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Intro to Java Midterm Review Dan Deutsch Daniel Deutsch.

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1 Intro to Java Midterm Review Dan Deutsch Daniel Deutsch

2 About Java Compiled vs. Interpreted Java is compiled to Java Bytecode, not interpreted Bytecode is then interpreted when the program runs Python is interpreted, so it translates as the program runs on the computer The Java API The Java Application Programming Interface is a library of code that you can use in Java programs It has things like the String class and the Scanner class Daniel Deutsch

3 Primitives and Reserved Words Primitives Types already built into Java. Variables can be these types They turn purple in jGrasp Integer types: byte, short, int, long Floating point types: float, double Other: boolean, char Reserved words You can’t use any of the words that Java has reserved to be “keywords” as a variable name All of the primitives, public, private, protected, static, void, class, switch, case, default, for, while, do, continue, if, else, return, new, this, throw, throws, Daniel Deutsch

4 Variable Names Variables must start with a letter, underscore or $ After the first letter, it can be a letter, an underscore, $ or a number Good examples: Num1, DAYS_IN_YEAR, $, money$$, _scanner Bad examples: 7even, hello world, private, num1* Daniel Deutsch

5 Characters Stored in memory as 8 bits Indicated by single quotes Each character maps to a number between 0 and 255 ‘a’ = 97 ‘A’ = 65 ‘0’ = 48 This means that ‘a’ > ‘A’ char c1 = ‘a’; System.out.println(c1 + 2); int c2 = c1 + 2; System.out.println(c2); char c3 = (char) (c1 + 2); System.out.println(c3); 99 c Daniel Deutsch

6 Arithmetic Operators have an order of operations () - negation *, /, % +, - = Be careful of integer division!! Casting helps to get around this int a = 5, b = 2; System.out.println(a / b); System.out.println((double) a / b); double c = 5.0; System.out.println(c / b); System.out.println(5.0 / b); double d = 8.825; int e = (int) d; System.out.println(e); System.out.println((double) (5 / 2)); Daniel Deutsch

7 Arithmetic The mod operator % gives you the remainder when dividing two numbers int mod = 33 % 4; Find the ones digit of the number 2358? int ones = 2358 % 10; The tens digit? int tens = 2358 / 10; tens = tens % 10; Misc. operators num++, num += 1, num = num + 1 What’s the difference between num++ and ++num ? num++ executes then line, then increments by 1 ++num increments by 1, then executes line int num = 0; System.out.println(num++); // num is now 1 System.out.println(++num); , *=, /=, +=, -=, %= Daniel Deutsch

8 Objects Objects are instantiations of Classes They have data associated with them and you can call methods on them Scanner scanner = new Scanner(“Dan 37.4 java”); // here we call the next() method on a scanner String d = scanner.next(); Java saves the location in memory where the Object’s data is. Primitives actually save the value of the variable int a = 5; String s = “Dan”; The Math class and the Character class provide static methods that you can call with the class name double d = Math.floor(3.25); You cannot instantiate them! Math math = new Math(); a 5 s Dan 3.0 Daniel Deutsch

9 Strings Strings are Objects, not primitive types Indicated with double quotes Length String s = “abcdefghijk”; int length = s.length(); indexOf int index = s.indexOf(‘c’); char bad = s.charAt(length); charAt char c = s.charAt(index); substring String s1 = s.substring(3); String s2 = s.substring(index, 6); 11 2 Crashes! c defghijk cdef toLowerCase String lower = “UPPER”.toLowerCase(); toUpperCase upper Daniel Deutsch

10 Strings Escape characters There are special characters which use two symbols to represent them \t is a tab \n is a newline character \\ is the \ character String s = “1\t2\n\t3\t\\”; System.out.println(s); 123\123\ Daniel Deutsch

