12. Common Errors, a few Puzzles

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.2 Common Errors, a few Puzzles Sources  Cay Horstmann, Computing Concepts with Java Essentials, Wiley, 1998  The Java Report, April 1999

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.3 Roadmap  Common errors … —Typical programming traps  A few Java puzzles... —The darker side of Java

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.4 Roadmap  Common errors … —Typical programming traps  A few Java puzzles... —The darker side of Java

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.5 Trap 1 What does this print? 25.0 Don’t assume that floating point numbers are exact representations of mathematical values! double f = 2e ; double g = 2e ; println(100*(f-g));

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.6 static void test(String name, boolean bool) { println(name + ": " + (bool?"true":"false")); } Trap 2 When are two Strings equal? == tests object identity; equals() tests object equality String s1 = new String("This is a string"); String s2 = new String("This is a string"); test("String==", s1 == s2); test("String.equals", s1.equals(s2)); String==: false String.equals: true

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.7 Trap 3 When are two Objects equal? object==: false object.equals: false The default implementation of equals() is ==, and should normally be overwritten Object x = new Object(); Object y = new Object(); test("object==", x == y); test("object.equals", x.equals(y));

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.8 Trap 4 When are two Strings equal? String==: true String.equals: true Literal strings with the same content refer to the same object! Always use equals() or compareTo() to compare strings. String s3 = "This is a string"; String s4 = "This is a string"; test("String==", s3 == s4); test("String.equals", s3.equals(s4));

© O. Nierstrasz P2 — Common Errors, a few Puzzles 12.9 Trap 5 Is “now” really before “later”? see you now Date later = new Date(now.getTime()); Explicitly create a clone if you need one: Calendar now = Calendar.getInstance(); Calendar later = now; later.set(Calendar.HOUR_OF_DAY, 1 + now.get(Calendar.HOUR_OF_DAY)); if (now.before(later)) { println("see you later"); } else { println("see you now"); }

© O. Nierstrasz P2 — Common Errors, a few Puzzles Trap 6  nothing  0 is even 1 is negative static void checkEven(int n) { boolean result = true; if (n>=0) if ((n%2) == 0) println(n + " is even"); else println(n + " is negative"); } What is printed when we run these checks? checkEven(-1); checkEven(0); checkEven(1); Always use braces to group nested if { } else { } statements! if (n>=0) { if ((n%2) == 0) { println(n + " is even"); } else { println(n + " is negative"); }

© O. Nierstrasz P2 — Common Errors, a few Puzzles Trap 7 The binomial coefficient is Is this a correct implementation? binomial(7,3) = 21  35 To avoid off-by-1 errors: 1. Count the iterations — do we always do k multiplications? (no) 2. Check boundary conditions — do we start with n/1 and finish with (n-k+1)/k? (no) Off-by-1 errors are among the most common mistakes in implementing algorithms. static int binomial(int n, int k) { int bc = 1; for (int i=1; i<k; i++) { bc = bc * (n+1-i) / i; } return bc; }

© O. Nierstrasz P2 — Common Errors, a few Puzzles Trap 8 For which values does this function work correctly? brokenFactorial(0) = 1 brokenFactorial(5) = 120 brokenFactorial(-1) =... (doesn’t terminate) Always use an inequality test to terminate a loop. static int brokenFactorial(int n) { int result=1; for (int i=0; i!=n; i++ ) { result = result*(i+1); } return result; }

© O. Nierstrasz P2 — Common Errors, a few Puzzles Some other common errors Magic numbers  Never use magic numbers; declare constants instead. Forgetting to set a variable in some branch  If you have non-trivial control flow to set a variable, make sure it starts off with a reasonable default value. Underestimating size of data sets  Don’t write programs with arbitrary built-in limits (like line-length); they will break when you least expect it. Leaking encapsulation  Never return a private instance variable! (return a clone instead) Bugs are always matter of invalid assumptions not holding

© O. Nierstrasz P2 — Common Errors, a few Puzzles Roadmap  Common errors … —Typical programming traps  A few Java puzzles... —The darker side of Java

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 1 A.p() Private methods are statically bound. A b = new B(); b.m(); class A { public void m() { this.p(); } private void p() { println("A.p()"); } } class B extends A { private void p() { println("B.p()"); } } Are private methods inherited? Which is called? A.p() or B.p()?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Static and Dynamic Types Consider: The static type of variable a is A — i.e., the statically declared class to which it belongs. The static type never changes. The dynamic type of a is B — i.e., the class of the object currently bound to a. The dynamic type may change throughout the program. Now the dynamic type is also A! A a = new B(); a = new A();

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 2 (part I) m(A,A) m(A,B) m(B,A) m(B,B) The static type of arguments is always used to resolve overloaded method calls. class A { } class B extends A { } void m(A a1, A a2) { println("m(A,A)"); }; void m(A a1, B b1) { println("m(A,B)"); }; void m(B b1, A a1) { println("m(B,A)"); }; void m(B b1, B b2) { println("m(B,B)"); }; B b = new B(); A a = b; How are overloaded method calls resolved? m(a, a); m(a, b); m(b, a); m(b, b); Which is considered: the static or dynamic argument type?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 2 (part II) What happens if we comment out: Will the examples still compile? If so, which methods are called? m(a,a) won’t compile m(b,b) won’t compile Prints: m(A,A) m(B,A) m(B,B) m(A,A)? m(B,B)? m(A,B)?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 3 How do static and dynamic types interact? A.m(A) B.m(B) Static types determine which message is sent. Dynamic types determine which method is called. a.m(a); a.m(b); b.m(a); b.m(b); class A { void m(A a) { println("A.m(A)"); } } class B extends A { void m(B b) { println("B.m(B)"); } } B b = new B(); A a = b; In which cases will B.m(B) be called?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 4 (part I) C.i = 0 C.j = 0 C.k = 0 C.l = 0 First, default values are assigned... then constructors are evaluated. How do default values and constructors interact? class C { int i = 100, j = 100, k = init(), l = 0; C() { i = 0; k = 0; } int init() { j = 0; l = 100; return 100; } } What gets printed? 0 or 100? C c = new C(); println("C.i = " + c.i); println("C.j = " + c.j); println("C.k = " + c.k); println("C.l = " + c.l);

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 4 (part II) B.i = 0 B.j = 0 First all super initialization is executed, then the subclass initialization abstract class A { int j = 100; A() { init(100); j = 200; } abstract void init(int value); } class B extends A { int i = 0, j = 0; B() { super(); } void init(int value) { i = value; } } B b = new B(); println("B.i = " + b.i); println("B.j = " + b.j); What gets printed? 0, 100 or 200?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Puzzle 5 The finally clause always takes precedence! 3 Does try or finally return? class A { int m() { try { return 1; } catch (Exception err) { return 2; } finally { return 3; } } Prints 1, 2, or 3? A a = new A(); println(a.m());

© O. Nierstrasz P2 — Common Errors, a few Puzzles What you should know!  When can you trust floating-point arithmetic?  To which “if” does an “else” belong in a nested if statement?  How can you avoid off-by-1 errors?  Why should you never use equality tests to terminate loops?  Are private methods inherited?  What are the static and dynamic types of variables?  How are they used to dispatch overloaded methods?

© O. Nierstrasz P2 — Common Errors, a few Puzzles Can you answer these questions?  When is method dispatching ambiguous?  Is it better to use default values or constructors to initialize variables?  If both a try clause and its finally clause throw an exception, which exception is really thrown?

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