Lecture 6

fall 2018

Narges Norouzi

Strings

What is a String

• Java String is a sequence of characters

• Example:

  • "Java"

  • "1365"

  • ""

  • "    " 

  • "?"

Creating String (1)

• Many ways of constructing a string:

Creating String (2)

• Many ways of constructing a string:

Creating String (3)

• Many ways of constructing a string:

String Creation

• Does Java create 2 String objects internally?

• Without “new” operator for String creation:

  • Java looks into the String pool (Collection of Strings)

    • Try to find objects with same String value

  • If object exists new variable points to existing object
  • If object does not exist new object is created
  • For efficiency ...

String Creation

String Creation

String methods

• Advantage of String class:

  • many built-in methods for String manipulation

Methods: valueof

• String.valueOf(x) – returns String

  representation of x.

• x: char, int, double, float

• Useful for converting different types to String

Methods: substring

• String.substring(x) – returns a new String by copying characters from an existing String.
• str.substring(i, k)
  • ​Returns substring of characters from index i to k-1
• str.substring(i)
  • Returns substring of characters from the index i to the end

Methods: concat

• Method 1: plus “+” operator

• Method 2: Using String’s “concat” method  

String immutability

• Strings in Java are immutable
• Meaning: cannot change its value, once created

String immutability

• With frequent modifications of Strings:

  • Create many new objects – uses up memory

  • Destroy many unused ones – increase workload

  • Overall: can slow down performance

String equality

String equality

• Checking if two Strings contain same value

5 minutes break

LINK

characters

Binary vs. text

• All data are in the end binary

  • (10001110110001) in base 2

• Bits represent encoded information,, executable instructions, or numeric data
• In order to store text files we need binary representation for all characters

text encoding

• In ASCII encoding, one byte

  represents one character

• Encoding is a rule where you map

  characters to integers:

  • ‘a’ = 97 (01100001) in base 2

Ascii Table

Resource: http://www.asciichars.com

Assignment 2 review

• Zybooks chapters we covered

• Chapter 2:

  • section 2.14

  • section 2.15

• Zybooks chapters we'll cover next

• Chapter 7: Objects and Classes

Questions?