Java Break Statement: For Loop, Switch

These Java examples use the break statement in for-loops, while and switch. Break can slow down a for-loop.
Break. A loop continues until a condition is reached. And control falls through each case in a switch. With break, a keyword, we stop control flow.
It stops the entire loop. No further iterations in a loop will occur after a break is reached. Continue, meanwhile, will stop just the current iteration.
For-loop break. This program uses the break statement in a for-loop. We loop over 100 numbers from 0 to 99. But when 5 is reached, a break statement executes.

And: The loop terminates before the value 6 is printed. The break is a way to stop a loop, eve none that has not reached its bounds.

Note: The break statement has no arguments. We cannot use labels with it. We cannot use it to break out of nested loops at once.

Return: A return sometimes is used in place of a break (but the enclosing method returns also).

Java program that uses break in for-loop public class Program { public static void main(String[] args) { // Loop over the first 100 integers. for (int i = 0; i < 100; i++) { System.out.println(i); // Break when the variable equals 5. if (i == 5) { break; } } } } Output 0 1 2 3 4 5
While-true loop. Break is most useful in while-loops. Here we use a while-true loop, which repeats infinitely until broken. If the random number is low, we break the loop.While

Note: Some researchers have found that breaking from a while-loop is more intuitive for students than using for-loops.

However: When we use this style of code, we must be careful that the loop terminates. An infinite loop is a hazard.

Java program that breaks from while-true loop public class Program { public static void main(String[] args) { // Use a while-true loop. while (true) { double number = Math.random(); System.out.println(number); // Break if random number is less than a certain value. if (number <= 0.2) { break; } } } } Output 0.5650302991616442 0.64936100765301 0.18994610761919195
Switch with break. A switch statement has case blocks. If a break (or return) statement does not terminate a case, control will fall through to the next case.

Note: Fall-through occurs even when the value does not match the case labels. Only the first case is matched.

Here: We do not use a break in case 0. But case 1 has a break. So when we switch on 0, id is incremented twice.

Java program that uses break in switch public class Program { static int getId(int index) { int id = 0; switch (index) { case 0: // No break is in this case block, so control falls through. id++; case 1: // Control breaks in this block. id++; break; case 2: // Set id to 100. id = 100; } return id; } public static void main(String[] args) { int id = getId(0); System.out.println(id); id = getId(1); System.out.println(id); id = getId(2); System.out.println(id); } } Output 2 1 100
Performance. Here we test the performance of a break statement in a loop. The two loops (sum1 and sum2) are not precisely equivalent.

Sum1: This version loops through a range and sums all elements in the range. It starts at 0 and ends at max.

Sum2: This loops through the entire array, and breaks when the max is reached. It sums all elements up to this point.

Result: The sum1 loop is much faster. It has no break statement, and the loop design is simpler.

So: When designing loops, it is best to specify a range, and avoid the break-keyword. Break may cause a slowdown.

Java program that benchmarks break-statement public class Program { static int sum1(int max, int[] array) { // Loop over range in a for-loop. int sum = 0; for (int i = 0; i < max; i++) { sum += array[i]; } return sum; } static int sum2(int max, int[] array) { // Loop over entire array, and use break to stop. int sum = 0; for (int i = 0; i < array.length; i++) { if (i >= max) { break; } sum += array[i]; } return sum; } public static void main(String[] args) { int[] values = { 10, 20, 30, 40, 50, 60, 70, 80 }; long t1 = System.currentTimeMillis(); // Version 1: use for-loop with range. for (int i = 0; i < 100000000; i++) { sum1(5, values); } long t2 = System.currentTimeMillis(); // Version 2: use for-loop with range and break. for (int i = 0; i < 100000000; i++) { sum2(5, values); } long t3 = System.currentTimeMillis(); // ... Times. System.out.println(t2 - t1); System.out.println(t3 - t2); } } Output 5 ms, sum1: For-loop 254 ms, sum2: For-loop with break
A review. Break is an important keyword in Java. It affects the flow of control. It influences what statements are next reached, in a loop or in a switch.ForSwitch
Other control statements, like continue or return, may take the place of break. These statements create branches in code. They change the next statement executed.
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