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John HurleySpring 2011Cal State LA
CS 201 Lecture 6:
Strings
A String consists of zero or more characters
String is a class that contains many methods
Strings are more complicated than they sound!
3
Constructing StringsString newString = new String(stringLiteral); String message = new String("Welcome to Java");
Since strings are used frequently, Java provides a shorthand initializer for creating a string:
String message = "Welcome to Java";
4
Strings Are Immutable
A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML";
5
Trace Code
String s = "Java"; s = "HTML";
: String
String object for "Java"
s
After executing String s = "Java";
After executing s = "HTML";
: String
String object for "Java"
: String
String object for "HTML"
Contents cannot be changed
This string object is now unreferenced
s
animation
6
Trace Code
String s = "Java"; s = "HTML";
: String
String object for "Java"
s
After executing String s = "Java";
After executing s = "HTML";
: String
String object for "Java"
: String
String object for "HTML"
Contents cannot be changed
This string object is now unreferenced
s
animation
7
Interned StringsSince strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. For example, the following statements:
8
Examples
display s1 == s2 is false s1 == s3 is true
A new object is created if you use the new operator. If you use the string initializer, no new object is created if the interned object is already created.
String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java"; System.out.println("s1 == s2 is " + (s1 == s2)); System.out.println("s1 == s3 is " + (s1 == s3));
: String
Interned string object for "Welcome to Java"
: String
A string object for "Welcome to Java"
s1
s2
s3
9
Trace Code String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java";
: String
Interned string object for "Welcome to Java"
s1
animation
10
Trace Code String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java";
: String
Interned string object for "Welcome to Java"
: String
A string object for "Welcome to Java"
s1
s2
11
Trace Code String s1 = "Welcome to Java"; String s2 = new String("Welcome to Java"); String s3 = "Welcome to Java";
: String
Interned string object for "Welcome to Java"
: String
A string object for "Welcome to Java"
s1
s2
s3
12
String Comparisons java.lang.String
+equals(s1: String): boolean
+equalsIgnoreCase(s1: String): boolean
+compareTo(s1: String): int
+compareToIgnoreCase(s1: String): int
+regionMatches(toffset: int, s1: String, offset: int, len: int): boolean
+regionMatches(ignoreCase: boolean, toffset: int, s1: String, offset: int, len: int): boolean
+startsWith(prefix: String): boolean
+endsWith(suffix: String): boolean
Returns true if this string is equal to string s1.
Returns true if this string is equal to string s1 case-insensitive.
Returns an integer greater than 0, equal to 0, or less than 0 to indicate whether this string is greater than, equal to, or less than s1.
Same as compareTo except that the comparison is case-insensitive.
Returns true if the specified subregion of this string exactly matches the specified subregion in string s1.
Same as the preceding method except that you can specify whether the match is case-sensitive.
Returns true if this string starts with the specified prefix.
Returns true if this string ends with the specified suffix.
13
String Comparisonsequals
String s1 = new String("Welcome“);String s2 = "welcome";
if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference }
14
String Comparisons, cont.compareTo(Object object)
String s1 = new String("Welcome");String s2 = "welcome";
if (s1.compareTo(s2) > 0) { // s1 is greater than s2 } else if (s1.compareTo(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2
public class StringDemo{public static void main(String[] args){
String s1 = new String("Welcome");String s2 = "welcome";String s3 = s1;String s4 = new String("Welcome");
if (s1.equals(s2)){ System.out.println("s1 equals s2");
}else System.out.println("s1 does not equal s2");
if (s1 == s2) {
System.out.println("s1 == s2"); }else System.out.println("s1 != s2");
if (s1.equals(s3)){ System.out.println("s1 equals s3");
}
if (s1 == s3) {System.out.println("s1 == s3");
}else System.out.println("s1 != s3");
if (s1.equals(s4)){ System.out.println("s1 equals s4");
}else System.out.println("s1 does not equal s4");
if (s1 == s4){ System.out.println("s1 == s4");
}else System.out.println("s1 != s4");
if (s1.compareTo(s2) < 0) { System.out.println("s1 < s2");
}
}}
16
String Length, Characters, and Combining Strings
java.lang.String
+length(): int
+charAt(index: int): char
+concat(s1: String): String
Returns the number of characters in this string.
Returns the character at the specified index from this string.
