In computer programming, it’s often desirable to execute a certain section of code based upon whether the specified condition is true or false (which is known only during the run time). For such cases, control flow statements are used.

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Java if (if-then) Statement

In Java, the syntax of the if-then statement is:

if (expression) {
    // statements
}

Here expression is a boolean expression. A boolean expression returns either true or false.

• if the expression is evaluated to true, statement(s) inside the body of if (statements inside parenthesis) are executed
• if the expression is evaluated to false, statement(s) inside the body of if are skipped from execution

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How if statement works?

Working of Java if statement

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Example 1: Java if Statement

class IfStatement{
    public static void main(String[] args){

        int number = 10;

        // checks if number is greater than 0
        if (number > 0) {
            System.out.println("The number is positive.");
        }
        System.out.println("This statement is always executed.");
    }
}

Output:

The number is positive.
This statement is always executed.

In the above example, we have a variable named number. Here, the test expression checks if the number is greater than 0 (number > 0).

Since the number is greater than 0. So the test expression evaluates to true. Hence code inside the body of if is executed.

Now, change the value of the number to a negative integer. Let’s say -5.

int number = -5;

If we run the above program with the new value of the number, the output will be:

This statement is always executed.

Here, the value of number is less than 0. So, the test expression number > 0 evaluates to false. Hence, the body of if is executed.

 

Note: We can also use strings as the boolean expression.

Example 2: Java if…else with String

class Main{
  public static void main(String[] args){
    // create a string variable
    String language = "Java";

    // if statement
    if(language == "Java") {
      System.out.println("This is best programming language.");
    }
  }
}

Output

This is best programming language.

Here, we are comparing two strings in the if block.

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Java if…else (if-then-else) Statement

The if statement executes a certain section of code if the test expression is evaluated to true. However, if the test expression is evaluated to false, it does nothing.

In this case, we can use an optional else block. Statements inside the body of else block are executed if the test expression is evaluated to false. This is known as the if-then-else statement in Java.

The syntax of the if-then-else statement is:

if (expression) {
   // codes
}
else {
  // some other code
}

Here, our program will do one task (task inside if block) if the test expression is true and another task (task inside else block) if the test expression is false.

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How if…else statement works?

Working of Java if-else statements

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Example 2: Java if else Statement

class IfElse{
    public static void main(String[] args){    	
        int number = 10;

        // checks if number is greater than 0	 
        if (number > 0) {
            System.out.println("The number is positive.");
        }
        else {
            System.out.println("The number is not positive.");
        }
 
        System.out.println("This statement is always executed.");
    }
}

Output:

The number is positive.
This statement is always executed.

In the above example, we have a variable named number. Here, the test expression checks if the number is greater than 0 (number > 0).

Since the value of the number is 10, the test expression evaluates to true. Hence code inside the body of if is executed.

Now, change the value of the number to a negative integer. Let’s say -5.

int number = -5;

If we run the program with the new value of the number, the output will be:

The number is not positive.
This statement is always executed.

Here, the value of number is -5. So the test expression evaluates to false. Hence code inside the body of else is executed.

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Java if..else..if Statement

In Java, we have an if…else…if ladder, that can be used to execute one block of code among multiple other blocks.

if (expression1) {
   // codes
}
else if(expression2) {
   // codes
}
else if (expression3) {
   // codes
}
.
.
else {
   // codes
}

Here, if statements are executed from the top towards the bottom. As soon as the test expression is true, codes inside the body of that the if statement is executed. Then, the control of the program jumps outside the if-else-if ladder.

If all test expressions are false, codes inside the body of else is executed.

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Example 3: Java if..else..if Statement

class Ladder{
    public static void main(String[] args){   

        int number = 0;

        // checks if number is greater than 0	 
        if (number > 0) {
            System.out.println("The number is positive.");
        }

        // checks if number is less than 0
        else if (number < 0) {
            System.out.println("The number is negative.");
        }
        else {
            System.out.println("The number is 0.");
        } 
    }
}

Output:

The number is 0.

In the above example, we are checking whether the number is positive, negative or zero. Here, we have two test expressions:

• number > 0 – checks if the number is greater than 0
• number < 0 – checks if the number is less than 0

Here, the value of number is 0. So both the test expression evaluates to false. Hence the statement inside the body of else is executed.

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Java Nested if..else Statement

In Java, it is also possible to if..else statements inside a if..else statement. It’s called nested if...else statement.

Here’s a program to find largest of 3 numbers:

Example 4: Nested if…else Statement

class Number{
    public static void main(String[] args){

        // declaring double type variables
        Double n1 = -1.0, n2 = 4.5, n3 = -5.3, largestNumber;

        // checks if n1 is greater than or equal to n2
        if (n1 >= n2) {

            // if...else statement inside the if block
            // checks if n1 is greater than or equal to n3
            if (n1 >= n3) {
                largestNumber = n1;
            }

            else {
                largestNumber = n3;
            }
        }
        else {

            // if..else statement inside else block
            // checks if n2 is greater than or equal to n3
            if (n2 >= n3) {
                largestNumber = n2;
            }

            else {
                largestNumber = n3;
            }
        }

        System.out.println("The largest number is " + largestNumber);
    }
}

Output:

The largest number is 4.5