add jml/esc source files

[ss1617.git] / jml_esc / Amount.java

diff --git a/jml_esc/Amount.java b/jml_esc/Amount.java
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+++ b/jml_esc/Amount.java
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+
+/* ESC/Java2 exercise
+
+   Objects of this class represent euro amounts. For example, an Amount 
+   object with
+     euros == 1
+     cents == 55
+   represents 1.55 euro. 
+
+   Specify the class with JML and check it ESC/Java2.  
+
+   NB there may be errors in the code that you have to fix to stop 
+   ESC/Java2 from complaining, as these complaints of ESC/Java2 
+   point to real bugs in the code. But keep changes to the code to
+   a minimum of what is strictly needed. 
+   Mark any changes you have made in the code with a comment,
+   eg to indicate that you replaced <= by <.
+
+   You should add enough annotations to stop ESC/Java2 complaining,
+   but you should ALSO specify invariants discussed below:
+
+   1) We do not want to represent 1.55 euro as an object with
+         euro  == 0
+         cents == 155
+      (Note that the "equals" method will not be correct if we allow 
+       this.)
+      Specify an invariant that rules this out.
+
+   2) We do not want to represent 1.55 euro as an object with
+         euros =  2
+         cents =  -45
+      Specify one (or more) invariant(s) that rule this out. But note that
+      we DO want to allow negative amounts, otherwise the method negate 
+      can't be allowed.
+      It may be useful to use the JML notation ==> (for implication) in 
+      your invariants.
+
+   The only JML keywords needed for this are
+      requires
+      invariant
+      ensures
+
+   If you want, you can also use
+      non_null
+
+   While developing your specs, it may be useful to use the keywords
+      assert
+   to add additional assertions in source code, to find out what 
+   ESC/Java2 can - or cannot - prove at a given program point.
+
+*/
+
+public class Amount{
+
+ private int cents;
+
+ private int euros;
+ 
+ public Amount(int euros, int cents){
+   this.euros = euros;
+   this.cents = cents;
+ }
+
+ public Amount negate(){
+   return new Amount(-cents,-euros); 
+ }
+
+ public Amount subtract(Amount a){
+   return this.add(a.negate());
+ }
+
+ public Amount add(Amount a){
+   int new_euros = euros + a.euros;
+   int new_cents = cents + a.cents; 
+   if (new_cents < -100) {  
+      new_cents = new_cents + 100;
+      new_euros = new_euros - 1;
+   } 
+   if (new_cents > 100) {  
+      new_cents = new_cents - 100;
+      new_euros = new_euros - 1;
+   } 
+   if (new_cents < 0 && new_euros > 0) { 
+       new_cents = new_cents + 100; 
+       new_euros = new_euros - 1;
+   } 
+   if (new_cents >= 0 && new_euros <= 0) {
+       new_cents = new_cents - 100; 
+       new_euros = new_euros + 1;
+   }
+   return new Amount(new_euros,new_cents);
+ }
+
+ public boolean equal(Amount a) {
+   if (a==null) return false;
+   else return (euros == a.euros && cents == a.cents);
+ }
+
+}