Java --- OOP as many states

The use of more than one member state

1.1 Features:

• Member variables: using the parent class
• Member method: Due to rewrite phenomenon so using a subclass
• Static member: With the object exists, who called who returns

public class Test1_UseManyTai {
    public static void main(String[] args) {
       // 创建多态对象测试
       Fu f = new Zi();// 口诀1 ：父类引用 指向 子类对象
       System.out.println(f.count); //1、成员变量用父类的！！！口诀2： 编译看左边，运行看右边(针对重写现象)。 
	       f.eat(); // 口诀2： 编译看左边，运行看右边(针对重写现象)。
		    //2、 编译看左边，eat()调用的是父类的方法声明，但是方法体使用的是子类的。
	       // 编译看左边更好的体现了多态可以用来 统一调用标准，就是都向 父类 看齐，必须使用父类提供了的功能。       
	       //3、静态方法能重写吗？？？---静态资源不能重写
	       f.sleep();//使用的仍然是父类的功能  
    }
}

// 创建父类
class Fu {
    int count = 10;
    public void eat() {
	       System.out.println("Fu..eat()...");
    }
	// 静态方法
    static public void sleep() {
	       System.out.println("Fu..sleep()...");
    }
}

// 创建子类
class Zi extends Fu {
	    int count = 20;
	    public void eat() {
	       System.out.println("Zi..eat()...");
    }
    // 静态方法  --  真的是重写现象吗---不存在！！
	    static public void sleep() {
	       System.out.println("Zi..sleep()...");
    }
}

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2 Permissions modifier

• Used to control a class, range or access class members.
  

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3 abnormal

3.1 Overview:

• Used to encapsulate target error messages.
• Makeup: Types, Tips, line number.

3.2 abnormal inheritance structure:

• Throwable - top-level parent class
• - Error: System Error, can not be repaired
• - Exception: correctable error
• –RunTimeException
• –ClassCastException
• –ClassNotFoundException

3.3 Exception Handling:

• Program encountered an exception, there are usually two approaches: the capture or throw up
• When you call a method throws an exception, call the location can not continue to throw up process can also catch the exception.

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4 abstract class

4.1 concept:

• Java can be defined method is not a method thereof, by which a subclass to a specific implementation.
• This method is not a method we call the body abstract methods abstract class containing method we call an abstract class.
• An abstract class can be understood as a special class method declaration is not only a method body.

//把共性的代码，向上提取，形成父类  -- 形成继承的结构好处是可以复用父类的代码
//抽象类是个特殊的类，特殊在哪儿？特殊在类里可以有普通方法，还可以有抽象方法。
抽象的 class Animal{
    //父类的方法体让子类重写了！！！--这时父类的方法体有点多余！！！父类干脆就不提供方法体--抽象方法
    抽象的 public void eat() ;
    public void sleep(){
       syso(....);
    }
}

class Dog extends Animal{
    public void eat(){//重写目的是修改父类原有的功能
       syso(.);
   }
}

4.2 Features:

• Java achieved by keyword abstract
• The method may be modified or class
• An abstract class can be no abstract methods (to be implemented by subclasses)?
• If the abstract methods in the class, that class must be defined as an abstract class
• Subclass inherits the abstract class in the future, or is an abstract class, or put all the abstract methods are rewritten
• Used for multiple states
• An abstract class can not be instantiated new

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Usage abstract class 5

public class Test4_UseAbstract {
    public static void main(String[] args) {
       //创建多态对象测试
       Father f = new Son();
	   //Father f2 = new Father();
    }
}

//创建父类
abstract class Father{
    //1、抽象类能创建对象吗？--不能  ！！
    //2、那么抽象类里的构造方法有啥用？？---方便子类创建对象！！
    public Father() {
	       System.out.println("Father()...");
    }
}

//创建子类
class Son extends Father{
    public Son() {
       super();//默认就存在
              System.out.println("Son()...");
    }
}