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KwickClips Java

Java: 196 short revision clips (page 4)

One idea per clip, with the answer written out underneath. Each clip links back to the full Java lesson it came from.

Three points: infix needs brackets, postfix needs none, and it is read once left to right.Why is postfix easier for a computer?42 secA table showing A + B * C in infix, postfix A B C * + and prefix + A * B C.Which operator is placed first when converting?43 secA stack trace for 2 3 4 * + showing 2 3 4, then 2 12, then 14.What do you do when you read an operand?39 secA worked answer: 8 2 - gives 6, and (A + B) * C gives A B + C * and * + A B C.Which popped value is the left operand?43 secA table comparing checking each item against halving, for 1,000 and 10,00,000 items.Why not just time the program with a stopwatch?40 secA table mapping 3n + 5 to O(n), 3n squared plus 5n plus 9 to O(n squared), and 50 to O(1).What is the Big O of 3n² + 5n + 9?41 secA table of loop shapes: single loop O(n), loop in loop O(n squared), i = i / 2 O(log n).What is the Big O of two separate loops, one after the other?38 secA table of linear search cases: best item first O(1), average middle O(n), worst last or missing O(n).Is linear search O(1) because it once found the item first?41 secThree points defining a package: groups related classes, works like a folder, stops name clashes.What is a package in Java?39 secA table: java.lang automatic with String and Math, java.util needs import with Scanner.Which package is imported automatically?41 secAn import java.util.Scanner line shown above a class, with the star form beside it.Where do import statements go?39 secAn Arrays.sort call sorting marks 72, 45 and 90 into 45, 72, 90.What does Arrays.sort(m) do?38 secAn assert line checking that marks are at most 100, with the AssertionError message it produces.What happens if an assert condition is false?38 secThree points: a thread is one path of execution, programs start in the main thread, many threads run together.What is a thread?40 secA thread class extending Thread with run overridden, and new T().start() in main.Which method do you call to begin a thread?40 secTwo threads updating the same counter c, and the same method marked synchronized.What is a race condition?40 secThree limits of arrays: size fixed at creation, no add or remove, shifting done by hand.Can a Java array grow after it is created?42 secCode creating an ArrayList, adding Tea and Vada, reading get(0) and printing size().Can an ArrayList hold int values directly?38 secA for-each loop adding each mark to a total, beside an Iterator with hasNext and next.Which loop is easiest for reading every item?42 secA table comparing array and ArrayList on size, what they hold, and how the count is read.Which would you use for the twelve months?40 secFour numbered steps: New Project, Java with Ant, JFrame Form, Design view.What is a JFrame?40 secA table of three Swing controls with the prefixes lbl, txt and btn.Why can my code not find the control after I changed its text?39 secA table matching JPanel to grouping, JTable to rows and columns, JOptionPane to a pop-up box.Which class groups controls together?39 secAn ActionPerformed method reading text fields, computing simple interest and setting the result.Why does nothing happen when I click the button?41 secA code line without its semicolon and the compiler message semicolon expected.Who catches a syntax error?39 secCode dividing 30 by a groups variable of 0, and the ArithmeticException slash by zero message.Why did a program with no compile error still crash?39 secCode showing a + b / 2 printing 65 beside (a + b) / 2 printing 45.Why does 40 + 50 / 2 give 65?38 secA loop with i < 5 printing sum 10, and the corrected i <= 5 printing 15.Why did the loop stop at 4?37 secA numbered algorithm for the sum of the digits of n, written in plain English.In which language is an algorithm written?41 secA table mapping algorithm steps Read, If, Repeat and Print to nextInt, if, while and println.What does a repeated algorithm step become in Java?42 secA test table: 18 gives NIVEN, 19 gives NOT NIVEN, 0 gives INVALID INPUT.Is one successful run enough?42 secThree points listing algorithm, commented program and output for test data.What are the three parts of the answer?40 secAn analysis table for a report card project with input, process and output rows.What should you write before coding a project?42 secA table turning student into a class, name and marks into fields, find total into a method.What becomes a class?39 secA total method being tested alone with marks 80, 72 and 91 giving 243.Why test each method immediately?37 secA four-point project report outline: problem and design, commented code, output and test log, limitations.What goes at the top of your code file?42 secA table of unary, binary and ternary operators with the examples -a, a + b and c ? x : y.How many operands does a binary operator take?39 secA trace table with a starting at 5, showing b = ++a giving 6 and b = a++ giving 5.What does b = a++ store when a is 5?39 secA trace of a starting at 10 through a += 5, a *= 2 and a %= 7.What does a *= 3 mean?39 secA ternary expression picking the larger of a = 45 and b = 72, giving 72.What does the ternary operator return when the condition is false?40 secThe same loop shown as a for loop and as a while loop with the parts moved.Where does the initialisation go in a while loop?37 secA table showing while running 0 times and do-while running once for i = 10 with i <= 9.Which loops can run zero times?40 secA table of finite, infinite and empty loops with the examples i = 1 to 5, while (true) and for (...);.Why does my loop never stop?40 secA trace table of k as 10, 7, 4, 1 with sum reaching 22 after four rounds.How many times does the loop run?41 secA table of INDIA showing charAt(0) as I, charAt(4) as A and charAt(5) as an exception.What is the last valid index of a string?41 secA reverse trace of a word beside a palindrome comparison using the equals method.Which line reverses the string?41 secA table of three counting tasks: vowels in AEIOU, words by spaces, and one letter compared directly.Why does the word counter start at 1?42 secA list of four board string programs: palindrome and reverse, vowels and words, frequency, initials.What do all these programs have in common?41 sec
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