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How can I make it so my while-loop only prints even numbers? Java Eclipse IDE

Beginner here. For my coding class, we have an assignment that requires us to print numbers 1-20, but configure it so that it only outputs even numbers. Here is what I have so far but I'm quite stuck. He says to put an if statement and use the "%" operator but I've no idea where to put them.
int counter = 1;
System.out.println("Part 2 - Even Numbers");
while (counter <= 20)
{
//if (counter
System.out.printf("%d ", counter);
counter++;
} // end while loop
Instructions for assignment
My Output
CORRECT Output

if(counter % 2 == 0){
System.out.printf("%d ", counter);
}
counter++;
% operator is mod operator, if counter % 2 == 0 , then counter is an even number

% is an arithmetic operator, it is called MODULO.
Modulo operator returns the remainder of 2 numbers. In this case, we use a modulo to find out whether a number is even or odd.
odd%2 returns 1
even%2 returns 0
The while loop loops through the first 20 elements. So we put an if statement before printing the element. If the counter is an even number i.e (counter%2 == 0) we print that.
This is the code that prints even numbers:
int counter = 0;
System.out.println("Part 2 - Even Numbers");
while (counter <= 20)
{
if (counter%2 == 0){
System.out.printf("%d ", counter);
}
counter++;
} // end while loop
This can also be done without using MODULO operator:
int counter = 0;
System.out.println("Part 2 - Even Numbers");
while (counter <= 20)
{
System.out.printf("%d ", counter);
counter+=2;
} // end while loop

use fori
public static void main(String[] args) {
for (int i = 1; i <= 20; i++) {
if (i % 2 == 0) {
System.out.println(i);
}
}
}
% is the remainder operation

Write a program in java to print the sum of all numbers from 50 to 250(inclusive of 50 and 250) that are multiples of 3 and not divisible by 9

/* when I run this code there is no error in fact output generated is also correct but I want to know what is the logical error in this code? please can any one explain what is the logical error. */
class abc
{
public static void main(String arg[]){
int sum=0;
//for-loop for numbers 50-250
for(int i=50;i<251;i++){
// condition to check if number should be divided by 3 and not divided by 9
if(i%3==0 & i%9!=0){
//individual number which are selected in loop
System.out.println(i);
//adding values of array so that total sum can be calculated
sum=sum+i;
}
}
//final display output for the code
System.out.println("the sum of intergers from 50 to 250 that are multiples of 3 and not divisible by 9 \n"+sum);
}
}

My philosophy is "less code == less bugs":
int sum = IntStream.rangeClosed(50, 250)
.filter(i -> i % 3 == 0)
.filter(i -> i % 9 != 0)
.sum();
One line. Easy to read and understand. No bugs.

Change this:
if(i%3==0 & i%9!=0){
to this:
if(i%3==0 && i%9!=0){
& = bitwise and operator
&& = logical operator
Difference between & and && in Java?

The only problems I saw were:
The variable sum was undeclared
Use && in place of &
int sum = 0;
for (int i = 50; i <= 250; i++) {
if (i % 3 == 0 && i % 9 != 0) {
System.out.println(i);
sum = sum + i;
}
}
System.out.println("the sum of intergers from 50 to 250 that are multiples of 3 and not divisible by 9 \n" + sum);

Well, instead of touching every single value from 50 to 250 like you would do here for(int i=50;i<251;i++), you can consider something like this...
int i = 48;
int sum = 0;
while(i < 250) {
i += 3;
if(i%9 != 0)
sum += i;
}
This is somewhat optimized in the sense that I am skipping over values that I know are not possible candidates.
But, there is a much bigger issue in your code. The following code block prints true, sure. But, it is a bad idea to depend on the & since that is not its job. The & is for bitwise AND whereas the && is for logical AND, which is what you are trying to do.
boolean t = true;
boolean f = false;
System.out.println(f&t);
Why?
In Java, if it is a && operation, as soon as you find the first false, you are sure that the expression will evaluate to false. Meanwhile, in your implementation, it would need to evaluate both sides. f&t will evaluate to false, but the JVM would need to look at both the f and t variables. Meanwhile, on using &&, it wouldn't even need to look at the t.

Supposed variable cannot be resolved as variable?

I'm doing a basic Java tutorial and below is the question.
Write a method that prints the numbers from 1 to 100. But for multiples of three print ÒFizzÓ instead of the number,and for the multiples of five print ÒBuzzÓ. For numbers which are multiples of both three and five print ÒFizzBuzzÓ."
My code is below
public static void fizzBuzz(){
for(int i = 0; i < 101; i= i +1 )
System.out.println(i);
if (i%3 == 0){
System.out.println("ÒFizzÓ");
}else if (i % 5 == 0){
System.out.println("ÒBuzzÓ");
}else if (i % 15 == 0){
System.out.println("ÒFizzBuzzÓ");
}
}
Eclipse tells me that "i" cannot be resolved as a variable. This is confusing to me as I thought I already defined "i" as an integer in my for loop? Thanks for taking the time to solve this newbie question :)

Add braces or your loop body ends after the first statement. Also, for your approach you need to test 15 first because it's a multiple of 3 and 5
for(int i = 0; i < 101; i++) { // <-- i++ is short for i = i + 1
System.out.println(i);
if (i % 15 == 0) {
System.out.println("ÒFizzBuzzÓ");
} else if (i % 5 == 0) {
System.out.println("ÒBuzzÓ");
} else if (i % 3 == 0) {
System.out.println("ÒFizzÓ");
}
}

