String permutation with recursion

Question

I am a java beginner and trying to do a string permutation practice from java programming book. I am defining two method:

public static void displayPermutation(String s)
public static void displayPermutation(String s1, String s2)

The first method simply invokes displayPermutation(" ", s). The second method uses a loop to move a character from s2 to s1 and recursively invokes it with a new s1 and s2. The base case is that s2 is empty and prints s1 to the console.

Can anyone help me to find what is the problem of the following code?

Her's example:

 public static void displayPermutation(String s) {
            displayPermuatation("", s);
        }

    private static void displayPermuatation(String s1, String s2) {
        //base case: when s2 is empty, print s1
        if (s2.isEmpty()) {
            System.out.println(s1);
        }
        else {          
        for (int i = 0; i < s2.length(); i++) {
                        //move a char from s1 to s2, and recursively invokes it with 
                        //new s1 and s2
            s1 = s1 + s2.charAt(i);
            s2 = s2.substring(0, i) + s2.substring(i+1);
            displayPermuatation(s1, s2);
        }
    }
   }

if s = "abc", it prints only: abc acb

it seems that in the first call of displayPermuatation("", "abc"), it does not finish the for loop.... any comments?

Thanks for all the comments below. I think the mistakes I made is because that passing object as argument to a method is actually passing the reference. it is not like primitive data (passing by value). When changing the object, it will affect following method call using that object.

Answer 1

Do not alter s1 and s2 in the loop, that causes the error. Simply pass those definitions as arguments to recursive function. Like this:

.
.
for (int i = 0; i < s2.length(); i++) {
        displayPermuatation(s1 + s2.charAt(i), s2.substring(0, i) + s2.substring(i+1));
    }
.
.

Answer 2

Problem with your code is that you are changing value of s1 and s2 in the loop which affects the following iterations in the loop, see the following code where I have fixed this issue.

public static void displayPermutation(String s) {
    displayPermuatation("", s);
}

private static void displayPermuatation(String s1, String s2) {
    // base case: when s2 is empty, print s1
    if (s2.isEmpty()) {
        System.out.println(s1);
    } else {
        for (int i = 0; i < s2.length(); i++) {
            // move a char from s1 to s2, and recursively invokes it with
            // new s1 and s2
            displayPermuatation(s1 + s2.charAt(i), s2.substring(0, i) + s2.substring(i + 1));
        }
    }
}