Packaging class (mutual conversion between types)

Packaging

• Packaging class in java.lang As a basic class , They appear to solve the problem that basic types cannot
• Directly participate in object-oriented development , So that basic types can " object " There is a form of .

   int a = 123;
    Integer i1 = new Integer(a);        // Convert a basic type to a reference type  
    Integer i2 = Integer.valueOf(a);

The wrapper class has a static method :parseXXX You can convert strings to basic types
The premise is that the string correctly describes the value that the basic type can save , Otherwise, an exception will be thrown :
NumberFormatException: The number format is abnormal

String num = "123";
int d = Integer.parseInt(num);
System.out.println(d);
double dou = Double.parseDouble(num);
System.out.println(dou);

   long lmax = Long.MAX_VALUE;   //long The maximum of 
    System.out.println(lmax);
    long lmin = Long.MIN_VALUE;   //long The minimum value of 
    System.out.println(lmin);

    double dmax = Double.MAX_VALUE;  //double The maximum of 
    System.out.println(dmax);
    double dmin = Double.MIN_VALUE;  //double The minimum value of 
    System.out.println(dmin);

JDK1.5 A new feature was introduced : Automatic disassembly box

• This feature is recognized by the compiler, not java Virtual machine approved , When the compiler compiles the code, it finds that there are basic types and
• The conversion code will be automatically added when the reference types are assigned to each other , In this way, we no longer need coding to complete mutual conversion in the source code

 int d = 123;
    /*  The following code will trigger : Auto boxing feature , The compiler will complete the conversion code  Integer i = Integer.valueOf(d); */
    Integer i = d;
    /*  Automatic dismantling  d = i.intValue(); */
    d = i;
    System.out.println(i);
    System.out.println(d);

Point p = new Point(1,2);

     The operation of outputting the content of an object to the console in development is very frequent , And this method outputs objects 
     The output is this object toString String returned by method .
    Object This method is defined in , therefore java All classes in have toString, But if 
     Improper rewriting , Then use Object The string format that will be returned when this method is provided is :
     Class name @ Address , This is of little help to our development 

   notes :java API Most of the provided classes have been overridden toString Method , Usually it's just ourselves 
     The definition class needs to rewrite this method itself when using it is not ideal 
     */
    System.out.println(p);
    /* java The result of any type connected with a string in is a string , The actual operation is to call   Of instances of this type toString Method and connect the returned string with the string  */
    String line = " This object is "+p;
    System.out.println(line);

    /* Object Provides a way equals, The purpose is to compare two objects " Content " Are they the same?   For reference type variables, the address of the object is saved ,"==" Compare the values of variables , therefore   Is to compare addresses , Only two reference type variables have the same address ( Point to the same object ) when   For true  and equals It doesn't matter whether they are the same object , What we judge is whether their contents are the same   notes :Object Of equals For internal use "==" Realization , So it doesn't make sense if the class doesn't rewrite this method . */
    Point p2 = new Point(1,2);
    System.out.println(p==p2); // Than the address , Determine whether it is the same object 
    System.out.println(p.equals(p2));