Which is better?Slide 2Answer: BSlide 4Slide 5Slide 6Answer: ASlide 8Slide 9Slide 10Slide 11Slide 12Answer: neitherSlide 14Slide 15Slide 16Slide 17Slide 18Answer: DSlide 20Answer: CSlide 22Slide 23ConclusionsThe EndWhich is better?Which is better?•Assume s1 and s2 are Strings:A. if (s1 == s2) { ... }B. if (s1.equals(s2)) { ... }Answer: B•s1 == s2 tests whether s1 and s2 reference the same string; s1.equals(s2) tests whether they reference equa l strings•String s1 = "ABC";String s2 = s1;String s3 = "ABC";String s4 = "AB" + "C"•All these strings are equal; but s4 is in a different memory location than the others, so the == test yields falseWhich is better?•Assume String s1;A. if (s1.equals("OK")) { ... }B. if ("OK".equals(s1)) { ... }Answer: B•s1.equals("OK") sends a message to s1 asking if it is equal to "OK"•"OK".equals(s1) sends a message to "OK" asking if it is equal to s1–This is legal, because "OK" is a String•If s1 is null, then: s1.equals("OK") gives a NullPointerException "OK".equals(s1) gives falseWhich is better?•Assume int numbers[ ] = new int[100];A. for (int i = 0; i < 100; i++) numbers[i] = i;B. for (int i = 0; i <= 99; i++) numbers[i] = i;Answer: A•for (int i = 0; i < 100; i++) is better thanfor (int i = 0; i <= 99; i++)•Three reasons:–The first is more traditional–The array size is 100, not 99, so it’s more obvious where the number came from•You have to do some arithmetic to get 99–If you change the array size, a search for 100 won’t find the loop that uses 99Which is better?•Assume int numbers[ ] = new int[100];A. for (int i = 0; i < 100; i++) numbers[i] = i;B. for (int i = 0; i < numbers.length; i++) numbers[i] = i;Answer: B•for (int i = 0; i < numbers.length; i++)is better thanfor (int i = 0; i < 100; i++)•If you later decide to change the size of the numbers array, you only need to do it in the declaration; the for loop that uses length will automatically adjustWhich is better?•Assume finished is a boolean variable:A. if (finished == true) {...}B. if (finished) {...}Answer: B•finished == true is redundant:–If finished is true, then finished==true will be true–If finished is false, then finished==true will be false•The extra words don’t gain you anything–finished==true might seem more readable to a beginner, but you quickly learn to read the shorter form–Brevity in programming, as in writing, is a virtue–You can avoid the possible mistake of saying if (finished = true) { ... }Which is better?•Assume foo, bar, and larger are integersA. if (foo > bar) larger = foo;else larger = bar;B. larger = foo > bar ? foo : bar;Answer: neither•For each of these, you have to look at the code carefully to make sure it is correctlarger = Math.max(foo, bar);is easier to read and more obviously correctWhich is better?A. String s = "Hello";B. String s = new String("Hello");Answer: A•"Hello" is special syntax to implicitly construct a string•String s = new String("Hello"); actually constructs two strings: "Hello" constructs the first string, then it is given as a parameter to an explicit constructor, which constructs the second stringWhich is better?•Suppose p is a Panel with a BorderLayout and okButton is a Button:A. p.add(okButton, BorderLayout.NORTH);B. p.add(okButton, "North");•Note: BorderLayout.NORTH == "North"Answer: A•p.add(okButton, BorderLayout.NORTH); is strongly recommended over the shorter form p.add(okButton, "North") -- but why?•Answer: better error detection–If you type p.add(okButton, "north"), there is no error, but it doesn’t do what you want–If you type p.add(okButton, BorderLayout.North) you will get a syntax error, because BorderLayout has no such variable as NorthWhich is better?•Suppose n is an int and s is a String:A. s = Integer.toString(n);B. s = String.valueOf(n);C. s = new Integer(n).toString();D. s = n + "";Answer: D•I prefer D (s = n + ""; ) because:–It’s a common idiom, therefore easily recognized–It’s short–It works for any typeWhich is better?•Assume n is an integer:A. if (n < 0) n = 0;B. if (n < 0) n = 0;C. if (n < 0) { n = 0;}Answer: C•If, later on, you want to add a statement, it’s easy to make this mistake with B:–if (n < 0) System.out.println("n was " + n); n = 0;•You won’t make this mistake with A or C–With C (using braces), you don’t have to change anything that’s already there–However, A (all on one line) is often convenientWhich is better?•Assume n is an integer:A. int factorial = 1;for (int i = 2; i < n; i++) { factorial *= n;}B. int factorial = 1;int i = 1;for (i = 2; i < n; i++) { factorial *= n; }Answer: A•In most cases, you don’t care about the index of a for loop outside the loop–You typically give it an initial value in the loop–You typically already have its final value in some variable or by some simple computation•If you don’t need a variable, you shouldn’t have that variable–It doesn’t help anything, and it might get in the wayConclusions•There are various ways to do things•One way may be better than another because:–It’s easier to read and understand–It’s more familiar—the way things are usually done–It’s less error prone, or provides better error detection–It’s more efficientThe
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