I know a funny story about Apple who lost a few million dollars because a developer updated a code with an if block but... the if statement had only one instruction and no curly brackets and he did not see it. Thus, the code he was willing to add when the condition was met were actually ALWAYS executed.
In your case, you won't lose money but you surely did the same mistake :
for(int i = 0; i < 101; i= i +1 ) {
System.out.println(i);
if (i % 15 == 0){
System.out.println("ÒFizzBuzzÓ");
} else if (i%3 == 0){
System.out.println("ÒFizzÓ");
} else if (i % 5 == 0){
System.out.println("ÒBuzzÓ");
}
}

When Java says something cannot be resolved as a variable, it is usually been used outside the scope it was declared or it was not declared at all.In your case, your braceless for-loop is causing the problem.

Trying to properly print an array in Java

I am new to Programming so bear with me if I do not properly present my issue. I have an assignment to create a program to assign integer values 1-25 to a 25 element integer array. I need to print the array as five separate lines with each line containing 5 elements separated by commas. The last element does not have a comma following it. The final output should be as follows:
1,2,3,4,5
6,7,8,9,10
11,12,13,14,15
16,17,18,19,20
21,22,23,24,25
The code that I came up with comes close, but it's not quite right. The code that I came up with is:
public class Test2 {
/**
* #param args
* the command line arguments
*/
public static void main(String[] args) {
int[] numbers = new int[25];
for (int i = 0; i < numbers.length; i++) {
numbers[i] = i + 1;
System.out.print(numbers[i] + ",");
if (i % 5 == 0 && i > 0)
System.out.println();
}
}
}
The printout that I get from my code is as follows:
1,2,3,4,5,6,
7,8,9,10,11,
12,13,14,15,16,
17,18,19,20,21,
22,23,24,25,
I am not sure why I am getting 1-6 on the first line as well as how to remove the comma at the end of each line. Any help pointing out my errors would be appreciated.

The error is that you are checking if int i is divisible by 5 (i % 5), not numbers[i] (numbers[i] % 5). This way, your code prints:
number 1 when i = 0,
number 2 when i = 1,
number 3 when i = 2,
number 4 when i = 3,
number 5 when i = 4,
number 6 when i = 5
and finally prints line break.
The correct code is:
int[] numbers = new int[25];
for (int i = 0; i < numbers.length; i++) {
numbers[i]=i+1;
System.out.print(numbers[i]);
if (numbers[i] % 5 == 0 && i > 0) {
System.out.println();
} else {
System.out.print(",");
}
}
The above code will print (as intended):
1,2,3,4,5
6,7,8,9,10
11,12,13,14,15
16,17,18,19,20
21,22,23,24,25

You're getting 6 numbers on the first line, because you start counting at i=0, and only print the newline once i=5; at which point the number you've just printed is 6, not 5 - you're printing i+1 in each iteration.
If you made your logic such that it printed EITHER a comma OR a newline but not both, you'd get rid of the commas at the ends of the lines.

You're close. Very close.
Consider what your condition is checking - you want to inspect a value i, and want to stop when that particular value is divisible by 5 but is nonzero.
The problem is that you have the wrong value - i isn't what you want, but numbers[i]. The reason: each number in numbers[i] is offset of i by 1.
What you want to do is check if numbers[i] is divisible by 5. You still need to check for a nonzero i, though.
if(numbers[i] % 5 == 0 && i > 0) {
System.out.println(numbers[i]);
} else {
System.out.print(numbers[i] + ",");
}

Replace
if (i % 5 == 0 && i > 0)
with
if (i % 5 == 4)

How to check if an integer can be divided by 3

How to check if my integer can be divided by 3 as below:
for(int i=0; i<24; i++){
//here, how to check if "i" can be divided by 3 completely(e.g. 3, 6, 15)?
}

Use the modulo operator.
if(i % 3 == 0)
Also see Modulo operation at Wikipedia

If you are using a loop, you can use the fact that every third number can be divided by 3.
for(int i = 0; i < 24; i += 3) {
System.out.println(i + " can be divided by 3");
System.out.println((i+1) + " cannot be divided by 3");
System.out.println((i+2) + " cannnot be divided by 3");
}
This avoids the need for a modulo and cuts the number of loops by a factor of 3.

Use the MOD operator
for(int i=0; i<24; i++){
if( i%3 == 0 )
// It is divisible by 3
}

Well, what you could do (it might be a bit faster; it is faster on my machine) is:
boolean canBeDevidedBy3 = ((int) (i * 0x55555556L >> 30) & 3) == 0;
instead of
boolean canBeDevidedBy3 = (i % 3) == 0;
However, the multiplication trick only works for -2 <= i <= 1610612735. This answer was inspired by this optimization question. But if I can give you a tip: use (i % 3) == 0. It's so much simpler, and will always work.

Check the remainder of i devided by 3
if (i % 3 == 0) {}

inside the loop:
if (i%3 == 0)
// it can be divided by 3
% is called "mod" or "modulus" and gives you the remainder when dividing two numbers.
These are all true:
6 % 3 == 0
7 % 3 == 1
7 % 4 == 3

if( i % 3 == 0 )
The % operator delivers you the rest of the division i / 3

if( i % 3 == 0 ){
System.out.println("can be divided by 3");
}else{
System.out.println("cant divide by 3");
}
Is this question for real?