Permission modifiers and code blocks

One 、 Permission modifier

1、 stay Java Four kinds of access rights are provided in , When decorated with different access modifiers , The decorated content will have different access rights ,

• public： Public
• protected： The protected
• ( Empty )： default
• private： Private

2、 Different access rights

	public	protected	（ Empty ）	private
In the same category	√	√	√	√
In the same bag ( Subclasses and unrelated classes )	√	√	√
Subclasses of different packages	√	√
Unrelated classes in different packages	√

3、 When writing code , Without special consideration , It is recommended to use the permission in this way ：

• Member variables use private , Hidden details .
• The construction method uses public , Easy to create objects .
• Member method usage public , Easy to call methods .

Two 、 Code block

1、 Building blocks of code

	 Format : {
    }
	 Location :  Class , Outside method 
	 perform :  Every time you call a constructor , Will execute 
	 Use scenarios :  Count how many objects of this type have been created 
	
	 for example :
	public class Person{
    
	    {
    
	         The construction code block is executed 
	    }
	}

2、 Static code block

	 Format :static{
    }
	 Location :  Class , Outside method 
	 perform :  When the class is loaded, execute , And only once 
	 Use scenarios :  For example, load driver , This kind of code that only needs to be executed once can be placed in static code blocks          
	
	public class Person {
    
		private String name;
		private int age;
	 // Static code block 
		static{
    
			System.out.println(" Static code blocks execute ");
		}
	}

3、 Local code block

	 Format :{
    }
	 Location :  In the method 
	 perform :  Calling method , Execute when the local code block is executed 
	 Use scenarios :  Save memory space , It doesn't make much sense 
	 for example :
	public static void main(String[] args) {
    
	     int a = 10;
	     // Local code block 
	     // The role of local code blocks is to limit the scope of variables , Disappear from memory early , To save memory 
	     // But now memory is not worth money , So the local code block has no use 
	     {
    
	         int b = 20;
	     }
	
	     //int a = 30; // You cannot define a variable with the same name in the same region 
	     // Don't complain , Think the scope is different 
	     int b = 40;
	 }