Returns a new string that concatenate this string with string s1. string.
17
Finding String LengthFinding string length using the length() method:
message = "Welcome";
message.length() (returns 7)
18
Retrieving Individual Characters in a String
Do not use message[0]
Use message.charAt(index)
Index starts from 0
W e l c o m e t o J a v a
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
message
Indices
message.charAt(0) message.charAt(14) message.length() is 15
19
String ConcatenationString s3 = s1.concat(s2);
String s3 = s1 + s2;
s1 + s2 + s3 + s4 + s5 same as(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);
Stringspublic static void main(String[] args){
String s1 = new String("Welcome");String s2 = " Shriners";
String s3 = s1.concat(s2);System.out.println(s3);
String s4 = s1 + s2;System.out.println(s4);
s1 = s1 + s2;System.out.println(s1);
if(s1.startsWith("Wel")) System.out.println("\"" + s1 + "\"" + " starts with \"Wel!\"");
System.out.println("Length of \"" + s1 + "\" is " + s1.length());
System.out.println("In the string \"" + s1 + "\", the character at position 4 is " + s1.charAt(4));
}
21
Extracting Substrings java.lang.String
+subString(beginIndex: int): String
+subString(beginIndex: int, endIndex: int): String
Returns this string’s substring that begins with the character at the specified beginIndex and extends to the end of the string, as shown in Figure 8.6.
Returns this string’s substring that begins at the specified beginIndex and extends to the character at index endIndex – 1, as shown in Figure 8.6. Note that the character at endIndex is not part of the substring.
22
Extracting SubstringsYou can extract a single character from a string using the charAt method. You can also extract a substring from a string using the substring method in the String class.
String s1 = "Welcome to Java";String s2 = s1.substring(0, 11) + "HTML";
W e l c o m e t o J a v a
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
message
Indices
message.substring(0, 11) message.substring(11)
23
Converting, Replacing, and Splitting Strings
java.lang.String
+toLowerCase(): String
+toUpperCase(): String
+trim(): String
+replace(oldChar: char, newChar: char): String
+replaceFirst(oldString: String, newString: String): String
+replaceAll(oldString: String, newString: String): String
+split(delimiter: String): String[]
Returns a new string with all characters converted to lowercase.
Returns a new string with all characters converted to uppercase.
Returns a new string with blank characters trimmed on both sides.
Returns a new string that replaces all matching character in this string with the new character.
Returns a new string that replaces the first matching substring in this string with the new substring.
Returns a new string that replace all matching substrings in this string with the new substring.
Returns an array of strings consisting of the substrings split by the delimiter.
24
Examples"Welcome".toLowerCase() returns a new string, welcome."Welcome".toUpperCase() returns a new string, WELCOME." Welcome ".trim() returns a new string, Welcome."Welcome".replace('e', 'A') returns a new string, WAlcomA."Welcome".replaceFirst("e", "AB") returns a new string, WABlcome."Welcome".replace("e", "AB") returns a new string, WABlcomAB."Welcome".replace("el", "AB") returns a new string, WABlcome.
25
Splitting a String
String[] tokens = "Java#HTML#Perl".split("#", 0);
for (int i = 0; i < tokens.length; i++)
System.out.print(tokens[i] + " ");
Java HTML Perl
displays
26
Finding a Character or a Substring in a String
java.lang.String
+indexOf(ch: char): int
+indexOf(ch: char, fromIndex: int): int
+indexOf(s: String): int
+indexOf(s: String, fromIndex: int): int
+lastIndexOf(ch: int): int
+lastIndexOf(ch: int,
fromIndex: int): int
+lastIndexOf(s: String): int
+lastIndexOf(s: String,
fromIndex: int): int
Returns the index of the first occurrence of ch in the string. Returns -1 if not matched.
Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched.
Returns the index of the first occurrence of string s in this string. Returns -1 if not matched.
Returns the index of the first occurrence of string s in this string after fromIndex. Returns -1 if not matched.
Returns the index of the last occurrence of ch in the string. Returns -1 if not matched.
Returns the index of the last occurrence of ch before fromIndex in this string. Returns -1 if not matched.
Returns the index of the last occurrence of string s. Returns -1 if not matched.
Returns the index of the last occurrence of string s before fromIndex. Returns -1 if not matched.
