Lecture Java methods: Object-oriented programming and data structures (2nd AP edition): Chapter 10 - Maria Litvin, Gary Litvin

Chapter 10 - Strings. In this chapter, the learning objectives are: Learn about literal strings, learn about string constructors and commonly used methods, understand immutability of strings, learn to format numbers into strings and extract numbers from strings, learn several useful methods of the Character class, learn about the StringBuffer class.

and Data Structures

Maria Litvin ● Gary Litvin

2nd AP edition with GridWorld

Objectives:

commonly used methods

extract numbers from strings

Character class

The String class

string of characters.

• The String class belongs to the java.lang

package, which is built into Java.

and methods.

Literal Strings

text in double quotes.

they are “just there.”

Literal Strings (cont’d)

as parameters.

String fileName = "fish.dat";

button = new JButton("Next slide");

if ("Start" equals(cmd))

Literal Strings (cont’d)

characters (described in Section 6.5) For example:

Immutability

none of its methods can change the string.

several references can point to the same

object safely: there is no danger of changing

an object through one reference without the others being aware of the change.

Immutability (cont’d)

create a new string and throw away the old one for every small change.

String s2 = new String();

private String errorMsg; errorMsg

is null

Empty strings

Constructors

• String ’s no-args and copy constructors are not used much.

(Chapter 12) into strings

String s1 = new String ();

String s2 = new String (s1);

String s1 = ""; String s2 = s1;

”Wind".charAt (2);

Returns:

Character positions in strings

are numbered starting from 0

String s2 = s.substring (i, j);

returns the substring of chars in

positions from i to j-1

String s2 = s.substring (i);

returns the substring from the i-th

char to the end

String date ="July 5, 2012 1:28:19 PM";

returns true if the string s1 matches s2, case-blind

int diff = s1.compareTo(s2);

returns the “difference” s1 - s2

int diff = s1.compareToIgnoreCase(s2);

returns the “difference” s1 - s2, case-blind

Methods — Replacements

String s2 = s1.trim ();

returns a new string formed from s1 by

removing white space at both ends

String s2 = s1.replace(oldCh, newCh);

returns a new string formed from s1 by replacing all occurrences of oldCh with newCh

String s2 = s1.toUpperCase();

String s2 = s1.toLowerCase();

returns a new string formed from s1 by

converting its characters to upper (lower) case

Integer and Double

are “wrapper” classes

from java.lang that

represent numbers as objects They also provide useful static methods.

Numbers to Strings (cont’d)

• The DecimalFormat class can be used for formatting numbers into strings.

Numbers to Strings (cont’d)

int m = 5, d = 19, y = 2007;

double amt = 123.5;

System.out.printf (

"Date: %02d/%02d/%d Amount = %7.2f\n", m, d, y, amt);

String s = String format(

"Date: %02d/%02d/%d Amount = %7.2f\n", m, d, y, amt);

Displays,

sets s to:

"Date: 05/19/2007 Amount 123.50"

Numbers from Strings

NumberFormatException if s does not

represent a valid number (integer, real

number, respectively).

String s1 = "-123", s2 = "123.45";

int n = Integer.parseInt(s1);

double x = Double.parseDouble(s2);

• Character also has methods that convert a letter to the upper or lower case.

int d = Character.digit (ch, radix);

returns the int value of the digit ch in the given

int radix

char ch = Character.forDigit (d, radix);

returns a char that represents int d in a given

int radix

The StringBuffer Class

mutable object

StringBuffer() // empty StringBuffer of the default capacity StringBuffer(n) // empty StringBuffer of a given capacity StringBuffer(str) // converts str into a StringBuffer

methods

• The toString method converts this

StringBuffer into a String

Review:

character into a literal string?

• Is "length".length() allowed syntax? If so,

what is the returned value?

efficient or less efficient?

Review (cont’d):

Review (cont’d):

few overloaded versions.

compareTo methods?

String object?

Review (cont’d):

a string.

String into an int ?

identify the category to which a given

character belongs.

StringBuffer classes?