Understanding the for loop in Java

xstarts at 0 so the first iteration has the condition y<=0, the second will have y<=1 and so on .. till y<=5

While the second one will have y<=5in every iteration, thats why you get xxxxx in every line.

In the first code you print x times the "x" String in each row.

for(int y=1; y<=x; y++) {
    System.out.print("x");
}

BTW, it prints the following (which is different than what you claim in the question):

*****
x****
xx***
xxx**
xxxx*

In the second code you print 5 times the "x" String in each row.

for(int y=1; y<=5; y++) {
    System.out.print("x");
}

As you can see the x is = 5 too

No, x iterates from 0 to 4, so in each iteration of the outer for loop, it has a different value.

In your first code your for(int y=1; y<=x; y++) for iterations of outer for loop is -
for(int y=1;y<=0;++y) (for first iteration of outer loop)
for(int y=1;y<=1;++y) (for second iteration of outer loop)
for(int y=1;y<=2;++y) (for third iteration of outer loop)
for(int y=1;y<=3;++y) (for fourth iteration of outer loop)
for(int y=1;y<=4;++y) (for fifth iteration of outer loop)

But in your second code its always -
for(int y=1; y<=5; ++y)
for all iterations of outer for loop.

It is very simple the will run for 5 time times and every itreation its value will be increamented by 1 i.e. from 0 to 4.

So in first loop inner loop will have the condition like this:

for (int y = 1; y <= x; y++) {
      System.out.print("x");
}

But since in first loop the value of x is 0 hence it literally means:

for (int y = 1; y <= 0; y++) {
      System.out.print("x");
}         

But in the last iteartion of outer loop the value of x is 4 hence this is equivalent to:

for (int y = 1; y <= 4; y++) {
     System.out.print("x");
}

So it iterates 4 times.

In your first example y is first less than x = 1 and during the next iteration it will be less x= 2 ... Because x values changes with your first for loop.

For the second example however you state that y have to be less than 5 which doesn't change at all.

They are different because in the first case your x varies from 0 to 4 based on : for(int x=0; x<5; x++)

In the case second case x is fixed at 5.

I have replaced your two inner for loops with a new stringRepeat function. It might be easier to understand this way.

public class ForLoopStarsAndCrosses {

    public static void main(String[] args) {
        /// the total length ot the x's and stars
        final int length = 5;
        // start with 1 x until one less than the length (in this case 4)
        for(int x = 1; x < length; x++) {
            // the number of stars equals the total length minus the number of x's
            final int nbStars = length - x;
            // add the x's and the stars
            final String output = stringRepeat("x", x) + stringRepeat("*", nbStars);
            System.out.println(output);
        }
        System.out.println();
    }

    /** Repeat the string s n times */
    private static String stringRepeat(String s, int n) {
        String result = "";
        for (int i = 0; i < n; i += 1) {
            result += s;
        }
        return result;
    }
}

Firstly, you should understand the difference between value and variable. Value is a constant as you write 5 but variable can be changeable.

For your question, first code and first round:

x = 1, y<= 1 and output: x

for(int y=1; y<=x; y++){
                System.out.print("x");
            } 

but for the second code and first round:

y<=5 so output is: xxxxx

for(int y=1; y<=5; y++){
            System.out.print("x");
        }

it is very simple.

The purpose of loops (can be for, while, do-while) is that, how many number of times the same set of statements to be executed under a specific condition. "for" is a definite loop where there will be an index starting with an integer, keep increment it until the condition is achieved. "while" is a indefinite loop where there is no index and it will be executed until the condition is achieved. "do-while" is a loop similar to "while", which will be executed atleast once and then validates the condition for the next iteration.

Based on the above details,

for(int x=0; x<5; x++){
        for(int y=1; y<=x; y++){

for(int x=0; x<5; x++){
        for(int y=1; y<=5; y++){

The diff between these two conditions is that, First condition: In the first iteration, value of x is 0 and for the second loop y is started with 1. Here when it compares the value with x which is 0 and 1<=0 which is false, this condition is failed and the statements under Y will not be executed and the control will go back control of x.

Second condition: In the first iteration, value of x is 0 and for the second loop y is started with 1. Here when it compares the value with 5, 1<=5 which is true, this condition is valid and the statements under Y will be executed and the control will go back control of x until y becomes 6 and condition checked is 6<=5 which is false, this condition is failed and the statements under Y will not be executed and the control will go back control of x.