There are two kind of loops in java, for and while.

The for loop has tree sections:

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

First section runs once when we enter the loop.

Second section is the gate keeper, if it returns true, we run what is in the loop, if it returns false, we exit the loop. It right after the first section for the first time then every time the loop is finished and the third section is run.

The third section is the final statement that will run every time the loop runs.

So in the case we have just seen, the loop will run 3 times. Here is the order of the commands:

int i = 0;
i < 3 // 0 < 3 true
      // Inside of loop
i++ // i is now 1
i < 3 // 1 < 3 true
      // Inside of loop
i++ // i is now 2
i < 3 // 2 < 3 true
      // Inside of loop
i++ // i is now 3
i < 3 // 3 < 3 false
      // Loop is done...

We can omit the first and third section of the loop (although it will be weird), and the loop will still work:

for (;i < 5;) {}

For cases where we want to use a loop that look like that, we use a while loop

The syntax is very similar to the previous for we looked at:

while (condition) {}

The condition will run for the first time when entering and every time the loop is done. If it returns false, the loop will not run.

If we want the loop to always run at least one, we can use do-while

do {

} while(condition);

Notice the ; in the end of the do-while.

Another version of for, is the foreach. The keyword we use is still for, but when we want to iterate on the elements inside an array we can simply use it:

int[] arr = {2, 0, 1, 3};
for (int el : arr) {
    System.out.println(el);
}

This is a short version and equivalent to:

int[] arr = {1, 9, 9, 5};
for (int i = 0; i < arr.length; i++) {
    int el = arr[i];
    System.out.println(el);
}

Notice that if you want to use the index of the element inside the loop, you have to use the longer version and can't use foreach.

These two keywords help us control the loop from within it. "break" will cause the loop to stop and will go immediately to the next statement after the loop:

int i;
for (i = 0; i < 5; i++) {
    if (i >= 2) {
        break;
    }
    System.out.println("Yuhu")
}
System.out.println(i);
// Output:
// Yuho
// Yuho
// 2

"continue" will stop the current iteration and will move to the next one. Notice that inside a for loop, it will still run the third section.

int i;
for (i = 0; i < 5; i++) {
    if (i >= 3) {
        break;
    }
    System.out.println("Yuhu");
    if (i >= 1) {
        continue;
    }
    System.out.println("Tata");
}
System.out.println(i);
// Output
// Yuhu
// Tata
// Yuhu
// Yuhu
// 3

Loop through and print out all even numbers from the numbers list in the same order they are received. Don't print any numbers that come after 237 in the sequence.

public class Main { public static void main(String[] args) { int[] numbers = { 951, 402, 984, 651, 360, 69, 408, 319, 601, 485, 980, 507, 725, 547, 544, 615, 83, 165, 141, 501, 263, 617, 865, 575, 219, 390, 984, 592, 236, 105, 942, 941, 386, 462, 47, 418, 907, 344, 236, 375, 823, 566, 597, 978, 328, 615, 953, 345, 399, 162, 758, 219, 918, 237, 412, 566, 826, 248, 866, 950, 626, 949, 687, 217, 815, 67, 104, 58, 512, 24, 892, 894, 767, 553, 81, 379, 843, 831, 445, 742, 717, 958, 609, 842, 451, 688, 753, 854, 685, 93, 857, 440, 380, 126, 721, 328, 753, 470, 743, 527}; } }

402 984 360 408 980 544 390 984 592 236 942 386 462 418 344 236 566 978 328 162 758 918