Nested if selection structure and switch selection structure

nesting if Selection structure

nesting if structure ： In a if The selection structure contains a complete if Selection structure
The law of execution ： Execute the corresponding code according to whether the condition is true
                 
The acquisition of random numbers ：
           Math.random(): Randomly generate a greater than or equal to 0.0 Less than 1.0 Of double Data of type
                    
            Math.random()*10: Randomly generate a greater than or equal to 0.0 Less than 10.0 Of double Data of type
                    
            (int)(Math.random()*10): Randomly generate a greater than or equal to 0 Less than 10 Of int Data of type
                    
  summary ： Pick one at random [num1,num2) Between int Type integer （num2>num1）
                int num =(int)(Math.random()*(num2-num1)+num1);

eg:

//  Randomly generate a greater than or equal to 19, Less than 51 Between int Number of types 
		int num4 = (int) (Math.random() * 32 + 19);
		System.out.println(num4);

 

switch Selection structure

switch choice ：
                 Rule of grammar ：
                    switch ( expression ){
                        case Constant 1：
                             // sentence ;
                             break;
                                                 
                        case Constant 2：
                             // sentence ;
                             break;
                             
                        case Constant 3：
                             // sentence ;
                             break;
                        ......     
                             
                        case Constant n：
                             // sentence ;
                             break;
                             
                        default:
                             // sentence ;
                             break;
                             
                    }
                 The law of execution ：
                        1) Match according to the value of the expression case The following constant values ,
                           After matching, execute the corresponding statement , Finish executing the following statement , perform break,
                           End the whole thing switch structure , So let's keep going switch Select the code after the structure
                        2) If the value of the expression does not match the value of all constants ,
                           Is executed default The words in it ,
                           Execute... After execution break Quit the whole switch Selection structure
                          
                 Be careful ：
                 1） The value of an expression can be int、short、byte、char、 Enumeration type 、String(JDK1.7 Later support )
                 2)case The following constant values cannot be the same
                3)default Sentences can be left blank . If you write that the value of the expression does not match all constant values ,
                   Is executed default The sentence in .default Statements can be written anywhere , Generally written in the book switch Last bit of                         Set up .
                 4)switch Choosing a structure can only deal with the case of equivalent judgment
                 5)break The function of is to end the whole switch Selection structure ,
                     If there is a lack of , The code will continue to run down , Carry out the next case The sentence in , This phenomenon is called " through                             The phenomenon ".
                    case Penetration phenomenon can be used reasonably  

Penetration phenomenon eg:

            /*
			 *  Enter month , Output season 
			 * 
			 * 12 1 2  In the winter  3 4 5  In the spring  6 7 8  In the summer  9 10 11  In the fall 
			 */
		int month=4;
			switch(month){
			case 12:
				//System.out.println(" In the winter ");
				//break;
			case 1:
				//System.out.println(" In the winter ");
				//break;
			case 2:
				System.out.println(" In the winter ");
				break;
			case 3:
				//System.out.println(" In the spring ");
				//break;
			case 4:
				//System.out.println(" In the spring ");
				//break;
			case 5:
				System.out.println(" In the spring ");
				break;
			case 6:
				//System.out.println(" In the summer ");
				//break;
			case 7:
				//System.out.println(" In the summer ");
				//break;
			case 8:
				System.out.println(" In the summer ");
				break;
			case 9:
				//System.out.println(" In the fall ");
				//break;
			case 10:
				//System.out.println(" In the fall ");
				//break;
			case 11:
				System.out.println(" In the fall ");
				break;

effect :