the String[] array in the int array
          int[] arr = new int[strArray.length];
          for(int i=0; i<arr.length; i++) {
              arr[i] = Integer.parseInt(strArray[i]);
          }
  
          //Sort the int array
          Arrays.sort(arr);
  
          //The elements in the sorted int array are stitched together into a string, which is stitched using StringBuilder
          StringBuilder sb = new StringBuilder();
          for(int i=0; i<arr.length; i++) {
              if(i == arr.length - 1) {
                  sb.append(arr[i]);
              } else {
                  sb.append(arr[i]).append(" ");
              }
          }
          String result = sb.toString();
  
          //Output Results
          System.out.println(result);
      }
  }
  

1.5 Automatic unpacking and packing (understanding)

• Automatic packing

  Convert basic data types to corresponding wrapper class types

• Auto-unboxing

  Convert wrapper class type to corresponding base data type

• Sample Code

  Integer i = 100;  // Automatic packing
  i += 200;         // i = i + 200;I + 200 auto-unboxing;i = i + 200;Is automatic packing
  

2. Time Date Class

2.1 Date class (application)

• An overview of the Date class

  Date represents a specific time, accurate to milliseconds

• Date class construction method

  Method Name	Explain
  public Date()	Assign a Date object and initialize it so that it represents the time it was allocated, accurate to milliseconds
  public Date(long date)	Assign a Date object and initialize it to represent the number of milliseconds specified from standard base time

• Sample Code

  public class DateDemo01 {
      public static void main(String[] args) {
          //public Date(): Assign a Date object and initialize it so that it represents the time it was allocated, accurate to milliseconds
          Date d1 = new Date();
          System.out.println(d1);
  
          //public Date(long date): Assign a Date object and initialize it to represent the number of milliseconds specified from standard base time
          long date = 1000*60*60;
          Date d2 = new Date(date);
          System.out.println(d2);
      }
  }
  

2.2Date Class Common Methods (Applications)

• common method

  Method Name	Explain
  public long getTime()	Gets the millisecond value of the date object from 00:00:00 on January 1, 1970 to the present
  public void setTime(long time)	Set the time, given the value in milliseconds

• Sample Code

  public class DateDemo02 {
      public static void main(String[] args) {
          //Create Date object
          Date d = new Date();
  
          //public long getTime(): Gets the millisecond value of the date object from 00:00:00 on January 1, 1970 to the present
  //        System.out.println(d.getTime());
  //        System.out.println(d.getTime() * 1.0 / 1000 / 60 / 60 / 24 / 365 + "year");
  
          //public void setTime(long time): Set the time, given a value of milliseconds
  //        long time = 1000*60*60;
          long time = System.currentTimeMillis();
          d.setTime(time);
  
          System.out.println(d);
      }
  }
  

2.3SimpleDateFormat class (application)

• An overview of the SimpleDateFormat class

  SimpleDateFormat is a specific class for formatting and parsing dates in a locale-sensitive manner.

  We focus on formatting and parsing dates

• SimpleDateFormat class construction method

  Method Name	Explain
  public SimpleDateFormat()	Construct a SimpleDateFormat using default mode and date format
  public SimpleDateFormat(String pattern)	Construct a SimpleDateFormat to use the given pattern and default date format

• Common methods of the SimpleDateFormat class

  • Formatting (from Date to String)
    • public final String format(Date date): Format the date into a date/time string
  • Parse (from String to Date)
    • public Date parse(String source): parses text from the beginning of a given string to generate a date

• Sample Code

  public class SimpleDateFormatDemo {
      public static void main(String[] args) throws ParseException {
          //Formatting: From Date to String
          Date d = new Date();
  //        SimpleDateFormat sdf = new SimpleDateFormat();
          SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd day HH:mm:ss");
          String s = sdf.format(d);
          System.out.println(s);
          System.out.println("--------");
  
          //From String to Date
          String ss = "2048-08-09 11:11:11";
          //ParseException
          SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
          Date dd = sdf2.parse(ss);
          System.out.println(dd);
      }
  }
  

2.4 Tool Case (Application)

• Case Requirements

  Defines a DateUtils tool class that contains two methods: converting a date to a string in a specified format;Parses the string to a date in the specified format, and then defines a test class (DateDemo), the method of the test date tool class