11 Strings Covert a String to an int or double String s = “72”; int a = Integer.parseInt(s); double d = Double.parseDouble(“38.08”); int b = Integer.parseInt(“seven”); Convert a number into a String int a = 7; String s = a + “”; If you need to compare two String s, never use ==. The == operator compares the values at the memory address. Objects store a location in memory where the data is stored, not the actual value of the variable. Use equals instead String s1 = “Java”, s2 = “Java”; if (s1.equals(s2)) System.out.println(“same!”); equalsIgnoreCase() Crashes! Daniel Deutsch

12 Scanner Scanner is a class that allows you to iterate over a stream of text, like System.in or a String It’s located in java.util.Scanner, which you have to import initialization Scanner kb = new Scanner(System.in); Scanner s = new Scanner(“Dan ”); hasNext hasNextLine next if (scanner.hasNext()) String d = s.next(); nextInt int a = s.nextInt(); nextDouble double d = s.nextDouble(); nextChar does not exist. Do this instead: // user types “yes” char c = kb.next().charAt(0); true “Dan” s.next(); s.hasNext(); ‘y’ Crashes! false Daniel Deutsch

13 Scanner Example “First Last” Scanner s = new Scanner(“First Last”); s.next(); s.nextInt(); s.nextDouble(); s.nextInt(); s.next(); “First” “3” crashes! Last s.hasNext(); false Daniel Deutsch

14 Math Class You can use the Math class to do useful math operations You call the methods with the Math class pow double d1 = Math.pow(2, 3); round double d2 = Math.round(7.8); ceil double d3 = Math.ceil(2.1); double d4 = Math.ceil(3.0); floor double d5 = Math.floor(5.6); sqrt double d6 = Math.sqrt(34); … Daniel Deutsch

15 Printf The printf command allows you to format a print statement in very specific ways You create a String that encodes the output format of the string %d for integers %f for floats %s for Objects (e.g. Strings ) % displays %, like the \ character works with normal Strings You can specify how many spaces wide you want the result to be and how many points after the decimal you want to see %.2f will have 2 digits after the decimal, padded with 0s. Dosen’t work with integers – why? %8d will make sure the width of the number is at least 8 characters %08d will pad the number with 0s if necessary The newline character \n is not included, like System.out.print(); Daniel Deutsch

16 Printf Examples int a = 7; System.out.printf(“%3d\n”, a); System.out.printf(“%03d\n”, a); double d = 3.2; System.out.printf(“%04d\n”, d); System.out.printf(“%.3f\n”, d); System.out.printf(“%06.3f%\n”, d); System.out.printf(“My name is %s\n”, “Dan”); double d2 = ; System.out.printf(“%f\n”, d2); System.out.printf(“%2f\n”, d2); “ 7” “007” crashes! “3.200” “03.200%” “My name is Dan” Daniel Deutsch

17 If Statements If statements are useful when you want to change the logic of your program based on values of variables or properties of Objects They must use expressions that evaluates to Booleans If true, it will only execute the next statement unless curly braces are used to create code blocks, regardless of indentation if (a == 3) System.out.println(“equal”); System.out.println(“always runs”); a = 3; “equals” “always runs” a = 2; “always runs”; If statements can combine with else statements for more control. If the if statement evaluates to false, the else statement runs. Otherwise only the if statement runs if (a == 3) System.out.println(“equal”); else System.out.println(“not equal”); a = 3 a = 2 Daniel Deutsch

18 If Statements If you have mutually exclusive options, you can use an if-else-if structure. If you include an else, exactly one of the options will be chosen char choice = kb.next().charAt(0); if (choice = ‘a’) System.out.println(“a chosen”); else if (choice == ‘b’) System.out.println(“b chosen”); char choice = kb.next().charAt(0); if (choice = ‘a’) System.out.println(“a chosen”); else if (choice == ‘b’) System.out.println(“b chosen”); else System.out.println(“neither”); ab c a b c Daniel Deutsch

