extras: add P16-P23, P25

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

P17.java

@@ -0,0 +1,9 @@
+public class P17 {
+    public static void main(String[] args) {
+        int a = 5;
+        int b = 6;
+        int c = a + b;
+
+        System.out.print(c);
+    }
+}