27
Finding a Character or a Substring in a String
"Welcome to Java".indexOf('W') returns 0."Welcome to Java".indexOf('x') returns -1."Welcome to Java".indexOf('o', 5) returns 9."Welcome to Java".indexOf("come") returns 3."Welcome to Java".indexOf("Java", 5) returns 11."Welcome to Java".indexOf("java", 5) returns -1."Welcome to Java".lastIndexOf('a') returns 14.
public class StringDemo5{ public static void main(String[] args){
String theString = "The baby fell out of the window\nI thought that her head would be split\nBut good luck was with us that morning\n" +"For she fell in a bucket of #$%*!\n\nHere we are in this fancy French restaurant\nI hate to be raising a snit\nBut waiter, I ordered cream vichysoisse\nand you " + "brought me a bowl full of #$%*!\n\n";
System.out.println("\nOld:\n" + theString);
String newString = theString.replace("#$%*!", "shaving cream\nBe nice and clean\nShave every day and you'll always look keen!\n");
System.out.println("New:\n" + newString);
}}
public class StringDemo3{ public static void main(String[] args){ String theString = "'Take some more tea,' the March Hare said to Alice, very earnestly.\n\n'I've had nothing yet,' Alice replied in " +
"an offended tone, 'so I can't take more.'\n\n'You mean you can't take LESS,' said the Hatter: 'it's very easy to take MORE than nothing.'" +
"\n\n'Nobody asked YOUR opinion,' said Alice.";
System.out.println("\n" + theString); System.out.println("\n\nall lower case: " + theString.toLowerCase() + "\n"); System.out.println("\n\nsubstring from index 3 to end: " + theString.substring(3) +
"\n"); System.out.println("\n\nsubstring starting at index 24, ending at 30: "
+theString.substring (24, 30) + "\n"); System.out.println("character at index 39 is: " + theString.charAt(39) + "\n");
System.out.println("\n\nfirst y is at: " + theString.indexOf('y')); System.out.println("\n\nsubstring from first y to last y: " +
theString.substring(theString.indexOf('y'), theString.lastIndexOf('y'))); }
}
30
StringBuilderThe StringBuilder class is a supplement to
the String class. StringBuilder is more flexible than String. You
can add, insert, or append new contents into a string buffer, whereas the value of a String object is fixed once the string is created.
You will often have to parse the StringBuilder to a String when you are done constructing it.
31
StringBuilder Constructors
java.lang.StringBuilder
+StringBuilder()
+StringBuilder(capacity: int)
+StringBuilder(s: String)
Constructs an empty string builder with capacity 16.
Constructs a string builder with the specified capacity.
Constructs a string builder with the specified string.
32
Modifying Strings in the Builder
java.lang.StringBuilder
+append(data: char[]): StringBuilder
+append(data: char[], offset: int, len: int): StringBuilder
+append(v: aPrimitiveType): StringBuilder
+append(s: String): StringBuilder
+delete(startIndex: int, endIndex: int): StringBuilder
+deleteCharAt(index: int): StringBuilder
+insert(index: int, data: char[], offset: int, len: int): StringBuilder
+insert(offset: int, data: char[]): StringBuilder
+insert(offset: int, b: aPrimitiveType): StringBuilder
+insert(offset: int, s: String): StringBuilder
+replace(startIndex: int, endIndex: int, s: String): StringBuilder
+reverse(): StringBuilder
+setCharAt(index: int, ch: char): void
Appends a char array into this string builder.
Appends a subarray in data into this string builder.
Appends a primitive type value as a string to this
builder.
Appends a string to this string builder.
Deletes characters from startIndex to endIndex.
Deletes a character at the specified index.
Inserts a subarray of the data in the array to the builder at the specified index.
Inserts data into this builder at the position offset.
Inserts a value converted to a string into this builder.
Inserts a string into this builder at the position offset.
Replaces the characters in this builder from startIndex to endIndex with the specified string.
Reverses the characters in the builder.
Sets a new character at the specified index in this builder.
33
The toString, capacity, length, setLength, and charAt Methods
java.lang.StringBuilder
+toString(): String
+capacity(): int
+charAt(index: int): char
+length(): int
+setLength(newLength: int): void
+substring(startIndex: int): String
+substring(startIndex: int, endIndex: int): String
+trimToSize(): void
Returns a string object from the string builder.
Returns the capacity of this string builder.
