Welcome to the Using Objects lab!

Transcription

1 Welcome to the Using Objects lab! Learning Outcomes By the end of this lab: 1. Be able to define chapter 3 terms. 2. Describe reference variables and compare with primitive data type variables. 3. Draw memory model diagrams contrasting primitive with reference types. Include int, Integer, String, Scanner, etc. 4. Describe char and Character. 5. Describe how calling an instance (non-static) method is different from calling a class (static) method. 6. Use API to lookup Scanner, String, Random, Integer, and Double methods. 7. Trace multiple instance methods calls for Scanner and String. 8. Trace multiple method calls. 9. Describe String concatenation and immutability. Exercise A: Terms Please write out (on paper or notepad) your understanding of each of these terms (which are mostly from zybooks). As you are doing the lab, consider whether your understanding is confirmed or clarified. Review your understanding with a TA. object vs instance vs class, binary, decimal, char, char literal, ASCII, escape sequence (\n, \t, \\, \"), primitive type vs. reference type, a string vs String class, reference, System.in, Integer, Double, wrapper class, Random, Scanner, static method call, non-static method call, API, immutable, package, import, debugging Exercise B: Trace and Explain Week 2 we saw primitive data type variables simply hold a value in memory. Declaring a variable m is giving that name to a location in memory. int m = 9; Can be drawn as: m 9 Or m 9 Week 3 we saw that reference variables contain the reference (location or address) of where to find the value in memory. Technically, references contain the information needed for the JVM to find the value. For simplicity, we can think of a reference as an address.

2 Integer p = new Integer(7); The value (e.g., 132) in the memory location we have named p is the reference (address) to go to find the actual value 7, in this case. Since p is a reference variable (declared as Integer class) we know the value 132 is another location in memory. Going to that location (location 132) we then find the value 7. When we follow the reference we "dereference" the variable. Since the actual number, 132 in this case, could change every time the program is run we typically draw the reference as an arrow showing the location in memory the reference refers to. Draw reference variables with the arrow. Sometimes references are referred to as pointers. With your partner, trace and explain each of the following (also found in traceexplain.txt ). Check your understanding using Java Visualizer. A. double num1 = 9.0; Double num2 = 9.0; When tracing these, draw a picture of each. Check your picture by viewing in Java Visualizer. In Java Visualizer, remember to check: options > Show String/Integer/etc objects, not just values. When explaining, describe where the compiler will do an implicit type conversion. B. int numa; numa = 3; Integer numb; numb = new Integer("3"); //Can you draw a diagram and then describe in detail what is happening here? //Sample Answer (select, copy and paste into notepad to read): numa is a primitive variable declared with data type int. The statement numa = 3; puts the int literal 3 into the variable numa. The statement Integer numb; declares numb to be a reference variable intended to hold a reference to an instance of Integer. The expression new Integer("3") instantiates/creates an instance/object of class Integer with the initial value of 3. The result of the expression is a reference/address/pointer to the location in memory of the newly created instance. The assignment statement puts the reference into the variable numb. C. String str; str = "Your Name"; //Is String a class or primitive data type? //Are the characters "Your Name" stored in the memory location referred to by str? //What is stored in str?

3 D. String abc = " a b c "; System.out.println( abc.trim()); //If you don't know what trim() does, where can you find the description the method (or any method)? //What are 3 other String methods? E. String str = " r g h "; String.trim(); //what is wrong with this and how do you fix? //once this is fixed and the string is trimmed, what happens to the trimmed string? F. int length = " strange looking".length(); //why does this work? System.out.println( length); G. String str = "HELLO"; str.tolowercase(); System.out.println( str); //Is the string printed out in all uppercase letters? Why/Why not? What does immutable mean? H. String str = "he" + (9+2) + 'o'; String str2 = str.touppercase(); System.out.println( str2); I. String str = "Read " + " Java carefully."; System.out.println( str.substring(10,15)); //What exactly will be print out? //What is the meaning of each of the parameters to the substring method? J. String str2 = "Careful reading will save " + " lots of time debugging."; System.out.println( str2.substring( str2.indexof('e') + 1, str2.indexof('o')).trim()); //What will be print out? Describe in your own words what the substring, indexof and trim methods do. H. char ch = Character.toUpperCase('a'); System.out.println( ch); I. Character ch = new Character('A'); System.out.println( ch.islowercase()); J. int num = '1'; //will this data type conversion work? System.out.println( num); K. Random rand = new Random(); //remember to import java.util.random int num1 = rand.nextint(); int num2 = rand.nextint(); int num3 = rand.nextint();

