Java Basics/16th lesson: Control Process/Java if condition statement

2018-03-07

1.IFIF (expression 1) {expression 2;} If the value of expression 1 is true, an expression of 2 is executed

 Public class HelloWorld {    publicstaticvoid  main (string[] args) {                   Booleantrue;         // Print Yes        if it is set up if (b) {            System.out.println ("yes");              }}}

2. Multiple expressions and an expression

 Public classHelloWorld { Public Static voidMain (string[] args) {Booleanb =false; //If you have multiple expressions, you must include them with braces        if(b) {System.out.println ("Yes1"); System.out.println ("Yes2"); System.out.println ("Yes3"); }                 //otherwise the expression 2 3 executes regardless of whether B is true                 if(b) System.out.println ("Yes1"); System.out.println ("Yes2"); System.out.println ("Yes3"); //If only one expression can be used without parentheses, it will look simple        if(b) {System.out.println ("Yes1"); }                 if(b) System.out.println ("Yes1"); }}

3.if may encounter pits during use

In line 6th, the If is followed by a semicolon; And the semicolon is a complete expression.
If B is true, the semicolon is executed and the Yes is printed
If B is false, the semicolon is not executed and then print Yes
That way, it looks like it'll print yes anyway.

 Public class HelloWorld {    publicstaticvoid  main (string[] args) {           Booleanfalse;          if (b);            System.out.println ("yes");}     }

4.if Else

else represents a case of non-establishment

 Public class HelloWorld {    publicstaticvoid  main (string[] args) {           Booleanfalse;          if (b)            System.out.println ("yes");         Else             System.out.println ("no");}     }

5.else if

else if is a multi-conditional judgment

 Public classHelloWorld { Public Static voidMain (string[] args) {//if only using if, 4 judgments are performed        inti = 2; if(i==1) System.out.println (1); if(i==2) System.out.println (2); if(i==3) System.out.println (3); if(i==4) System.out.println (4); //If you use else if, once in 18 rows, the judgment is established, 20 rows and 22 lines of judgment will not be executed, saving the computing resources        if(i==1) System.out.println (1); Else if(i==2) System.out.println (2); Else if(i==3) System.out.println (3); Else if(i==4) System.out.println (4); }}

6.switch

The Swich statement is equivalent to another expression of if else, and switch can use Byte,short,int,char,string,enum

Note: At the end of each expression, there should be a break;
Note: string is not supported before Java1.7, Java supports switch with string from 1.7, and compiles string into hash value, in fact, integer
Note: Enum is an enumeration type and is explained in detail in the enumeration section

 Public classHelloWorld { Public Static voidMain (string[] args) {//If you use the IF Else        intDay = 5; if(day==1) System.out.println ("Monday"); Else if(day==2) System.out.println ("Tuesday"); Else if(day==3) System.out.println ("Wednesday"); Else if(day==4) System.out.println ("Thursday"); Else if(day==5) System.out.println ("Friday"); Else if(day==6) System.out.println ("Saturday"); Else if(day==7) System.out.println ("Sunday"); ElseSystem.out.println ("What the hell is this?" "); //If you use the switch        Switch(day) { Case1: System.out.println ("Monday");  Break;  Case2: System.out.println ("Tuesday");  Break;  Case3: System.out.println ("Wednesday");  Break;  Case4: System.out.println ("Thursday");  Break;  Case5: System.out.println ("Friday");  Break;  Case6: System.out.println ("Saturday");  Break;  Case7: System.out.println ("Sunday");  Break; default: System.out.println ("What the hell is this?" "); }             }}

Java Basics/16th lesson: Control Process/Java if condition statement