Statements

Java Programming Conditional blocks

Loop blocks

Conditional blocks allow a program to take a different path depending on some condition(s). These allow a program to perform a test and then take action based on the result of that test. In the code sections, the actually executed code lines will be highlighted.

Theif block executes only if the boolean expression associated with it is true. The structure of anif block is as follows:

Here is a double example to illustrate what happens if the condition is true and if the condition is false:

Code section 3.22: Twoif blocks. intage=6;System.out.println("Hello!");if(age<13){System.out.println("I'm a child.");}if(age>20){System.out.println("I'm an adult.");}System.out.println("Bye!");

Output for Code section 3.22 Hello!I'm a childBye!

If only one statement is to be executed after anif block, it does not have to be enclosed in curly braces. For example,if (i == 0) i = 1; is a perfectly valid portion of Java code. This works for most control structures, such aselse andwhile. However Oracle'sJava Code Conventions explicitly state that the braces should always be used.

Theif block may optionally be followed by anelse block which will execute if that boolean expression is false. The structure of anif block is as follows:

Anelse-if block may be used when multiple conditions need to be checked.else-if statements come after theif block, but before theelse block. The structure of anif block is as follows:

Here is an example to illustrate:

Code listing 3.3: MyConditionalProgram.java publicclassMyConditionalProgram{publicstaticvoidmain(String[]args){inta=5;if(a>0){// a is greater than 0, so this statement will executeSystem.out.println("a is positive");}elseif(a>=0){// a case has already executed, so this statement will NOT executeSystem.out.println("a is positive or zero");}else{// a case has already executed, so this statement will NOT executeSystem.out.println("a is negative");}}}

Output for code listing 3.3 a is positive

Keep in mind thatonly a single block will execute, and it will be the first true condition.

All the conditions are evaluated whenif is reached, no matter what the result of the condition is, after the execution of theif block:

Code section 3.23: A new value for the variable a. inta=5;if(a>0){// a is greater than 0, so this statement will executeSystem.out.println("a is positive");a=-5;}elseif(a<0){// a WAS greater than 0, so this statement will not executeSystem.out.println("a is negative");}else{// a does not equal 0, so this statement will not executeSystem.out.println("a is zero");}

Output for code section 3.23 a is positive

Conditional expressions use the compound?: operator. Syntax:

This evaluatesboolean expression1, and if it istrue then the conditional expression has the value ofexpression1; otherwise the conditional expression has the value ofexpression2.

Example:

Code section 3.24: Conditional expressions. Stringanswer=(p<0.05)?"reject":"keep";

This is equivalent to the following code fragment:

Code section 3.25: Equivalent code. Stringanswer;if(p<0.05){answer="reject";}else{answer="keep";}

Theswitch conditional statement is basically a shorthand version of writing manyif...else statements. Theswitch block evaluates achar,byte,short, orint (orenum, starting in J2SE 5.0; orString, starting in J2SE 7.0), and, based on the value provided, jumps to a specificcase within the switch block and executes code until thebreak command is encountered or the end of the block. If the switch value does not match any of the case values, execution will jump to the optionaldefault case.

The structure of aswitch statement is as follows:

Here is an example to illustrate:

Code section 3.26: A switch block. inti=3;switch(i){case1:// i doesn't equal 1, so this code won't executeSystem.out.println("i equals 1");break;case2:// i doesn't equal 2, so this code won't executeSystem.out.println("i equals 2");break;default:// i has not been handled so far, so this code will executeSystem.out.println("i equals something other than 1 or 2");}

Output for code section 3.26 i equals something other than 1 or 2

If a case does not end with thebreak statement, then the next case will be checked, otherwise the execution will jump to the end of theswitch statement.

Look at this example to see how it's done:

Code section 3.27: A switch block containing a case without break. inti=-1;switch(i){case-1:case1:// i is -1, so it will fall through to this case and execute this codeSystem.out.println("i is 1 or -1");break;case0:// The break command is used before this case, so if i is 1 or -1, this will not executeSystem.out.println("i is 0");}

Output for code section 3.27 i is 1 or -1

Starting in J2SE 5.0, theswitch statement can also be used with anenum value instead of an integer.

Thoughenums have not been covered yet, here is an example so you can see how it's done (note that the enum constants in the cases do not need to be qualified with the type:

Code section 3.28: A switch block with an enum type. Dayday=Day.MONDAY;// Day is a fictional enum type containing the days of the weekswitch(day){caseMONDAY:// Since day == Day.MONDAY, this statement will executeSystem.out.println("Mondays are the worst!");break;caseTUESDAY:caseWEDNESDAY:caseTHURSDAY:System.out.println("Weekdays are so-so.");break;caseFRIDAY:caseSATURDAY:caseSUNDAY:System.out.println("Weekends are the best!");break;}

Output for code section 3.28 Mondays are the worst!

Starting in J2SE 7.0, theswitch statement can also be used with anString value instead of an integer.

Code section 3.29: A switch block with a String type. Stringday="Monday";switch(day){case"Monday":// Since day == "Monday", this statement will executeSystem.out.println("Mondays are the worst!");break;case"Tuesday":case"Wednesday":case"Thursday":System.out.println("Weekdays are so-so.");break;case"Friday":case"Saturday":case"Sunday":System.out.println("Weekends are the best!");break;default:thrownewIllegalArgumentException("Invalid day of the week: "+day);}

Output for code section 3.29 Mondays are the worst!

Statements

Java Programming Conditional blocks

Loop blocks

• Book:Java Programming