Returns the character at the specified index.
Returns the number of characters in this builder.
Sets a new length in this builder.
Returns a substring starting at startIndex.
Returns a substring from startIndex to endIndex-1.
Reduces the storage size used for the string builder.
34
ExamplesstringBuilder.append("Java");stringBuilder.insert(11, "HTML and ");stringBuilder.delete(8, 11) changes the builder to Welcome Java.stringBuilder.deleteCharAt(8) changes the builder to Welcome o Java.stringBuilder.reverse() changes the builder to avaJ ot emocleW.stringBuilder.replace(11, 15, "HTML") changes the builder to Welcome to HTML.stringBuilder.setCharAt(0, 'w') sets the builder to welcome to Java.
StringsConsider the chorus of the Irish song “Rare
Old Mountain Dew”:diddly eidel diddle dumdiddly eidel diddle dumdiddly eidel diddly eidel dum day
diddly eidel diddle dumdiddly eidel diddle dumdiddly eidel diddly eidel dum dayThis contains several substrings that are
repeatedHow can we generate this using loops?
Strings and StringBuilderspublic class MountainDew{
public static void main(String[] args){
String diddly = "diddly eidel";
String diddle = "diddle dum";
String dum = "dum day";
StringBuilder sb = new StringBuilder();
for(int counter = 0; counter < 2; counter++){
for(int counter2 = 0; counter2 <= 3; counter2++){
sb.append(diddly + " ");
if(counter2 < 2) {
sb.append(diddle);
sb.append("\n");
}
else if(counter2 == 3) sb.append(dum);
}
if(counter == 0) sb.append("\n\n");
}
System.out.println(sb);
}
}
StringBuilderpublic class StringBuilderDemo{
public static void main(String[] Args){StringBuilder sb = new StringBuilder("Bakey");System.out.println(sb);
sb.insert(4, "ry");System.out.println(sb);
sb.deleteCharAt(5);System.out.println(sb);
// this will not do what you expect!sb.append(" " + sb.reverse());System.out.println(sb);
sb.delete(sb.indexOf(" "), sb.length());System.out.println(sb);
StringBuilder sb2 = new StringBuilder(sb);sb.deleteCharAt(5);sb2.reverse();sb.append(sb2);
System.out.println(sb);
sb.insert(5," ");System.out.println(sb);
sb.replace(0,0,"Y");System.out.println(sb);
sb.deleteCharAt(1);System.out.println(sb);
sb.reverse();System.out.println(sb);
}}
Dialog Boxes
GUI construction is taught in CS202, but we will cover a few GUI rudiments in 201
The first is the JOptionPane class, which provides pop-up I/O boxes of several kinds
Need to include this at the very top of your class:import javax.swing.JOptionPane;
JOptionPane.showMessageDialog()displays a dialog box with text you specify
Dialog Boxesimport javax.swing.JOptionPane;
public class DialogBox{public static void main(String[] args){
for(int i = 0; i < 4; i++) { JOptionPane.showMessageDialog(null, "This is number " + i);
}}
}
Dialog BoxesJOptionPane.showInputDialog() shows a
dialog box that can take inputThe input is a StringWe will learn soon how to parse from String
to other types
Dialog Boxesimport javax.swing.JOptionPane;
public class InputBox{public static void main(String[] args){
String input = JOptionPane.showInputDialog(null, "Please enter some input ");
JOptionPane.showMessageDialog(null, "You entered: \"" + input + "\"");
String input2 = input.concat(" " + JOptionPane.showInputDialog(null, "Please enter some more input "));
JOptionPane.showMessageDialog(null, "You entered: \"" + input2 + "\"");}
}
Casting Strings to Numeric TypesRecall that input from
JOptionPane.showInputDialog is a String
Cast to integer: Integer.parseInt(intString);Example: String input =
int age = Integer.parseInt(JOptionPane.showInputDialog(null, “Please enter your
age”);Cast to double:
Double.parseDouble(doubleString);
Casting to Numeric Types
Note the capitalization in the method names. Double and Integer are not quite the same as double and integer.You’ll understand when you are a little
older…
Parsing to Integerimport javax.swing.JOptionPane;
public class NumericCastDemo{
public static void main(String[] args){
int age = Integer.parseInt(JOptionPane.showInputDialog(null, "Please enter your age"));
if(age < 30)
JOptionPane.showMessageDialog(null, age + " is pretty young.");
else if(age > 100) JOptionPane.showMessageDialog(null, "really?");
else JOptionPane.showMessageDialog(null, "That's OK. Methuseleh lived to be " + (270 - age) +
" years older than you are now.");
} // end main()
} // end class
