Weird java for-each behavior when swapping variables

With the follow code, I expect four elements, but get only three. Clearly I'm doing something crazy silly.

import java.util.HashSet;

import java.util.Set;

public class MyTest {
    public static void main(String[] args) {
        String[] myStrings = new String[] { "a", "b" };

        Set<String> set = new HashSet<String>();
        for (String x : myStrings) {
  开发者_运维技巧          for (String y : myStrings) {
                String temp = x;
                x = y;
                y = temp;

                set.add(x + y);
            }
        }

        System.out.println(set);
    }
}

result:

[ba, aa, ab]

---

Sets don't allow duplicates, and you're adding 'ba' twice. Your loop control if confusing, but here's the simplified trace, with rvalues expanded.

x = "a"

y = "a"
temp = "a"
x = "a"
y = "a"
add("aa")

y = "b"
temp = "a"
x = "b"
y = "a"
add("ba")

x = "b"

y = "a"
temp = "b"
x = "a"
y = "b"
add("ab")

y = "b"
temp = "a"
x = "b"
y = "a"
add("ba")

---

You are modifying the variable of the outer loop each time you run through the inner loop. So x can become corrupted.

In particular the penultimate execution of the body assigns 'a' to x. Next time round the loop we miss out on "bb", instead getting a duplicat "ab".

Using final is quite handy.

---

Playing with loop variables is the issue.

Do this:

    Set<String> set = new HashSet<String>();
    for (String x : myStrings) {
        for (String y : myStrings) {
            String temp = x;
            x = y;
            y = temp;
            System.out.println(x+y);
            set.add(x + y);
        }
    }

You will see you are adding ab twice, which in a Set means overwriting. Then change the loop to use something other than loop variables like:

    Set<String> set = new HashSet<String>();
    for (String a : myStrings) {
        for (String b : myStrings) {
            String x = a;
            String y = b;
            String temp = x;
            x = y;
            y = temp;
            System.out.println(x+y);
            set.add(x + y);
        }
    }

And you will get all 4 things you want. I have to do some more reading about how traversing an array in the way you have done works, but that is clearly the culprit.

---

You're changing the outer loop variable, so the output is unexpectedly missing "bb".

---

for (String x : myStrings) {
    for (String y : myStrings) {
        String temp = x;
        x = y;
        y = temp;

        set.add(x + y);
    }
}

Why are you assigning things to x and y when they are both for loop variables? If you wish to loop through x and y independently, get rid of those 3 lines to swap x and y. You are modifying the variables in a way that is probably different than you expect.

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Maybe it's because you're putting the resulting strings into a hash set, which doesn't preserve order. Try with a linked hash set.

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The problem is simple -- where is "bb" in the output. It should generate 4 strings.