Learn Java

What is control flow

Statements execute top to bottom, line following line. Control flow statements interrupt this normal code execution.

When you need to make a decision, repeat statements, or branch elsewhere, you use Java's control structures.

• If or Switch statements - the outcome of a condition determines which branch to take
• Looping - repeat the same statements
• Break, continue or return statements - program must branch immediately

How it's done

Choice statements

• if-then
• if-then-else
• switch

Repetitive loops

• for
• while
• do while
• Branching
• break
• continue
• return

Using booleans

• Choice and looping statements make use of booleans.
• A boolean condition is used and if this condition evaluates to true the statements inside the block are executed.
• If the condition evaluates to false, the statements are skipped.
• The code outside the control statements is executed line by line regardless.

Using booleans

• hasMoved is a boolean variable
• If hasMoved is true then counter will be incremented.
• If hasMoved is false the code within the highlighted brackets will not execute.
• the print statement will always execute.

Choice

• if and switch
• Anatomy of an if
• Anatomy of a switch
• Which one to choose
• Examples

If & Switch

• if
• switch
• Both use the comparison of values to determine whether or not to execute statements inside their blocks
• If the comparison returns true then the statements are executed
• Otherwise, the statements are ignored

If

• It can be very simple and only execute statements if one boolean condition is met.
• Or it can include an alternative path when else is also used
• Can use multiple else if statements in conjunction with the if statement
• If statements can be nested inside each other

If

• The keyword if
• Boolean condition - inside the parentheses ( )
• Curly braces { }
• Code to potentially execute - inside the curly braces
• Optional else

Switch

• There's always a value outside the switch that is being compared
• Comparison is done in order - if checking for > 90 and > 80, do > 90 first
• Each case should have a break statement to close that case before another starts
• A default case handles values that the cases don't get a match for

The syntax ...

Switch

• The keyword switch followed by the boolean condition inside parentheses ( )
• The rest should all be inside curly braces { }
• case comparion value:
• Statement to be executed if this case is true
• break;
• Lastly default: (this is for when there's no match)

Switch

Which one ...

Use what you are comfortable with but as you progress in Java keep in mind the following-

• The if statement can become harder to follow if nested too many times
• In JDK1.6 the switch works only with byte, short, char and int
• But install JDK7 and you can use the switch on Strings, enum types and some Wrapper classes also
• When working with very large or complex data the switch is said to be faster

Examples

Simple if

If else

Nested if

Switch

Switch using char

Switch using int

Looping

• What are loops
• For loops
• While loops
• Do while loops
• Which one to choose
• Examples

What are loops

Loops are used when you wish to repeat lines of code, eg:

• Adding together a list of numbers
• Reading in the contents of a file
• Searching for a word in a paragraph of text
• Loops generally work with a pre-defined condition. While this condition is met, the loop continues executing.
• The condition is a boolean expression eg:
• i < 10
• ticked == false

for loops

There are two forms of for loops

• General - can be used when you have any type of code to repeat
• Enhanced - used to iterate through a Collection or array
• The difference in their structure lies inside the parentheses ( ) General for loop ... Enchanced for loop ...

General for

• The keyword for
• Loop conditions - inside the parentheses ( )
• Initialization value
• Termination expression
• Increment expression
• Curly braces { }
• Code to execute - inside the curly braces { }

Enhanced for

Used for iterating through Collections or arrays.

• The Collection or array type
• an iteration variable (your choice)
• semi-colon :
• The Collection or array name

While

• The keyword while
• Boolean condition - inside the parentheses ( )
• Inside the curly braces { } :
• Statements to be executed if the condition evaluates to true
• AND LAST, a statement that changes the condition (without this the loop would never stop)

Do while

• The boolean condition is evaluated at the end of the loop
• Therefore the very first time the loop is entered, the statements will execute regardless of the condition
• After this first time, the loop is only entered again if the condition evaluates to true
• The statement must include an exit condition

Which one to use

• The for loop is used when you know beforehand how many times you wish to go through the loop
• The while loop is for when you don't know beforehand how many times to continue looping, or whether the loop should even execute at all
• The do while loop is used when you always want to execute the statements inside the loop at least once, but don't know if they should be executed any more times after that

Examples

For

While

Do while

Branching

• What is branching
• Break
• Continue
• Return
• When to use which

What is branching

• Branching is when the road suddenly veers off to the left and there's nowhere else to go
• In programming, it means to suddenly re-route the flow of execution from one point to another
• The 3 mechanisms used for branching are:
• break
• continue
• return

Break

Break statements terminate a loop. The 2 types are:

• unlabelled - terminates the inner-most control statement it is located in ( eg switch, for, while, do while)
• labelled - terminates a labelled statement

Unlabelled break

• The inner for loop exits prematurely when i equals 3
• For every other value of i the inner loop executes normally
• The outer loop is unaffected

Labelled break

• The outer for loop exits prematurely when i equals 3
• When this happens the inner loop automatically stops also

Continue

If you wish to continue running a loop but stop processing the remainder of the code in the loop body, use continue. The 2 types are:

• unlabelled - discontinues the current iteration of the control and enters the next one
• labelled - terminates a labelled statement

Unlabelled continue

• The loop will print the value of i for every value except 6
• When i equals 6 any code after the continue statement will not be executed

Labelled continue

• Both for loops continue to execute but the inner doesn't print anything when i equals 4

Return

The return statement exits from a method and returns execution to where the method was called from. It has 2 forms:

• The first returns a value to the program
• The second doesn't - it just causes the method to exit

Return

Returning a value to the program:

Just exiting the method:

Which to use

• Use this when you need to terminate an entire loop

• Use this to break out of a loop's iteration early

• Used to break out of a method or return a value to the program