Java Conditional statements

Java has the following conditional statements:

• Use if to specify a block of code to be executed if a specified condition is true.

• Use else to specify a block of code to be executed, if the same condition is false.

• Use else if to specify a new condition to test, if the first condition is false.

• Use switch to specify many alternative blocks of code to be executed.

The if Statement

Use the if statement to specify a block of Java code to be executed if a condition is true.

Syntax:

if (condition) {
  // block of code to be executed if the condition is true
}

Example:

if (20 > 18) {
  System.out.println("20 is greater than 18");
}

The If-else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

Syntax:

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

Example:

int age = 20;
if (age < 18) {
  System.out.println("Eligible to vote");
} else {
  System.out.println("Not eligibile to vote");
}

The else-if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax:

if (condition1) {
  // block of code to be executed if condition1 is true
} else if (condition2) {
  // block of code to be executed if the condition1 is false and condition2 is true
} else {
  // block of code to be executed if the condition1 is false and condition2 is false
}

Example:

int age = 13;
if (age < 13) {
  System.out.println("Child");
} else if (19 <= age) && (age >= 13) {
  System.out.println("Teenager");
} else {
  System.out.println("Adult");
}
// Outputs "Teenager"

Short-Hand If..Else

There is also a short-hand if-else, which is known as the ternary operator because it consists of three operands.

It can be used to replace multiple lines of code with a single line, and is most often used to replace simple if-else statements:

Syntax:

variable = (condition) ? expressionTrue :  expressionFalse;

Example:

int age = 20;
String result = (age > 18) ? "Eligible to vote" : "Not eligible to vote";
System.out.println(result);

// Outputs "Eligible to vote"

The Switch Statement

Instead of writing many if..else statements, you can use the switch statement.

The switch statement selects one of many code blocks to be executed:

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

This is how it works:

• The switch expression is evaluated once.

• The value of the expression is compared with the values of each case.

• If there is a match, the associated block of code is executed.

• The break and default keywords are optional, and will be described later in this chapter

The example below uses the weekday number to calculate the weekday name:

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

Example:

int day = 4;
switch(day) {
  case 1:
    System.out.println("Monday");
    break;
  case 2:
    System.out.println("Tuesday");
    break;
  case 3:
    System.out.println("Wednesday");
    break;
  case 4:
    System.out.println("Thursday");
    break;
  case 5:
    System.out.println("Friday");
    break;
  case 6:
    System.out.println("Saturday");
    break;
  case 7:
    System.out.println("Sunday");
    break;
}
// Outputs "Thursday"

The break keyword

When Java reaches a break keyword, it breaks out of the switch block.

This will stop the execution of more code and case testing inside the block.

When a match is found, and the job is done, it's time for a break. There is no need for more testing.

Note: A break can save a lot of execution time because it "ignores" the execution of all the rest of the code in the switch block.

The default keyword

The default keyword specifies some code to run if there is no case match:

Example

int day = 4;
switch (day) {
  case 6:
    System.out.println("Today is Saturday");
    break;
  case 7:
    System.out.println("Today is Sunday");
    break;
  default:
    System.out.println("Looking forward to the Weekend");
}
// Outputs "Looking forward to the Weekend"