Muscle Java Object Oriented Day 1

instance

What is a class? (Review) Something like a design document ・ What to define -Status (status): Variable (field) -Behavior (behavior): method

--Entity created from design document (class) --Multiple generation is possible

Instance (code generation)

Class name instance variable name(freedom) =new class name(argument)[Constructor specification]；

Store the generated instance in a variable Notable here! !! __ Class type (reference type) instance variable __

All Java variables are statically typed to declare class types

constructor

--A special method that performs initialization processing when creating an instance --Constructor features --The method name is the same as the class name --No return value --There may be multiple constructors with different arguments --Constructor is optional

//Person class

public class Person {
  public Person(){
   age = 1;
  }
  public Person(int a){
   age = a;
  }
  public Person(String a){
   age = a;
  }
  public void printAge(){
   System.out.println("I"+age+"I'm old");
  }
 }
}

//main class

public static void main(String args[]){
  Person p1 = new Person();
  p1.printAge();
  
  int num1 = 29;
  Person p2 = new Person(num1){
  p2.printAge();
  }

  String num2 = "29";
  Person p3 = new Person(num2){
  p3.printAge();
  }
}

static definition

Access to static fields and static methods --Can be used without instantiating fields and methods with static keywords

Static definition restrictions

--static field --Only one variable value can be held → Overwritten each time it is stored --static method --You cannot use methods without static or fields without static in the same class from within static methods. --Accessible by creating an instance in a static method

Class cooperation

--When using classes from other packages --Specify directly in the code with "package name.class name" --Used in import declaration --import Other package name. Class name; --import package name. * (All classes);