Conditional 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.

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

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

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!");

Hello!
I'm a child
Bye!

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

An else-if block may be used when multiple conditions need to be checked. else-if statements come after the if block, but before the else block. The structure of an if block is as follows:

Here is an example to illustrate:

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

a is positive

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

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

inta=5;
if(a>0){
// a is greater than 0, so this statement will execute
System.out.println("a is positive");
a=-5;
}elseif(a<0){
// a WAS greater than 0, so this statement will not execute
System.out.println("a is negative");
}else{
// a does not equal 0, so this statement will not execute
System.out.println("a is zero");
}

a is positive

Conditional expressions use the compound ?: operator. Syntax:

This evaluates boolean expression1, and if it is true then the conditional expression has the value of expression1; otherwise the conditional expression has the value of expression2.

Example:

Stringanswer=(p<0.05)?"reject":"keep";

This is equivalent to the following code fragment:

Stringanswer;
if(p<0.05){
answer="reject";
}else{
answer="keep";
}

The switch conditional statement is basically a shorthand version of writing many if ...else statements. The switch block evaluates a char , byte , short , or int (or enum , starting in J2SE 5.0; or String , starting in J2SE 7.0), and, based on the value provided, jumps to a specific case within the switch block and executes code until the break 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 optional default case.

The structure of a switch statement is as follows:

Here is an example to illustrate:

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

i equals something other than 1 or 2

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

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

inti=-1;
switch(i){
case-1:
case1:
// i is -1, so it will fall through to this case and execute this code
System.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 execute
System.out.println("i is 0");
}

i is 1 or -1

Starting in J2SE 5.0, the switch statement can also be used with an enum value instead of an integer.

Though enums 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:

Dayday=Day.MONDAY;// Day is a fictional enum type containing the days of the week
switch(day){
caseMONDAY:
// Since day == Day.MONDAY, this statement will execute
System.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;
}

Mondays are the worst!

Starting in J2SE 7.0, the switch statement can also be used with an String value instead of an integer.

Stringday="Monday";
switch(day){
case"Monday":
// Since day == "Monday", this statement will execute
System.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);
}

Mondays are the worst!

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