Java LanguageBasic Control Structures

Remarks

All control structures, unless otherwise noted, make use of block statements. These are denoted by curly braces {}.

This differs from normal statements, which do not require curly braces, but also come with a stiff caveat in that only the line immediately following the previous statement would be considered.

Thus, it is perfectly valid to write any of these control structures without curly braces, so long as only one statement follows the beginning, but it is strongly discouraged, as it can lead to buggy implementations, or broken code.

Example:

// valid, but discouraged
Scanner scan = new Scanner(System.in);
int val = scan.nextInt();
if(val % 2 == 0)
    System.out.println("Val was even!");


// invalid; will not compile
// note the misleading indentation here
for(int i = 0; i < 10; i++)
    System.out.println(i);
    System.out.println("i is currently: " + i);

If / Else If / Else Control

if (i < 2) {
  System.out.println("i is less than 2");
} else if (i > 2) {
  System.out.println("i is more than 2");
} else {
  System.out.println("i is not less than 2, and not more than 2");
}

The if block will only run when i is 1 or less.

The else if condition is checked only if all the conditions before it (in previous else if constructs, and the parent if constructs) have been tested to false. In this example, the else if condition will only be checked if i is greater than or equal to 2.

If its result is true, its block is run, and any else if and else constructs after it will be skipped.

If none of the if and else if conditions have been tested to true, the else block at the end will be run.

For Loops

for (int i = 0; i < 100; i++) {
    System.out.println(i);
}

The three components of the for loop (separated by ;) are variable declaration/initialization (here int i = 0), the condition (here i < 100), and the increment statement (here i++). The variable declaration is done once as if placed just inside the { on the first run. Then the condition is checked, if it is true the body of the loop will execute, if it is false the loop will stop. Assuming the loop continues, the body will execute and finally when the } is reached the increment statement will execute just before the condition is checked again.

The curly braces are optional (you can one line with a semicolon) if the loop contains just one statement. But, it's always recommended to use braces to avoid misunderstandings and bugs.

The for loop components are optional. If your business logic contains one of these parts, you can omit the corresponding component from your for loop.

int i = obj.getLastestValue(); // i value is fetched from a method
    
for (; i < 100; i++) { // here initialization is not done
    System.out.println(i);
}

The for (;;) { function-body } structure is equal to a while (true) loop.

Nested For Loops

Any looping statement having another loop statement inside called nested loop. The same way for looping having more inner loop is called 'nested for loop'.

    for(;;){
        //Outer Loop Statements
        for(;;){
            //Inner Loop Statements
        }
        //Outer Loop Statements
    }

Nested for loop can be demonstrated to print triangle shaped numbers.

for(int i=9;i>0;i--){//Outer Loop
    System.out.println();
    for(int k=i;k>0;k--){//Inner Loop -1
        System.out.print(" ");
    }
    for(int j=i;j<=9;j++){//Inner Loop -2
        System.out.print(" "+j);
    }
 }

While Loops

int i = 0;
while (i < 100) { // condition gets checked BEFORE the loop body executes
    System.out.println(i);
    i++;
}

A while loop runs as long as the condition inside the parentheses is true. This is also called the "pre-test loop" structure because the conditional statement must be met before the main loop body is performed every time.

The curly braces are optional if the loop contains just one statement, but some coding style conventions prefers having the braces regardless.

do...while Loop

The do...while loop differs from other loops in that it is guaranteed to execute at least once. It is also called the "post-test loop" structure because the conditional statement is performed after the main loop body.

int i = 0;
do {
    i++;
    System.out.println(i);
} while (i < 100); // Condition gets checked AFTER the content of the loop executes.

In this example, the loop will run until the number 100 is printed (even though the condition is i < 100 and not i <= 100), because the loop condition is evaluated after the loop executes.

With the guarantee of at least one execution, it is possible to declare variables outside of the loop and initialize them inside.

String theWord;
Scanner scan = new Scanner(System.in);
do {
    theWord = scan.nextLine();
} while (!theWord.equals("Bird"));

System.out.println(theWord);

In this context, theWord is defined outside of the loop, but since it's guaranteed to have a value based on its natural flow, theWord will be initialized.

For Each

Java SE 5

With Java 5 and up, one can use for-each loops, also known as enhanced for-loops:

List strings = new ArrayList();
        
strings.add("This");
strings.add("is");
strings.add("a for-each loop");
        
for (String string : strings) {
    System.out.println(string);
}

For each loops can be used to iterate over Arrays and implementations of the Iterable interface, the later includes Collections classes, such as List or Set.


The loop variable can be of any type that is assignable from the source type.

The loop variable for a enhanced for loop for Iterable<T> or T[] can be of type S, if

  • T extends S
  • both T and S are primitive types and assignable without a cast
  • S is a primitive type and T can be converted to a type assignable to S after unboxing conversion.
  • T is a primitive type and can be converted to S by autoboxing conversion.

Examples:

T elements = ...
for (S s : elements) {
}
TSCompiles
int[]longyes
long[]intno
Iterable<Byte>longyes
Iterable<String>CharSequenceyes
Iterable<CharSequence>Stringno
int[]Longno
int[]Integeryes

If / Else

int i = 2;
if (i < 2) {
  System.out.println("i is less than 2");
} else {
  System.out.println("i is greater than 2");
}

An if statement executes code conditionally depending on the result of the condition in parentheses. When condition in parentheses is true it will enter to the block of if statement which is defined by curly braces like { and }. opening bracket till the closing bracket is the scope of the if statement.