Parsing to Doubleimport javax.swing.JOptionPane;
public class NumericCastDemo{
public static void main(String[] args){
double age = Double.parseDouble(JOptionPane.showInputDialog(null, "Please enter your age"));
if(age < 30)
JOptionPane.showMessageDialog(null, age + " is pretty young.");
else if(age > 100) JOptionPane.showMessageDialog(null, "really?");
else JOptionPane.showMessageDialog(null, "That's OK. Methuseleh lived to be " + (270 / age) +
" times as old as you are now.");
} // end main()
} // end class
Imports
We have already discussed javax.swing.JOptionPane methods to show input and message dialogs
These required the following line at the top of the class:Import javax.swing.JOptionPane;
If you omit this line, you will get an error message like this:SwitchDemo.java: 7: cannot find symbolSymbol:variable JOptionPane
Imports
Java classes are organized in packagesLate in this class or early in CS202 you will start using packages for multiple classes
Javax.swing is a package of GUI-related classes that is included with all distributions of Java
JOptionPane is a class within this package
JOptionPane.showMessageDialog() and JOptionPane.showInputDialog are methods of the class
Imports
Including a package in your program adds a small cost, slowing down the compiler as it looks through the imported package
Thus, things like javax.swing are not included automatically; you must specify that you need them
You will eventually be importing your own packages, too.
Don’t leave unused imports in your code
public class ValueOfDemo{public static void main(String[] args){
double d = 1234.56789;// this would cause an error: int precision=d.indexOf('.');
String stringD = String.valueOf(d);int stringLength = stringD.length();int locationOfDecimalPoint = stringD.indexOf(".");System.out.println("length of string " + stringD + " is " + stringLength + "
location of decimal point is " + locationOfDecimalPoint); int precision = stringLength - locationOfDecimalPoint -1;System.out.println(precision + " digits after the decimal point");
}}
Validating Data TypeWe have already discussed how to make sure that numeric input from Scanner is within a desired range
int age;do {
System.out.println("How old are you?");age = sc.nextInt();
} while (age < 0 || age > 100);
Validating Scanner InputSo far, our programs have crashed if casts to numeric types failed:
Try this with input “two point five”:import java.util.Scanner;public class ReadDouble2 {
public static void main(String[] args) {Scanner input = new Scanner(System.in);Double stuff = 0.0; do {
System.out.print("Input a double. Enter 0 to quit:");
stuff = input.nextDouble();System.out.println("\nYou entered: " +
stuff);} while (stuff != 0.0);
}}
Validating Scanner InputHere is the simplest way to validate Scanner input for data type (that is, to make sure you get input that can be cast to double, integer, etc.)
Scanner has hasNext…() methods that see if there is a parseable tokenhasNextInt()hasNextDouble()hasNextBoolean()
Also need next() to skip over bad input
Validating Scanner InputTry this one with input “nine.three”, then with input “nine point three:
import java.util.Scanner;
public class ReadDouble3 {
public static void main(String[] args) {Scanner input = new Scanner(System.in);Double inpDouble = 10.0;
// bad code ahead!!do {
System.out.print("Input a double. Enter 0 to quit:");if(input.hasNextDouble()){
inpDouble = input.nextDouble();System.out.println("\nYou entered: " + inpDouble);}
else input.next();
} while (inpDouble != 0.0);
}}
Validating Scanner InputIn order to get good output, we need to arrange things in a
slightly more complex way:
import java.util.Scanner;
public class ReadInt{public static void main(String[] args){
Scanner sc = new Scanner(System.in);int x;System.out.println("Enter an integer");while (!sc.hasNextInt()) {
sc.nextLine();System.out.println("No, an integer!");
}x = sc.nextInt();System.out.println(x);
}}
55
Convert Character and Numbers to Strings
The String class provides several static valueOf methods for converting characters, arrays of characters, and numeric values to Strings. These methods are named valueOf with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a string, use String.valueOf(5.44). The return value is string consists of characters ‘5’, ‘.’, ‘4’, and ‘4’.
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