Lesson 4: The If Statement

Now that we can define data and move it around, how do we check what it is? The if statement is the most common way to do that.

Let's break down a common if statement:

int foo = 3;
int bar = 9;
if (foo == 9) {
    System.out.println(bar);
}

This checks if foo's value is equal to 9, if it is then it prints the value of bar. If we ran this program, it would output nothing, because the statement foo == 9 evaluates to false when int foo = 3;.

A very important thing to understand here is that the if statement only takes a boolean. If we tried to do if (3) we would get an error, because 3 is not a boolean. (If you recall, a boolean is a data type that can only be true or false).

There are a few operators that compare values and "return" a boolean (they are not methods though). Here's a table of them (they should look familiar):

Operator	Name
==	Equals
!=	Not equals
> or <	Greater/less than
>= or <=	Greater/less than or equal to

Here's a few examples of these:

// This program prints fizz is greater than buzz!

double fizz = 3.9;
int buzz = 3;
if (if fizz > buzz) {
    System.out.println("fizz is greater than buzz!");
}

// This program prints Programming is cool!

double baz = 3;
double bin = 9;
if (Math.pow(baz, 2) == 9) {
    System.out.println("Programming is cool!");
}

There are also two conditional operators that compare booleans:

Operator	Name	Example
&&	Conditional AND	true && true evaluates to true
❘❘	Conditional OR	true && false evaluates to false

Also important to note is the logical compliment operator: !. It just turns a true value to false. For example:

boolean foo = false;
if (!foo) {
    System.out.println("foo is false!");
}

This example would output foo is false! because !false is true.

Else and Else If

What if you want one if statement to depend on the value of another?

You could do the following if you want the second statement to get run only if the first statement doesn't run:

// This will only print Second statement.

boolean foo = false;
int bar = 3;
if (foo) {
    System.out.println("First statement.")
}

if (!foo && bar == 3) {
    System.out.println("Second statement.")
}

The above works, but it's much more elegant to use the else if statement:

// This will print Second statement.

boolean foo = false;
int bar = 3;
if (foo) {
    System.out.println("First statement.")
} else if (bar == 3) {
    System.out.println("Second statement.")
}

Or, if you don't care that bar == 3, just that the previous statement didn't run, you can use the else statement:

// This will print Second statement.

boolean foo = false;
if (foo) {
    System.out.println("First statement.")
} else {
    System.out.println("Second statement.")
}

Notice that else and else if must immediately follow the end of an if statement body.

Equality Caveats

An important thing to note is that the equality operator (==) compares primitive types' values, but it does not compare the value of non-primitive types, it compares their pointer (memory address) instead.

Because of this, you shouldn't compare non-primitive types using == if you want to compare their values.

There are 8 primitive types, and they all start with lower case letters: int, double, boolean, short, long, float, char, and byte. The only non-primitive we've covered is String.

It is for that reason that the following does not print anything:

String foo = "foo";
String fooTwo = "foo";
if (foo == fooTwo) {
    System.out.println("This will never print anything.");
}

This is because String is not a primitive, and we're not comparing the values of those two strings. Instead, you should call a method on those strings: equals. For example, calling foo.equals("foo");" would return true. Please note that not all objects have an equals method.

If we want to make the earlier example work, we would do the following:

String foo = "foo";
String fooTwo = "foo";
if (foo.equals(fooTwo)) {
    System.out.println("This will get printed out.");
}

Conclusion

Go ahead and do the following PracticeIt problems:

1. If statement syntax

2. If statements and method parameters

3. if statements and method parameters