The else block is optional and can be omitted. It runs if the if statement is false and does not run if the if statement is true Because in that case if statement executes.

See also: Ternary If

Switch statement

The switch statement is Java's multi-way branch statement. It is used to take the place of long if-else if-else chains, and make them more readable. However, unlike if statements, one may not use inequalities; each value must be concretely defined.

There are three critical components to the switch statement:

  • case: This is the value that is evaluated for equivalence with the argument to the switch statement.
  • default: This is an optional, catch-all expression, should none of the case statements evaluate to true.
  • Abrupt completion of the case statement; usually break: This is required to prevent the undesired evaluation of further case statements.

With the exception of continue, it is possible to use any statement which would cause the abrupt completion of a statement. This includes:

  • break
  • return
  • throw

In the example below, a typical switch statement is written with four possible cases, including default.

Scanner scan = new Scanner(System.in);
int i = scan.nextInt();
switch (i) {
    case 0:
        System.out.println("i is zero");
        break;
    case 1:
        System.out.println("i is one");
        break;
    case 2:
        System.out.println("i is two");
        break;
    default:
        System.out.println("i is less than zero or greater than two");
}

By omitting break or any statement which would an abrupt completion, we can leverage what are known as "fall-through" cases, which evaluate against several values. This can be used to create ranges for a value to be successful against, but is still not as flexible as inequalities.

Scanner scan = new Scanner(System.in);
int foo = scan.nextInt();
switch(foo) {
    case 1:
        System.out.println("I'm equal or greater than one");
    case 2:
    case 3:    
        System.out.println("I'm one, two, or three");
        break;
    default:
        System.out.println("I'm not either one, two, or three");
}

In case of foo == 1 the output will be:

I'm equal or greater than one
I'm one, two, or three

In case of foo == 3 the output will be:

I'm one, two, or three
Java SE 5

The switch statement can also be used with enums.

enum Option {
    BLUE_PILL,
    RED_PILL
}

public void takeOne(Option option) {
    switch(option) {
        case BLUE_PILL:
            System.out.println("Story ends, wake up, believe whatever you want.");
            break;
        case RED_PILL:
            System.out.println("I show you how deep the rabbit hole goes.");
            break;
            
    }
}
Java SE 7

The switch statement can also be used with Strings.

public void rhymingGame(String phrase) {
    switch (phrase) {
        case "apples and pears":
            System.out.println("Stairs");
            break;
        case "lorry":
            System.out.println("truck");
            break;
        default:
            System.out.println("Don't know any more");
    }
}

Ternary Operator

Sometimes you have to check for a condition and set the value of a variable.

For ex.

String name;

if (A > B) {
    name = "Billy";
} else {
    name = "Jimmy";
}

This can be easily written in one line as

String name = A > B ? "Billy" : "Jimmy";

The value of the variable is set to the value immediately after the condition, if the condition is true. If the condition is false, the second value will be given to the variable.

Break

The break statement ends a loop (like for, while) or the evaluation of a switch statement.

Loop:

while(true) {
   if(someCondition == 5) {
       break;
   }
}

The loop in the example would run forever. But when someCondition equals 5 at some point of execution, then the loop ends.

If multiple loops are cascaded, only the most inner loop ends using break.

Try ... Catch ... Finally

The try { ... } catch ( ... ) { ... } control structure is used for handling Exceptions.

String age_input = "abc";
try {
    int age = Integer.parseInt(age_input);
    if (age >= 18) {
        System.out.println("You can vote!");
    } else {
        System.out.println("Sorry, you can't vote yet.");
    }
} catch (NumberFormatException ex) {
    System.err.println("Invalid input.  '" + age_input + "' is not a valid integer.");
}

This would print:

Invalid input. 'abc' is not a valid integer.

A finally clause can be added after the catch. The finally clause would always be executed, regardless of whether an exception was thrown.

try { ... } catch ( ... ) { ... } finally { ... }

String age_input = "abc";
try {
    int age = Integer.parseInt(age_input);
    if (age >= 18) {
        System.out.println("You can vote!");
    } else {
        System.out.println("Sorry, you can't vote yet.");
    }
} catch (NumberFormatException ex) {
    System.err.println("Invalid input.  '" + age_input + "' is not a valid integer.");
} finally {
    System.out.println("This code will always be run, even if an exception is thrown");
}

This would print:

Invalid input. 'abc' is not a valid integer.
This code will always be run, even if an exception is thrown

Nested break / continue

It's possible to break / continue to an outer loop by using label statements:

outerloop:
for(...) {
    innerloop:
    for(...) {
        if(condition1)
            break outerloop;

        if(condition2)
            continue innerloop; // equivalent to: continue;
    }
}

There is no other use for labels in Java.

Continue Statement in Java

The continue statement is used to skip the remaining steps in the current iteration and start with the next loop iteration. The control goes from the continue statement to the step value (increment or decrement), if any.

String[] programmers = {"Adrian", "Paul", "John", "Harry"};

    //john is not printed out
    for (String name : programmers) {
        if (name.equals("John"))
            continue;
        System.out.println(name);
    }

The continue statement can also make the control of the program shift to the step value (if any) of a named loop:

Outer: // The name of the outermost loop is kept here as 'Outer'
for(int i = 0; i < 5; )
{
    for(int j = 0; j < 5; j++)
    {
        continue Outer;
    }
}