Statements

Scope

Scope, also called a block, is defined with curly braces. This restricts the code to be visible only within the scope. You can think of scope as a parent-child relationship:

String hello = "Hello";

{
    // Child scope can access parent scope variables
    String message = hello + " World"; // This works
}

// You can access your `hello` variable, since you're in the same scope
System.out.println(hello); // This works

// You cannot access the `message` variable, since it's in a child scope
System.out.println(message); // This fails.

Selection

Selection statements are all about choosing what to execute.

If

The if statement evaluates a conditional, which must resolve to either true or false. Then, you provide the scope of your if statement, which executes if the conditional evaluates to true:

if (8 > 4) { // 8 > 4 is your condition, which is true
    // your scope is executed when the conditional is true
}

if (4 > 8) { // this resolves to false
    // so your scope is skipped over
}

Else

The else statement is used in tandem with an if statement, providing scope for an if statement that resolves to false:

if (4 == 5) { // your conditional is "if 4 equals 5", which is false
    // so your if scope is skipped over
}
else {
    // your else scope will be executed
}

You can also chain if-else statements together:

int a = 4;
int b = 5;

if (a == b) { // checked first
    // executes if a equals b
}
else if (a > b) { // checked if the first conditional isn't true
    // executes if the a > b
}
else {
    // executes if all if conditionals above are false
}

Iteration

Iteration statements are all about executing something multiple times

While

The while statement executes everything within scope until a condition is false. Use a while loop if you need to execute some code until something happens:

while (true) {
    // do something here until condition is false
}

One thing to notice: the above code will execute indefinitely, since true will never be false. Let's look at a while loop that would be more beneficial. This while loop would execute the scope of the while loop 10 times:

int i = 0;

while (i < 10) { // Will execute as long as i < 10
    // Increment i each execution
    i++;
}

This loop will never execute, since 1 is not less than 1:

int i = 1;

while (i < 1) { // Will execute as long as i < 1
    i++;
}

Do

The do statement can be used in tandem with a while statement. Use a do-while loop if you need to execute some code at least once, then continue execution until something happens. This do-while loop would execute the scope of the loop 10 times:

int i = 0;
do {
    // Increment i each execution
    i++;
} while (i < 10);

You can use a do-while loop to ensure that the scope is executed at least once. This do-while loop would execute the scope of the loop 1 time, then exit:

int i = 1;
do {
    i++;
} while (i < 1);

For

The for statement has lots of uses, but primarily it's somewhat of a bounded while statement. Use a for loop if you need to execute some code a specific number of times.

There are 3 parts to a for loop:

• Initializer: runs once at the beginning
• Conditional: evaluates before each iteration

• Increment: increment code to execute during the loop

  • ++i increments before each conditional check and scope execution
  • i++ increments after each conditional check and scope execution

Below if a for loop that demonstrates the 3 parts:

• Initializer: int i = 0
• Conditional: i < 10
• Increment: i++

These parameters will execute the scope of the for loop 10 times:

for (int i = 0; i < 10; i++) {
    // Execute some code
}

Foreach

The foreach loop is used for iterating over a collection. The foreach loop is similar to a for loop, and in Java it looks nearly the same. The syntax is easier to read and so is preferable in many scenarios when you have a collection. Use a foreach loop if you need to execute some code on each element in a collection of elements.

Below is a foreach loop that iterates of an array of 2 email addresses. These parameters execute the scope of the foreach loop 2 times, once for each email address:

String[] emails = new [] { "a@me.com", "b@me.com" };

for (String email : emails) {
    //do something with the `email`, like send a marketing message
}

If we added a 3rd email address to the emails array, the foreach loop scope would execute 3 times.