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Penn CIT 591 - Strings Lecture

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Strings, Etc.About StringsUseful String methods IUseful String methods IIUseful String methods IIIUseful String methods IVUseful String methods VUnderstanding “index”Useful String methods VIFinally, a useless String methodStrings are immutableSlide 12About StringBuffersStringBuffer constructorsUseful StringBuffer methods IUseful StringBuffer methods IIUseful StringBuffer methods IIIUseful StringBuffer methods IVUseful StringBuffer methods VWhen to use StringBuffersSlide 21About StringTokenizersStringTokenizer constructorsStringTokenizer methodsExample use of StringTokenizerSlide 26About StreamTokenizersThe EndJan 14, 2019Strings, Etc.Part I: Strings2About StringsThere is a special syntax for constructing strings: "Hello"Strings, unlike most other objects, have a defined operation (as opposed to a method): " This " + "is String " + "concatenation"3Useful String methods Ichar charAt(int index)Returns the character at the given index position (0-based)boolean startsWith(String prefix)Tests if this String starts with the prefix Stringboolean endsWith(String suffix)Tests if this String ends with the suffix String4Useful String methods IIboolean equals(Object obj)Tests if this String is the same as the obj (which may be any type; false if it’s not a String)boolean equalsIgnoreCase(String other)Tests if this String is equal to the other String, where case does not matterint length()Returns the length of this string; note that this is a method, not an instance variable5Useful String methods IIIint indexOf(char ch)Returns the position of the first occurrence of ch in this String, or -1 if it does not occurint indexOf(char ch, int fromIndex)Returns the position of the first occurrence of ch, starting at (not after) the position fromIndexThere are two similar methods that take a String instead of a char as their first argument6Useful String methods IVint lastIndexOf(char ch)Returns the position of the last occurrence of ch in this String, or -1 if it does not occurint lastIndexOf(char ch, int fromIndex)Returns the position of the last occurrence of ch, searching backward starting at position fromIndexThere are two similar methods that take a String instead of a char as their first argument7Useful String methods VString substring(int beginIndex)Returns a new string that is a substring of this string, beginning with the character at the specified index and extending to the end of this string. String substring(int beginIndex, int endIndex)Returns a new string that is a substring of this string, beginning at the specified beginIndex and extending to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex8Understanding “index”With charAt(index), indexOf(x), and lastIndexOf(x), just count characters (starting from zero)With substring(from) and substring(from, to), it works better to count positions between charactersSo, for example, substring(4, 8) is "said", andsubstring(8, 12) is ", \"H" "She said, \"Hi\"" 0 1 2 3 4 5 6 7 8 9 10 11 12 13"She said, \"Hi\""0 1 2 3 4 5 6 7 8 9 10 11 12 13 14If indexOf(',') is 8, then substring(0, indexOf(',')) is "She said"and substring(indexOf(',') + 1) is " \"Hi\""9Useful String methods VIString toUpperCase()Returns a new String similar to this String, in which all letters are uppercaseString toLowerCase()Returns a new String similar to this String, in which all letters are lowercaseString trim()Returns a new String similar to this String, but with whitespace removed from both ends10Finally, a useless String methodString toString()Returns this StringWhy do we have this method?Consistency--Every Object has a toString() method11Strings are immutableA String, once created, cannot be changedNone of the preceding methods modify the String, although several create a new StringStatements like this create new Strings: myString = myString + anotherCharacter;Creating a few extra Strings in a program is no big dealCreating a lot of Strings can be very costlyJan 14, 2019Strings, Etc.Part II: StringBuffers13About StringBuffersA StringBuffer has a capacity (the number of characters it can hold) and a length (the number of characters it is currently holding)If the capacity is exceeded, the StringBuffer is copied to a new location with more roomStringBuffers are used to implement String concatenationWhenever you say String s = "ab" + "cd", Java creates a StringBuffer containing the characters a and b, appends the characters c and d to it, and converts the result back to a StringAs you might guess, this isn’t terribly efficient, but it’s fine if you don’t overdo it14StringBuffer constructorsStringBuffer()Constructs a StringBuffer with a capacity of 16 charactersStringBuffer(int capacity)Constructs a StringBuffer with the requested capacityStringBuffer(String str)Constructs a StringBuffer containing the String str15Useful StringBuffer methods IStringBuffer append(X)Appends X to the end of this StringBuffer; also (as a convenience) returns this StringBufferThe append method is so heavily overloaded that it will work with any argument; if the argument is an object, its toString() method is used16Useful StringBuffer methods IIint length()Returns the number of characters in this StringBuffervoid setLength(int newLength)Sets the number of characters in this StringBuffer; this may result in truncation of characters at the end, or addition of null characters17Useful StringBuffer methods IIIchar charAt(int index)Returns the character at the location index void setCharAt(int index, char ch)Sets the character at location index to chStringBuffer reverse()The sequence of characters in this StringBuffer is replaced by the reverse of this sequence, and also returned as the value of the method18Useful StringBuffer methods IVStringBuffer insert(int offset, X)Insert X starting at the location offset in this StringBuffer, and also return this StringBuffer as the value of the method. Like append, this method is heavily overloadedStringBuffer deleteCharAt(int index)Deletes the character at location indexStringBuffer delete(int start, int end)Deletes chars at locations start through end-119Useful StringBuffer methods VString substring(int start)Returns a new String of


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Penn CIT 591 - Strings Lecture

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