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Some transformation operations (instance methods) on Strings

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• Transform a string into lower case letters
  • The instance method used to transform all upper case letters in a string s into lower case letters is:

    s.toLowerCase( ) returns: a new string where all upper case letters in s are replaced by lower case letters Note: the original string s is unchanged

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    Examples:

    "AbC".toLowerCase( ) returns "abc" "A&C".toLowerCase( ) returns "a&c"

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• Transform a string into upper case letters
  • The instance method used to transform all lower case letters in a string s into upper case letters is:

    s.toUpperCase( ) returns: a new string where all lower case letters in s are replaced by upper case letters Note: the original string s is unchanged

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    Examples:

    "AbC".toLowerCase( ) returns "ABC" "Ab%".toLowerCase( ) returns "AB%"

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    Java program that illustrates toLowerCase() and toUpperCase():

    public class Case1 { public static void main( String[] args ) { String s, r; s = "AbC"; r = s.toLowerCase( ); System.out.println( "\"" + s + "\"" + ".toLowerCase( ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\""); s = "Ab%"; r = s.toLowerCase( ); System.out.println( "\"" + s + "\"" + ".toLowerCase( ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\"\n\n"); /* ================================================== */ s = "AbC"; r = s.toUpperCase( ); System.out.println( "\"" + s + "\"" + ".toUpperCase( ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\""); s = "Ab%"; r = s.toUpperCase( ); System.out.println( "\"" + s + "\"" + ".toUpperCase( ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\""); } }

    Output:

    "AbC".toLowerCase( ) = "abc" Afterwards: s = "AbC" *** s is UNCHANGED ! "Ab%".toLowerCase( ) = "ab%" Afterwards: s = "Ab%" *** s is UNCHANGED ! "AbC".toUpperCase( ) = "ABC" Afterwards: s = "AbC" *** s is UNCHANGED ! "Ab%".toUpperCase( ) = "AB%" Afterwards: s = "Ab%" *** s is UNCHANGED !

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Case1.java To run:          java Case1

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• Transform a string by replacing a substring once
  • The instance method used to replace the first occurence of a substring old inside a string s by the substring new is:

    s.replaceFirst( old, new ) returns: a new string where the first occurence of "old" in s is replaced by "new" Note: the original string s is unchanged

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    Examples:

    "Hello World, Bye World".replaceFirst( "World", "Class" ) returns "Hello Class, Bye World"

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• Transform a string by replacing all occurences of a substring
  • The instance method used to replace all occurences of a substring old inside a string s by the substring new is:

    s.replaceAll( old, new ) returns: a new string where all occurence of "old" in s is replaced by "new" Note: the original string s is unchanged

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    Examples:

    "Hello World, Bye World".replaceAll( "World", "Class" ) returns "Hello Class, Bye Class"

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    Java program that illustrates replaceFirst() and replaceAll():

    public class Replace1 { public static void main( String[] args ) { String s, r; s = "Hello World, Bye World"; r = s.replaceFirst( "World", "Class" ); System.out.println( "\"" + s + "\"" + ".replaceFirst( \"World\", \"Class\" ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\"\n\n" ); /* ================================================== */ r = s.replaceAll( "World", "Class" ); System.out.println( "\"" + s + "\"" + ".replaceFirst( \"World\", \"Class\" ) = " + "\"" + r + "\"" ); System.out.println( "Afterwards: s = " + "\"" + s + "\"" ); } }

    Output:

    "Hello World, Bye World".replaceFirst( "World", "Class" ) = "Hello Class, Bye World" Afterwards: s = "Hello World, Bye World" "Hello World, Bye World".replaceAll( "World", "Class" ) = "Hello Class, Bye Class" Afterwards: s = "Hello World, Bye World"

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Case1.java To run:          java Case1

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• A shared property of all instance methods in the String class --- that make strings in Java immutable
  • Property of all instance mentods in the String class:

    The instance methods in the String class do not change the implicit parameter Examples: s = "AbC"; s.toLowerCase( ) returns: "abc" Afterwards: s = "AbC" toLowerCase( ) does not change the implicit parameter s (s is still equal to "AbC") s = "Hello World, Bye World"; s.replaceFirst( "World", "Class" ) returns: "Hello Class, Bye World" Afterwards: s = "Hello World, Bye World" replaceFirst( ) does not change the implicit parameter s (s is still equal to "Hello World, Bye World")

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  • Consequence: Strings in Java are immutable

    Suppose you create a string "abc" in Java: String s = "abc"; Suppose we want to update some letters inside this string.... $64,000 question: Can this be done ???            Answer: NO. Reason: Because the instance variables in the String class are private, you must use some (public) instance method in the String class to access and update the characters in a string Because all instance methods in the String class do not change the implicit parameter, there is no way for the user to change the string.