UF 06: JAVA

Objects and strings

Alta Formazione Professionale - ITT Marconi

January 2021

2.1. Objects - Instances of Classes

• will follow the book course.csawesome.org EXCEPT Java Swing Gui
• will add challenges later

1) What are Objects and Classes?

• class
• instances
• objects

• new instances / objects

• methods
• attributes / properties

• calling methods
• dot operator .

2) Intro to Objects (with Turtles)

3) Creating Turtle Objects

4) Programming Challenge : Turtle Drawing

2.2. Creating and Initializing Objects: Constructors

• new again
• default constructor

1) Overloading Constructors

2) The World Class Constructors

3) The Turtle Class Constructors

• passing a reference to world

4) Object Variables and References

• null

5) Constructor Signatures

• parameter list
• formal parameters

6) Formal and Actual Parameters

12) Programming Challenge: Custom Turtles

2.3. Calling Methods Without Parameters

1) Procedural Abstraction

• methods use attributes
• NullPointerException

2) Programming Challenge : Draw a Letter

2.4. Calling Methods With Parameters

1) Tracing Methods

2) Programming Challenge : Turtle House

2.5. Calling Methods that Return Values

1) Get Methods

• attributes
• set methods

2) toString() Methods

3) Methods with Arguments and Return Values

4) Programming Challenge : Turtle Distances

2.6. Strings

char

• single character
• single quote literal 'c'
• two bytes
• primitive

String

• many characters
• double quote literal "ciao mondo"
• object
• immutable
• NOTE: even if on Java Tutor strings appears as a primitive (blue box), they are not!

1) String Operators - Concatenation

• + operator
• produces a NEW String

String x = "ciao";
String y = " mondo";
String z = x + y;
System.out.println(z);

ciao mondo

2.7. String Methods

indexes start from 0 included

//                0123456789
String stringa = "ciao mondo";

1) String Methods: length, substring, indexOf

.length()

"Ciao".length()

4

//  012345678
   "un dubbio".length()

9

//                  012?.....
System.out.println("un\ndubbio");

un
dubbio

//  012 345678
   "un\ndubbio".length()

9

.substring(a,b)

• from a included
• until b excluded
• you may get IndexOutOfBoundsException

//                  0123456
System.out.println("cartone".substring(2,5))

rto

No chars:

//                  0123456
System.out.println("cartone".substring(2,2))

One char:

//                  0123456
System.out.println("cartone".substring(2,3))

r

//                  0123456
System.out.println("cartone".charAt(2))

r

//                  0123456
System.out.println("cartone".substring(2,3).getClass())

class java.lang.String

char c = "cartone".charAt(2);        // OK
char c = "cartone".substring(2,3);   // WRONG

String s = "cartone".substring(2,3); // OK
String s = "cartone".charAt(2);      // WRONG

.indexOf(s)

indexOf returns the FIRST occurrence of string s

//                  0123456 
System.out.println("cartone".indexOf("to"));

3

WARNING: ALWAYS ask yourself if your algorithm still works if you have duplicates:

//                  0123456789... 
System.out.println("arzigogolato".indexOf("go"));

4

2) CompareTo and Equals

Equals:

• .equals(otherString) method

System.out.println("Ciao".equals("Ciao")); // correct way

true

// case sensitive

System.out.println("Ciao".equals("Ciao"));

true

== operator

• compares references
• not char by char !

Don't use == with strings!

WARNING: sometimes seems to work because for efficiency Java may cache strings so two identical copies actually share the same memory address

System.out.println("Ciao" == "Ciao");  // WRONG: HERE WORKS *BY LUCK*

true

compareTo

• negative
• zero
• positive

System.out.println("Mario".compareTo("Wario"));

-10

System.out.println("Wario".compareTo("Mario"));

10

System.out.println("Mario".compareTo("Mario"));

0

Different lengths:

System.out.println("Mario".compareTo("MarioAndLuigi"));

-8

System.out.println("MarioAndLuigi".compareTo("Mario"));

8

Comparison with < and > is not supported:

// System.out.println("Mario" < "Wario");   // error

More strings

Seems like CSAwsome doesn't have many much material on strings, so

DO THESE EXERCISES: https://codingbat.com/java/String-1

2.8. Wrapper Classes - Integer and Double

• primitive -> object conversion
• autoboxing

Integer i = new Integer(2);  // the old way
Double d = new Double(3.5);

Integer i = 2;    // Autoboxing in Java 9+
Double d = 3.5;

i.getClass()

class java.lang.Integer

QUESTION: does this work?

2.getClass()

overflow / underflow

• Integer.MIN_VALUE
• Integer.MAX_VALUE

2.9. Using the Math Class

• static methods
• sqrt, pow, abs, max, min

Square root

Math.sqrt(5)

2.23606797749979

power

To raise to a power, you can call pow:

Math.pow(3,2)  // 3 power 2

9.0

WARNING: in Java 3^2 does NOT mean power raise!

absolute value

Math.abs(-4)

4

min and max

Math.min(2,6)

2

Math.min(7,3)

3

Math.max(7,3)

7

Note in Java you can only pass two values:

Math.max(2,6,8)  // WRONG: does not work in Java!

random numbers

System.out.println(Math.random());  // number changes at each call
System.out.println(Math.random());

0.9017304454541571
0.1058621772838938

Use Random with a seed for repeatable results:

int seed  = 10;
Random generator = new Random(seed);
System.out.println("seed = " + seed);
System.out.println(generator.nextDouble());  
System.out.println(generator.nextDouble());

System.out.println();  

seed = 10;
Random generator = new Random(seed);
System.out.println("seed = " + seed);
System.out.println(generator.nextDouble());   // get the same as before  
System.out.println(generator.nextDouble());

System.out.println();  

seed = 1234;                                  // changed seed
Random generator = new Random(seed);  
System.out.println("seed = " + seed);
System.out.println(generator.nextDouble());   // different sequence
System.out.println(generator.nextDouble());

seed = 10
0.7304302967434272
0.2578027905957804

seed = 10
0.7304302967434272
0.2578027905957804

seed = 1234
0.6465821602909256
0.9513577109193919