19 If Statement Examples int a = 2, b = 3; if (a == 2 && b == 3) System.out.println(1); else if (!(a != 2 || b != 3)) System.out.println(2); 1 This will never be called, no matter what a and b are. Why? (a == 2 && b == 3) is the same as !(a != 2 || b != 3) if (a == 8 && b == 5) System.out.println(“a is 8”); System.out.println(“b is 5”); Misleading indentation! Only executes next statement! a = 2, b = 3 if (a == 8 && b == 5) x++; y++; a = 8, b = 5 if (a == 8 && b == 5) { System.out.println(“a is 8”); System.out.println(“b is 5”); } a = 2, b = 3 a = 8, b = 5 a = 2, b = 3 Daniel Deutsch

20 If Statement Examples if (a <= 5) if (b <= 2) Sysytem.out.println(“tiny”); else { System.out.println(“huge”); System.out.println(“end”); } Misleading indentation! Rewrite it if (a <= 5) if (b <= 2) System.out.println(“tiny”); else { System.out.println(“huge”); System.out.println(“end”); } else goes with the most recent if a = 2, b = 1 a = 2, b = 3 a = 6, b = 1 if (a <= 5) { if (b <= 2) System.out.println(“tiny”); } else System.out.println(“huge”); System.out.println(“end”); { } This is the same thing! Just clearer (and better) if (a <= 5) { if (b <= 2) System.out.println(“tiny”); } else System.out.println(“huge”); System.out.println(“end”); Misleading indentation! Rewrite it a = 2, b = 1 a = 2, b = 5 a = 6, b = 1 Daniel Deutsch

21 Switch Statements Switch statements are interesting control structures. They are like if statements, but multiple clauses can be executed Unless there is a break, the program will keep executing The default case is for when no other case is selected char choice = kb.next().charAt(0); switch (choice) { case ‘a’: case ‘A’: x++; break; case ‘b’: y++; default: z++; } a a a a a a A A A A A A b b b b b b b c c c c c c c Daniel Deutsch

22 For Loops For loops are useful when code needs to be repeated and you know how many times it needs to execute. You may not know it will execute 5 times every time the code runs, but it will execute s.length() times There are 3 parts that create the for loop They are executed in the following order: 1. Initialization 2. condition 2.1 If true, the body 2.2 Otherwise, end 3. Afterthought 4. Go to step 2 for (int i = 0; i < s.length(); i++) initialization condition afterthought Daniel Deutsch

23 For Loop Examples for (int i = 0; i < 10; i++) System.out.print(i + “ “); String s = “I love Java”; for (int i = 0; i < s.length(); i += 2) System.out.print(s.charAt(i)); for (int n = 1; n < 1000; n *= 2) System.out.print(n + “ “); for (int i = 10; i 0; i--) System.out.print(i + “ “); for (int i = 0; i < 10; i++) System.out.println((i * 2) + “ “); System.out.println(“finished body”); Nothing! … 18 Finished body { } 0 Finished body 2 Finished body 4 … 18 Finished body … 1 < > 01 2 … 9 i = I lv a a n = 1 1 * 2 2 2* * … 512 Daniel Deutsch

24 While and Do-While Loops While loops are useful when you don’t know how many times the loop will run. Could be based on user input Do-while loops are useful when you don’t know how many times it will run, but it will run at least once int num = 0; while (num < 5) { if (kb.next().equals(“y”)) num++; } do { char choice = kb.next().charAt(0); // display menu } while (choice != ‘q’); Daniel Deutsch

25 File Structure The class declaration goes first Method declarations of the class go in the body Any imports/package declarations go outside of the class public class MyClass { } public static void main(String[] args) { } // code goes here public static int add(a, int b) { return a + b; } class declaration Memorize this! main method other methods import java.util.Scanner; imports Daniel Deutsch

26 Testing White box testing: every line of code get executed. The white box means you can see the code and make the test cases based on the code Test boundary cases Test invalid and valid cases Regression testing: rewriting code or adding code and making sure that everything that worked before works now Black box testing: try many values that are valid/invalid. You don’t know what the program is doing, so you try many cases if (0 <= a && a <= 10) Test -1, 0, 1, 9, 10, and 11. Probably values in between Daniel Deutsch


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