Bellwork Strings 11/25/13

Transcription

1 Bellwork Strings 11/25/13 1. How have we been using Strings in Java for? 2. Why does String begin with a capital letter? 3. From your homework, what are the problems that we run into when we try to compare Strings?

2 Strings AP Computer Science

3 String Class in Java java.lang Class String java.lang.object java.lang.string We do not have to import the String class since it comes from java.lang. An object of the String class represents a string of characters. abcde ;

4 Ways to create a String object Create a String literal: String str = "abc"; With the key word new since it is an object String str = new String( abc );

5 Empty Strings An empty string has no characters; Its contents are null. String s1 = "; String s2 = new String(); Empty strings

6 String Indexes String is a sequence of characters. Each letter in the string has its own index location and can be accessed by it. index location c o m p u t e r The length of the String is how many letters it contains: 8 The last index of the String is length 1; 7

7 Index Locations Index locations are from 0 to length-1 String s = strawberry ; strawberry // 10 letters in the word // index from 0 to 9 int len = s.length() // 10 int lastindex = s.length()-1 //9 s is the String object Objects call methods with a dot operator

8 String Methods Chart Use the computer to find the String methods list on your chart. Explain what the method does, provide an example, and provide sample output. You will use this chart throughout the Strings Unit and to study for the AP exam. 10-8

9 Let s Review Useful String Methods int length() returns the length of the string String substring(int start, int end) returns the substring beginning at start and ending at (end 1) String substring(int start) returns the substring beginning at start and ending at the end of the string int indexof(string otherstring) returns the index of the first occurrence of otherstring returns -1 if str is not found boolean equals(string otherstring) Compares string to otherstring. Returns true if equal (otherwise false). Never test for string equality by using == Java Concepts 4.6 (Strings), Appendix C (pg. 643)

10 length() int length() Returns the length of a string This is a count of the number of characters in the string, including spaces A string with a length of 0 is called the empty string Examples: String msg = "Hello, World!", str = ""; int msglen = msg.length(); int strlen = str.length(); msglen contains 13 strlen contains 0 Java Concepts 4.6 (Strings)

11 substring String substring(int start, int end) Returns a substring of the string The substring is composed of the characters beginning at position start and ending at position (end 1). In other words, you tell it the position of the first character you want and the position of the first character you do not want. The first position of the string is always position 0 Examples: H e l l o, W o r l d! Position: String msg = "Hello, World!", str1, str2; String str1 = msg.substring(0, 5); str1 contains "Hello" String str2 = msg.substring(7, 12); str2 contains "World" Java Concepts 4.6 (Strings)

12 substring String substring(int start) Returns a substring of the string The substring is composed of the characters beginning at position start and ending at the end of the string Examples: H e l l o, W o r l d! Position: String msg = "Hello, World!", str1, str2; String str1 = msg.substring(12); String str2 = msg.substring(7); str1 contains "!" str2 contains "World!" Java Concepts 4.6 (Strings)

13 indexof int indexof(string otherstring) Returns the position of the first occurrence of otherstring in the string Returns -1 if otherstring is not found in the string Examples: H e l l o, W o r l d! Position: String msg = "Hello, World!"; int pos1 = msg.indexof("or"); int pos2 = msg.indexof("x"); pos1 contains 8 pos2 contains -1 Java Concepts 4.6 (Strings)

14 equals boolean equals(string otherstring) Returns true if the string equals otherstring or returns false if they are not equal The comparison is case sensitive. Note: you should use equalsignorecase(string otherstring) if you want to ignore case during the comparison. Do not use == to compare strings. This will usually not return the desired result Examples: String originalstr = "North", str1 = "South", str2 = "north"; if (originalstr.equals(str1)) if (originalstr.equals(str2)) if (originalstr.equalsignorecase(str2)) false false true Java Concepts (Comparing Strings)

15 Let s try out these String methods String myteam = Carolina Panthers ; String myteamlower = carolina panthers ; String answer; int stringlength, stringpos, stringcompare; Write the Java code to answer the following questions: 1. What is the length of myteam? stringlength = myteam.length(); stringlength contains What is the position of the first occurrence of "n" in myteam? stringpos = myteam.indexof("n"); stringpos contains 6 3. What are the first 3 characters of myteam? answer = myteam.substring(0, 3); answer contains Car 4. If myteam equals myteamlower, print They are equal. if ( myteam.equals(myteamlower) ) false, nothing prints System.out.println( They are equal ); Java Concepts 4.6 (Strings), Appendix C (pg. 643)

16 String Methods Chart Use the computer to find the rest of the String methods listed on your chart.

17 StringBuilder Class AP Computer Science

18 The StringBuilder Class Use when you know a String will be modified. Like String, it is included in the java.lang package. Does NOT have to be imported.

19 The StringBuilder Class The StringBuilder class is similar to the String class. However, you may change the contents of StringBuilder objects. You can change specific characters, insert characters, delete characters, and perform other operations. A StringBuilder object will grow or shrink in size, as needed, to accommodate the changes.

20 The StringBuilder Class You must create a new StringBuilder object to use the StringBuilder class. StringBuilder message = new StringBuild();

21 StringBuilder Constructors StringBuilder() This constructor gives the object enough storage space to hold 16 characters. StringBuilder(int length) This constructor gives the object enough storage space to hold length characters. StringBuilder(String str) This constructor initializes the object with the string in str. The object will have at least enough storage space to hold the string in str, plus an extra 16 empty spaces trailing the string.

22 String Methods Chart Use the computer to find the rest of the StringBuilder methods listed on your chart.