1. CS2011 Introduction to Programming I Strings
Chengyu Sun
California State University, Los Angeles

2. Class A class can be considered as a user-defined type For example:
public class Point {
double x;
double y; }

3. Object Objects are instances of a class Type Instances Primitive Type
Values
Class
Objects

4. Static Method A class may have methods
Methods declared as static are associated with the class
For example:
Math.random()
Character.isDigit()
<class_name>.<method_name>

5. Instance Method
Methods without static are associated with each object (i.e. instance) of a class, so they are called instance methods

6. Instance Method Example
Class
Object
Create an object
Scanner in = new Scanner( System.in );
int n = in.nextInt();
in.close();
Instance Methods
<object_name>.<method_name>

7. String String is a Java class representing a sequence of character
String values (i.e. objects) are text enclosed in double quotes, e.g.
String message = "Welcome to Java";
Text processing is a very common task in programming,
so it's very important for us to know how to handle Strings.

8. Some String Instance Methods
length()
charAt()
concat()
toUpperCase()
toLowerCase()
trim()
substring()

9. How To Read API Documentation
Constructors
Fields
Methods
Static, Instance, Concrete, Deprecated
Arguments and return value

10. Search Inside a String indexOf() lastIndexOf()
Example: split a name into first name and last name
Handle erroneous input (i.e. no whitespace)
Handle leading/trailing whitespaces
Handle multiple whitespaces

11. Read String from Console
Scanner methods
next()
nextLine()
Example: read name from input in the SplitName example

12. Formatting Output double amount = 12618.98;
double interestRate = ;
double interest = amount * interestRate;
System.out.printf("Interest is $%4.2f",
interest);

13. Format Specifier % 4 .2 f Width, i.e. minimum Number of characters
To be written to output
Start of a
format specifier
% 4 .2 f
Conversion Character
Precision, i.e. number of
Digits after the decimal point
for floating-point numbers
d: decimal integer
f: floating-point number
c: character
s: String
Java printf() Quick Reference

14. String Comparison String s1 = "ab"; String s2 = "abc".substring(0,2);
System.out.println(s1);
System.out.println(s2);
System.out.println(s1 == s2); // true or false??

15. Value vs. Reference Variables of primitive types hold values
int a = 1;
int b = a;
a = 2;
System.out.print(a); //??
System.out.print(b); //??
Point a = new Point();
a.x = 1;
Point b = a;
a.x = 2;
System.out.print(a.x); //??
System.out.print(b.x); //??
Variables of primitive types hold values
Variables of class types hold references

16. Reference
An object reference can be thought as the location of the object in memory, or a pointer pointing to the object
Create a new object in memory
Point a = new Point();
Point b = a;
Assign the location of
the object to a
Copy the location of
the object to b

17. Reference Comparison s1 s1 "ab" "ab" s2 s2 "ab" s1 == s2 s1 == s2 true
false

18. String Comparison By Value
equals()
equalsIgnoreCase()
compareTo()
compareToIgnoreCase()

19. More String Comparisons
startsWith()
endsWith()
contains()

20. Interned Strings String s1 = "abc"; String s2 = "abc";
Sytem.out.println( s1 == s2 ); // true
In order to improve memory efficiency, JVM keeps a "pool" of the most recently used String literals
If two String literals are the same, they refer to the same String object in the pool
But, what if we change s1, e.g. s1 = "xyz" ??

21. String Are Immutable
The content of a String cannot be changed – when it seems like a String is changed, it's actually a new String being created
"xy" s1 s1
"ab"
"ab"
String s1 = "ab";
s1 = "xy";

22. String Conversion String s int i String s double d Integer.parseInt(s)
Double.parseDouble(s)
String s
double d
d + ""

23. Example: Hex2Dec2
Convert a 2-digit hexadecimal number to a decimal value
Hex
Digit
Decimal Value
1 - 9
1 – 9 A 10 B 11 C 12 D 13 E 14 F 15

24. Readings
Chapter 4 of the textbook (there will be a quiz next week)