added the solution for longest common substring in java

Java/LongestCommonSubstring.java

@@ -0,0 +1,49 @@
+// Calculate the lenght of longest Common substring
+//Solution:
+public class LongestCommonSubstring{
+
+    public static void main(String []args){
+       String a = "abcdxyz";
+       String b = "xyzabcd";
+       int i,j;
+       int lena = a.length();
+       int lenb = b.length();
+       int[][] arr = new int[lena+ 1][lenb+1];
+
+       for(i = 0; i<= lena; i++ )
+       {
+           for(j = 0;j<=lenb; j++)
+           {
+               if(i==0 || j==0)
+               arr[i][j] = 0;
+           }
+       }
+       for(i = 1; i<= lena; i++ )
+       {
+           for(j = 1;j<=lenb; j++)
+           {
+               if(a.charAt(i-1) == b.charAt(j-1))
+               {
+                   arr[i][j] = 1 + arr[i-1][j-1];
+               }
+               else
+               arr[i][j] = 0;
+           }
+       }    
+       int max =0;
+       for(i = 0; i<= lena; i++ )
+       {
+           for(j = 0;j<=lenb; j++)
+           {
+
+                    if(arr[i][j]>max)
+                    {
+                        max = arr[i][j];
+                    }
+           }
+       }
+
+
+       System.out.println("Longest Common Substring : "+max+" length");
+   }
+}