There are four kinds of loops in Java:

The while loop
The do-while loop
The for loop
The for-each loop (enhanced for)

The while loop

It’s executed while the given expression is evaluated to true:


  while (expression) 
     statement_or_block

while (expression)

statement_or_block

If the expression is initially false, the statement or block is never executed.

The do-while loop

It’s always executed at least once and keeps executing while the given expression is evaluated to true:


  do
     statement_or_block
  while (expression);

statement_or_block

while (expression);

The for loop


  for ([initialization_expr]; [termination_expr]; [increment_expr])
     statement_or_block

for ([initialization_expr]; [termination_expr]; [increment_expr])

statement_or_block

The four things you must remember about this loop are:

The initialization expression is executed just once, as the loop begins
When the termination expression evaluates to false, the loop terminates
The increment expression is invoked after each iteration. This expression can only be ++, --, an assignment (=), a method invocation or an object creation
All 3 expressions are optional:


  // infinite loop example
  for ( ; ; ) { 
  }

// infinite loop example

for ( ; ; ) {

}

The for-each loop (enhanced for)

This loop is used to iterate arrays or Collection objects:


  for (type element : collection)
     statement_or_block

for (type element : collection)

statement_or_block

The two things you must remember about this loop are:

final is the only allowed modifier for element
You can’t use an pre-existing variable for element

Example:


  // iterating an array
  int[] numbers =  {1,2,3};  
  for (int i : numbers) {
     System.out.println(i);
  }

  // iterating a collection
  ArrayList<Integer> list = new ArrayList<>();
  list.add(1);
  list.add(2);
  list.add(3);
  for (Integer e : list) {
     System.out.println(e);
  }

// iterating an array

int[] numbers = {1,2,3};

for (int i : numbers) {

System.out.println(i);

}

// iterating a collection

ArrayList<Integer> list = new ArrayList<>();

list.add(1);

list.add(2);

list.add(3);

for (Integer e : list) {

System.out.println(e);

}

Unreachable Code

Using false as the evaluation expression for a loop has different consequences depending on the kind of loop. Expect questions related to this topic in the exam:

Loop	Result
`while (false) {` `…` `}`	Compiler error
`do {` `…` `} while (false);`	Ok. The code block is executed once
`for (…; false; …) {` `…` `}`	Compiler error

To complete your preparation about Java loops, read the lesson about break and continue.

Loops

The while loop

The do-while loop

The for loop

The for-each loop (enhanced for)

Unreachable Code

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