JAVA classes and objects (the difference between instance variables and class variables) (the difference between instance methods and class methods) explanation

This articleIntroductionJAVAClasses and objects(The difference between instancevariablesand class variables)(The difference between instance methods and class methods)Explanation

Instance variables

• Instance variables are declared in a class, but outside methods, constructors and statement blocks;

• When an object is instantiated, the value of each instance variable is determined;

• Instance variables are created when the object is created, and when the object is destroyed Destroyed at the time;

• The value of the instance variable should be referenced by at least one method, constructor or statement block, so that the outside can obtain the instance variable information through these methods;

• Instance variables are visible to methods, constructors, or statement blocks in the class. In general, instance variables should be made private. Instance variables can be made visible to subclasses by using access modifiers

• Instance variables have default values. The default value of numeric variables is 0, the default value of Boolean variables is false, and the default value of reference type variables is

  null. The value of a variable can be specified at the time of declaration or in the constructor;

• Instance variables can be accessed directly through

  variable name. But in static methods and other classes, you should use the fully qualified name: ObejectReference.VariableName.

Class variables (static variables)

• Class variables are also called static variables, and are represented by the

  static keyword## in the class #Declaration, but must be outside the method constructor and statement block.

No matter how many objects a class creates, the class only has one copy of the class variable.
Static variables are rarely used except when declared as
• constants

  . Constants refer to variables declared as public/private, final and static types. Constants cannot be changed after initialization.

Static variables are stored in the static storage area. Often declared as constants, variables are rarely declared using static alone.
Static variables are created at the beginning of the program and destroyed at the end of the program.
Have similar visibility to instance variables. But in order to be visible to users of the class, most static variables are declared as public types.
Default values ​​are similar to instance variables. The default value of numeric variables is 0, the default value of Boolean variables is false, and the default value of reference types is null. The value of a variable can be specified when declaring it or in the constructor. In addition, static variables can also be initialized in static statement blocks.
Static variables can be accessed through:

• ClassName.VariableName.

When a class variable is declared as a public static final type, the class variable name must use uppercase letters. If the static variable is not of public or final type, its naming method is consistent with the naming method of instance variables and local variables.

• class TiXing{
      float up,height;
      static float down;
      
      TiXing(float x,float y,float z){
          up=x;
          height=y;
          down=z;
      }
  }
  
  public class ep3_9{
      public static void main(String args[]){
          TiXing one=new TiXing(1,2,3);
          System.out.println("one's down is:"+one.down);
          TiXing two=new TiXing(4,5,6);
          System.out.println("one's down is:"+one.down);
          System.out.println("two's down is:"+two.down);
      
          System.out.println("TiXing's down is:"+TiXing.down);
      }
  }

Instance method and class method access to instance variables and class variables Instance methods can operate on the instance variables of the current object or on class variables. Instance methods are called by instance objects.

Class methods cannot access instance variables, only class variables. Class methods are called by the class name or instance object. This or
• super

  keywords

  class TiXing{
       private float up,height;
       private static float down;
      
      TiXing(float x,float y,float z){
          up=x;
          height=y;
          down=z;
      }
      public void display(){
          System.out.println("up is:"+up+"height is:"+height+"down is:"+down);
      }
      public static void change(float number){
          down=number;
          //System.out.println("height:"+height);//出错
      }
  }
  
  public class ep3_9{
      public static void main(String args[]){
          TiXing one=new TiXing(1,2,3);
          one.display();
          TiXing two=new TiXing(4,5,6);
          one.display();
          two.display();
      
          //TiXing.display();//出错
          one.change(101);
          one.display();
          two.change(102);
          two.display();    
      }
  }

cannot appear in class methods.

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