about.md

About

In Java there are four looping constructs, two that are iteration centric: For and For-each and the other two are condition centric: While and Do-while.

For

A for loop is a control flow statement that allows you to efficiently execute a block of code multiple times. It achieves this repetition through a concept called iteration, where a variable (often called a loop counter) takes on different values within a specified range. This loop continues executing the code block until the iteration reaches its end.

for (initialization; test; update) {
    body;
}

The initialization sets an initial state for the loop and is executed exactly once at the start of the loop. Typically it declares and assigns a variable used in the test expression and update statement. For example:

int i = 1

The test expression tests if the loop should end. If it evaluates to true, the body and then the update expression will be executed. If it evaluates to false, neither the body nor the update statement will be executed and execution resumes after the loop's closing bracket. Typically it checks the variable assigned in the initialization block.

For example:

i <= 10

After executing the loop body, the update expression is executed. Typically it increments or decrements the loop variable by some value. For example:

i++

A for loop printing out the first four squares would look like this:

for (int i = 1; i <= 4; i++) {
    System.out.println("square of " + i + " is " + i * i);
}

The output would be:

square of 1 is 1
square of 2 is 4
square of 3 is 9
square of 4 is 16

Iterating through every element in a collection is usually performed using a for-each, but it can be done with a for loop like this:

for (int i = 0; i < array.length; i++) {
    System.out.print(array[i]);
}

A for loop does have some advantages over a for-each loop:

• You can start or stop at the index you want.
• You can use any (boolean) termination condition you want.
• You can skip elements by customizing the incrementing of the loop variable.
• You can process collections from back to front by counting down.
• You can use for loops in scenarios that do not involve collections.

For-each

A for-each loop provides a mechanism for executing a block of code for each element in a collection.

for (declaration: collection) {
    body;
}

The declaration declares the variable used to hold the values assigned from the collection.

The collection is an array or collection holding the values that will be assigned to the loop variable.

The body contains the statements that will be executed once for each value in the collection.

For example:

char[] vowels = {'a', 'e', 'i', 'o', 'u'};

for(char vowel: vowels) {
    System.out.println(vowel);
}

which outputs:

a
e
i
o
u

Generally a for-each loop is preferrable over a for loop for the following reasons:

• A for-each loop is guaranteed to iterate over all values.
• A for-each loop is more declarative meaning the code is communicating what it is doing, instead of how it is doing it.
• A for-each loop is foolproof, whereas with for loops it is easy to have an off-by-one error (think of using < versus <=).
• A for-each loop works on all collection types, including those that do not support using an index to access elements (eg. a Set).

While

The while loop continually executes a block of statements while a particular condition is true.

while (condition) {
    body;
}

The condition It's a statement that can be true or false. As long as the condition is true, the loop keeps running. The body it's the code that gets executed repeatedly until the condition becomes false.

For example:

int counter = 1;

while (counter <= 5) {
    System.out.println(counter);
    counter++;
}

which outputs:

1
2
3
4
5

Generally good rule of thumb is to use a while loops when you don't know exactly how many times you need to loop beforehand.

Do-while

As while loops do-while loops are condition centric loops, but unlike a regular while loop, a do-while loop guarantees that the code inside the loop will run at least once, no matter what.

do {
    body;
} while (condition);

For example:

int counter = 1;

do {
    System.out.println(counter);
    counter++;
} while (counter < 2);

which outputs:

1

Break

The break statement acts as an "exit door" for a looping construct. When encountered within the loop's body, break immediately terminates the loop's execution.

For example:

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;
    }

    System.out.println(i);
}

which outputs:

0
1
2
3
4

Continue

The continue statement in the other hand acts similar to a "skip button" in a looping construct. When encountered within a loop's body, continue skips the remaining statements in the current iteration.

For exmaple:

for (int i = 0; i < 5; i++) {
    if (i < 3) {
        continue;
    }

    System.out.println(i);
}

which outputs:

3
4