4 //Will the nextint() method return the same value each time? //What is the range of possible values returned? //Each time you run this will the values returned be the same or different? L. Random numgenerator = new Random(); System.out.println( "number: " + Random.nextInt()); //What is wrong and how do you fix? M. Random numgenerator = new Random(); int num1 = numgenerator.nextint(10); int num2 = numgenerator.nextint(10); int num3 = numgenerator.nextint(10); //Does the value returned from nextint method call change each time? //What are the possible values returned? //If you re-run this same code are the results the same? N. Random rand = new Random(); rand.setseed( 365); int num1 = rand.nextint(6); int num2 = rand.nextint(6); int num3 = rand.nextint(6); //How does setting a seed affect the results? //What would you change to get the values 1-6, such as for simulating a dice roll? To learn more about Scanner see O. java.util.scanner stdin = new java.util.scanner( System.in); String str = stdin.nextline(); //what does this do, precisely? P. //to use just the class name Scanner within your program, remember to import java.util.scanner //will this work? why or why not? Scanner stdin = new Scanner( System.in); int num = stdin.nextdouble(); Q. //Exactly what is printed out? Scanner input = new Scanner("This is the input."); String str = input.next(); str = input.next(); System.out.println( str); R. Scanner input = new Scanner("one 1 2.0"); System.out.println( input.nextdouble()); //how would this code need to change so that 2.0 is print out? S. Scanner input = new Scanner("one 1 2.0");

5 String str = input.next(); System.out.println( input.nextdouble()); T. Scanner input = new Scanner("1 2 3\n 4 5 6"); String str = input.next(); System.out.println( input.next()); //str refers to what sequence of characters? U. Scanner input = new Scanner("1 2 3\n 4 5 6"); String str = input.nextline(); //str refers to what sequence of characters? V. Scanner input = new Scanner("1 2 3\n 4 5 6"); System.out.println( input.next()); String str = input.nextline(); System.out.println( "#" + str + "#"); //str refers to what sequence of characters? why? W. Scanner input = new Scanner("1 2 3\n 4 5 6"); System.out.println( input.nextline()); input.next(); X. //What is printed out? public class M { public static int methoda(int a) { return a + 2; public static void main(string []args) { double c = 5.2; System.out.println( methodb( c)); public static int methodb(double b) { return methoda((int) (b + 1.0)); Exercise C: Fix Order and Indenting of Lines Using the command-line tools, reorder lines to fix Scanner2.java Output:

6 Job:43125 name: Sara fee: Exercise D: Body Mass Index Body Mass Index (BMI) is used to estimate the amount of muscle, fat and bone in an individual. The BMI is defined as the body mass (kilograms) divided by the square of the body height (meters). Extend BMI.java to calculate a BMI based on height in meters and weight in kilograms. Height (meters): 1.6 Weight (kilograms): 65 BMI: Adapt to take input in inches and pounds. Find the appropriate formulas and constants on the web. There are calculators on the web that you may use to compare results. Express any constant values, such as meters in an inch, as a constant: final double METERS_IN_INCH = ; Height (inches): (meters) Weight (pounds): (kilograms) BMI: Adapt further to take input in feet, inches and pounds. Height (feet): 5 Height (inches): (meters) Weight (pounds): (kilograms) BMI: Exercise E: Write Program Write a program to use a Scanner to read the string, reading each number, " ControlType: " and print out as: building (522.0, 174.0) hero (397.0, 287.0) fire (541.0, 387.0) using control type: 1 Exercise F: Hello World Run this HelloWorld.java program and see if you can figure out how it works. 1. What does the seed for Random do? 2. What does += mean for Strings? 3. What does the following do? (char)(rand.nextint(27) + '`') 4. Why was '`' chosen?

7 5. Why was 27 chosen? 6. Why were those specific seeds chosen? Exercise G: Quiz To help you assess your own mastery of the material, see if you can answer these questions without looking back and without using a tool such as Java Visualizer. Use a tool to check your answers. 1. Draw a picture of memory as you trace the following code: Integer i = new Integer(3); Integer j = i; i = 4; int k = 5; k = j; 2. Use Random to generate an integer in the range -3 to 3, inclusive of both bounds. 3. What is printed out? String str = " This is a puzzle. \n"; str.substring( str.indexof( 'a'), 13).toUpperCase().trim(); System.out.println( str); 4. What is printed out? Scanner input = new Scanner("This\nhas\n4\nlines\n"); input.next(); System.out.println( "%"+ input.nextline() + "%"); input.nextline(); System.out.println("#"+ input.nextline() + "#"); System.out.println("*"+ input.nextline() + "*"); 5. What is printed out? public class Color { public static void red() { System.out.print("RED "); public static void blue() { red(); System.out.print("BLUE "); public static void main(string []args) { blue(); green(); red(); public static void green() { blue(); System.out.print("GREEN ");

8 blue(); Additional Learning Materials When you have mastered everything in this lab and previous labs, then you are welcome to learn from additional learning resources available on the web and beyond this course: Note: Due to programs and zybooks being individual work, it is Not appropriate to work on them during the Team Lab. Contributions by Marc Renault, Yien Xu Lab 2018 Jim Williams

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