Java Basics. SP17 ICS 111 Ed Meyer

Transcription

1 Java Basics SP17 ICS 111 Ed Meyer

2 Assignment 01 Write a Java program that: Prints a 2-line computer-related joke or a 4-line poem Uses print and println at least once Follows the Coding Standard Full details in Digital Dropbox Due Tues. Jan. 24 by 11:55pm Submit only the.java file 2

3 Quiz 01 Due Wed. Jan. 25 by 11:55pm Password: quiz01 15 Questions Multiple choice, True/False Based on lectures: Week 01 Day 02 Week 02 Day 01 and Day 02 (today) Attempts: 1 Time: 20 minutes 3

4 Last Time Print complex messages Escape Sequences \ Concatenation + Programming concepts Variables Constants Data types Declaring vs Initializing 4

5 Today Using String variables Arithmetic Expressions Assignment and Comparisons 5

6 String Variables What is a String? is a sequence of characters or words It is NOT a primitive data type Made up of characters 6

7 Breaking Down a String String message = "It's over 9,000!"; I t ' s o v e r 9, 0 0 0! The string is 16 characters long. 7

8 More on Strings Strings are very commonly used They are special They have object properties They have primitive type properties Can be declared like a primitive data type 8

9 Strings are Useful Help us label output Give instructions to the user Build more complex strings using concatenation 9

10 The String Data Type It is a Java object String JavaDoc It may be declared As a Java object As a primitive data type 10

11 Declaring and Initializing a String As you would expect from previous examples String message = "I choose you, Pikachu!"; Declaring and initializing as a primitive data type prim_data_type var_name = value; 11

12 Printing a Previously Declared String String message = "I choose you, Pikachu!"; // Pretend there is more code here System.out.println(message); Notice you don't need the quotes when you use the variable name. 12

13 When do you put the quote? If you want to use a string that is inside a variable No quotes are needed, just use the variable name If you are hard-coding a message You need to quote the message If you are initializing a string variable You need to quote the message 13

14 Printing Two Strings String location = "a bottle"; System.out.print("This message was found in"); System.out.println(location); This message was found ina bottle 14

15 Printing Two Strings String location = "a bottle"; System.out.print("This message was found in "); System.out.println(location); This message was found in a bottle 15

16 Arithmetic Expressions 16

17 Arithmetic Expressions add + subtract multiply * divide / modulus % Asterisk: Shift + 8 Follows the PEMDAS rule as you would expect Modulus has same priority as * and / 17

18 Adding Example int i = 20; int j = 5; int total = 0; total = i + j; These are declaration and initializing statements. 18

19 Adding Example int i = 20; int j = 5; int total = 0; total = i + j; This is an assignment statement. Take the thing on the right, and store it in the left. 19

20 More about Modulus % Calculate the remainder of a division Examples: 23 % 5 =? 20

21 More about Modulus % Calculate the remainder of a division Examples: 23 % 5 = 3 17 % 3 =? 21

22 More about Modulus % Calculate the remainder of a division Examples: 23 % 5 = 3 17 % 3 = 2 16 % 4 =? 22

23 More about Modulus % Calculate the remainder of a division Examples: 23 % 5 = 3 17 % 3 = 2 16 % 4 = 0 23

24 Assignment, Comparisons, and Output

25 Why Declare All Your Variables Because Java says... If you use the same piece of data more than once, declaring a variable is very useful. Declare all variables at the beginning Set aside all the boxes you will need in the program 25

26 Declaration Example int i = 20; int j = 5; int resulta = 0; int results = 0; int resultm = 0; int resultd = 0; You can reuse the same code with different values of i and j. resulta = i + j; results = i j; resultm = i * j; resultd = i / j; 26

27 Comparisons If we compare two items, the result will be true or false Boolean! Primitive data type: boolean 27

28 Boolean Operators Comparison Symbol Greater than > Less than < Greater than or Equal to >= Less than or Equal to <= Equal to == (double equals) 28

29 Comparison Example int x = 7; int y = 19; boolean isgreater = true; boolean issmaller = false; isgreater = x > y; issmaller = x < y; 30

30 Assignment and Comparison In programming, = does not mean "is equal to" = means assignment Take the thing on the right, and put it in the left == means "is equal?" It is a question It compares and gives a Boolean result 31

31 Assignment and Comparison Example int x = 5; int y = 20; int total = 0; boolean isequal = true; total = x + y; isequal = x == y; 32

32 Assignment and Comparison Example isequal = x == y; Evaluate the stuff on the right first Take the thing on the right, and put it in the left 33

33 What will print? String output = ""; output = output + "Hi, my name is"; output = output + "Ed"; System.out.print(output); Hi, my name ised 34

34 More on System.out A program is worthless without feedback print and println have parameters Parameters are things inside parenthesis ( ) Information required to execute an action System.out.println() will print a blank line System.out.print() will throw an error If there is nothing in the parenthesis Java expects a parameter to print 35

35 A Bad Example int x = 5; int y = 20; int total = 0; boolean isequal = true; total = x + y; isequal = x == y; System.out.println(total); System.out.println(isEqual); 36

36 Let's Do Some Examples 37