Permutations of a string
Calling static methods


   public class Permutation{



Permutation doesn't really fit.  This isn't a permutation.  It's something that creates permutations.  Consider names like Permuter or PermutationUtils.  


        Permutation p = new Permutation();
        String str = "1234";
        int n = str.length();
        p.permute(str, 0 ,n-1);

     }

     public static void permute(String str, int l, int r){



You don't need to call static methods from an object.  This could be just 

        String str = "1234";
        int n = str.length();
        Permutation.permute(str, 0, n - 1);
     }

     public static void permute(String str, int l, int r) {


There's no need to instantiate an object.  

Beyond that, I'd actually prefer 

         PermutationUtils.permute("1234");
     }

     public static void permute(String str) {
         permute(str, 0, str.length() - 1);
     }

     private static void permute(String str, int l, int r) {


The caller shouldn't have to know details about how the method is called in my opinion.  

Instantiating and using an object

class Permuter {

    private final String original;
    private final List&lt;String&gt; permutations = new ArrayList&lt;&gt;();

    public Permuter(String original) {
        this.original = original;
    }

    public List&lt;String&gt; permute() {
        if (permutations.empty()) {
            permute(original, 0, original.length() - 1);
        }

        return permutations;
    }


That's an example of how the class could be defined if it was meant to be instantiated.  Then you could call it like 

    public static void main(String[] args) {
        Permuter p = new Permuter("1234");
        for (String permutation : p.permute()) {
            System.out.println(permutation);
        }
    }


This also returns the permutations and only displays them in the main method.  This is generally a better practice.  This way if what they really wanted was the permutations, you don't have to rewrite the whole code to give it to them.  

You may want to consider using a longer variable name than p.  It's OK here, as it's only used on adjacent lines.  But in a longer method, single letter variables names can get confusing.  

Don't do unnecessary work


         for(int i=l;i&lt;=r;i++){
             str = swap(str,i,l);
             permute(str, l+1, r);
             str = swap(str,i,l);
         }



This pattern is what you would use if str were a mutable, like a StringBuilder or array.  Since swap creates a new string, you could just say 

         for (int i = l; i &lt;= r; i++) {
             permute(swap(str, i, l), l + 1, r);
         }


It's not necessary to call swap twice, since it doesn't change the original string.  This way str never changes.  

I also changed the spacing.  It's easier to see where the variables start and end if there are spaces around the operators.  

If efficiency matters

If efficiency matters, consider using a mutable instead.  This version creates a new string every time swap is called.  With a mutable, it could create a new string only when it needed to add to the list or display.