• code implementation

  • Tool class

  public class DateUtils {
      private DateUtils() {}
  
      /*
          Converts the date to a string in the specified format
          Return value type: String
          Parameters: Date date, String format
       */
      public static String dateToString(Date date, String format) {
          SimpleDateFormat sdf = new SimpleDateFormat(format);
          String s = sdf.format(date);
          return s;
      }
  
  
      /*
          Resolves a string to a date in the specified format
          Return value type: Date
          Parameters: String s, String format
       */
      public static Date stringToDate(String s, String format) throws ParseException {
          SimpleDateFormat sdf = new SimpleDateFormat(format);
          Date d = sdf.parse(s);
          return d;
      }
  
  }
  

  • Test Class

  public class DateDemo {
      public static void main(String[] args) throws ParseException {
          //Create Date object
          Date d = new Date();
  
          String s1 = DateUtils.dateToString(d, "yyyy year MM month dd day HH:mm:ss");
          System.out.println(s1);
  
          String s2 = DateUtils.dateToString(d, "yyyy year MM month dd day");
          System.out.println(s2);
  
          String s3 = DateUtils.dateToString(d, "HH:mm:ss");
          System.out.println(s3);
          System.out.println("--------");
  
          String s = "2048-08-09 12:12:12";
          Date dd = DateUtils.stringToDate(s, "yyyy-MM-dd HH:mm:ss");
          System.out.println(dd);
      }
  }
  

2.5 Calendar class (application)

• An overview of Calendar classes

  Calendar provides methods for converting specific moments to a set of calendar fields and for manipulating calendar fields

  Calendar provides a class method, getInstance, to obtain generally useful objects of this type.

  This method returns a Calendar object.

  Its calendar field has been initialized with the current date and time: Calendar rightNow = Calendar.getInstance();

• Common Calendar Class Methods

  Method Name	Explain
  public int get(int field)	Returns the value of a given calendar field
  public abstract void add(int field, int amount)	Adds or subtracts a specified amount of time from a given calendar field according to calendar rules
  public final void set(int year,int month,int date)	Set the year, month and day of the current calendar

• Sample Code

  public class CalendarDemo {
      public static void main(String[] args) {
          //Get Calendar Class Object
          Calendar c = Calendar.getInstance();
  
          //public int get(int field): Returns the value of a given calendar field
          int year = c.get(Calendar.YEAR);
          int month = c.get(Calendar.MONTH) + 1;
          int date = c.get(Calendar.DATE);
          System.out.println(year + "year" + month + "month" + date + "day");
  
          //public abstract void add(int field, int amount): Adds or subtracts a specified amount of time from a given calendar field according to calendar rules
          //Demand 1:3 years ago today
  //        c.add(Calendar.YEAR,-3);
  //        year = c.get(Calendar.YEAR);
  //        month = c.get(Calendar.MONTH) + 1;
  //        date = c.get(Calendar.DATE);
  //        System.out.println(year + year + month + month + date + day);
  
          //Demand 2:10 days ago
  //        c.add(Calendar.YEAR,10);
  //        c.add(Calendar.DATE,-10);
  //        year = c.get(Calendar.YEAR);
  //        month = c.get(Calendar.MONTH) + 1;
  //        date = c.get(Calendar.DATE);
  //        System.out.println(year + year + month + month + date + day);
  
          //public final void set(int year,int month,int date): sets the month and year day of the current calendar
          c.set(2050,10,10);
          year = c.get(Calendar.YEAR);
          month = c.get(Calendar.MONTH) + 1;
          date = c.get(Calendar.DATE);
          System.out.println(year + "year" + month + "month" + date + "day");
  
      }
  }
  

February Day 2.6 Case (Application)

• Case Requirements

  Get the number of days in February of any year

• code implementation

  public class CalendarTest {
      public static void main(String[] args) {
          //Keyboard Enter Any Year
          Scanner sc = new Scanner(System.in);
          System.out.println("Please enter a year:");
          int year = sc.nextInt();
  
          //Set year, month, day of calendar object
          Calendar c = Calendar.getInstance();
          c.set(year, 2, 1);
  
          //March 1 goes forward one day, the last day of February
          c.add(Calendar.DATE, -1);
  
          //Get the output for this day
          int date = c.get(Calendar.DATE);
          System.out.println(year + "In February of the year" + date + "day");
      }
  }
  

3. Exceptions

3.1 Anomalies (Memory)

• Overview of anomalies

  An exception is a malfunction in the program

• Abnormal Architecture

3.2JVM default way of handling exceptions (understand)

• If something goes wrong with the program and we don't do anything, the JVM will do it by default in two steps:

• Information such as the name of the exception, the cause of the error and the location of the exception is output to the console

• Program Stop Execution

3.3try-catch handling exceptions (application)

• Define Format

  try {
  	Code with possible exceptions;
  } catch(Exception Class Name Variable Name) {
  	Handling code for exceptions;
  }
  

• Execute process

  • The program starts with code inside the try
  • If an exception occurs, it jumps to the corresponding catch to execute
  • After execution, the program can continue

• Sample Code

  public class ExceptionDemo01 {
      public static void main(String[] args) {
          System.out.println("start");
          method();
          System.out.println("End");
      }
  
      public static void method() {
          try {
              int[] arr = {1, 2, 3};
              System.out.println(arr[3]);
              System.out.println("Is it accessible here");
          } catch (ArrayIndexOutOfBoundsException e) {
  //            System.out.println("The array index you accessed does not exist, please go back and modify it to the correct index");
              e.printStackTrace();
          }
      }
  }
  

3.4 Throwable member method (application)

• common method

  Method Name	Explain
  public String getMessage()	Returns the detailed message string for this throwable
  public String toString()	Return this throwable short description
  public void printStackTrace()	Output exception error information to console

• Sample Code

  public class ExceptionDemo02 {
      public static void main(String[] args) {
          System.out.println("start");
          method();
          System.out.println("End");
      }
  
      public static void method() {
          try {
              int[] arr = {1, 2, 3};
              System.out.println(arr[3]); //new ArrayIndexOutOfBoundsException();
              System.out.println("Is it accessible here");
          } catch (ArrayIndexOutOfBoundsException e) { //new ArrayIndexOutOfBoundsException();
  //            e.printStackTrace();
  
              //public String getMessage(): Returns the detailed message string for this throwable
  //            System.out.println(e.getMessage());
              //Index 3 out of bounds for length 3
  
              //public String toString(): Returns this short description that can be thrown
  //            System.out.println(e.toString());
              //java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
  
              //public void printStackTrace(): Output exception error information to console
              e.printStackTrace();
  //            java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
  //            at com.itheima_02.ExceptionDemo02.method(ExceptionDemo02.java:18)
  //            at com.itheima_02.ExceptionDemo02.main(ExceptionDemo02.java:11)
  
          }
      }
  }
  

3.5 Difference between compile-time and run-time exceptions (memory)

• Compile-time exceptions

  • Are Exception classes and their subclasses
  • Processing must be displayed or the program will fail to compile

• Runtime Exceptions

  • Are both RuntimeException classes and their subclasses
  • No display handling is required or can be handled as an exception during compilation

3.6 throws handling exceptions (application)

• Define Format

  public void Method() throws Exception Class Name {
      
  }
  

• Sample Code

  public class ExceptionDemo {
      public static void main(String[] args) {
          System.out.println("start");
  //        method();
          try {
              method2();
          }catch (ParseException e) {
              e.printStackTrace();
          }
          System.out.println("End");
      }
  
      //Compile-time exceptions
      public static void method2() throws ParseException {
          String s = "2048-08-09";
          SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
          Date d = sdf.parse(s);
          System.out.println(d);
      }
  
      //Runtime Exceptions
      public static void method() throws ArrayIndexOutOfBoundsException {
          int[] arr = {1, 2, 3};
          System.out.println(arr[3]);
      }
  }
  

• Matters needing attention

  • This throws format follows the parentheses of the method
  • Compile-time exceptions must be handled, either try...catch...or throws. If throws are used, who calls whom to handle them in the future
  • Runtime exceptions can be left untreated and we need to come back to modify the code after a problem occurs

Differences between 3.7 throws and throws (memory)

3.8 Custom Exceptions (Apply)

• Custom exception class

  public class ScoreException extends Exception {
  
      public ScoreException() {}
  
      public ScoreException(String message) {
          super(message);
      }
  
  }
  

• Teachers

  public class Teacher {
      public void checkScore(int score) throws ScoreException {
          if(score<0 || score>100) {
  //            throw new ScoreException();
              throw new ScoreException("You gave the wrong score, it should be 0-100 Between");
          } else {
              System.out.println("Normal results");
          }
      }
  }
  

• Test Class

  public class Demo {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          System.out.println("Please enter a score:");
  
          int score = sc.nextInt();
  
          Teacher t = new Teacher();
          try {
              t.checkScore(score);
          } catch (ScoreException e) {
              e.printStackTrace();
          }
      }
  }