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Converting String to array of char and vice versa

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• Previously discuss: String objects
  • Strings in Java:

    String A; A = "abH"; // Assign string to string variable

    How a String is stored in memory:

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  • Now compare with how an array of characters is stored in memory:

    As you can see, they are identical !!!

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    A string and an array of characters is different in Java because the language definition forces the user to use different syntax to gain access to the information stored inside a string and inside an array of characters.

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• Strings VS. array of char in general (applicable to every programming language)
  • Fact:

    The content of Strings and arrays of char are very similar Because of their similarity, Strings and arrays of char are treated identically in some programming languages Example: A string and an array of char are identical in the C/C++ programming language (In C/C++, the programming language defines an pointer arithmetic on pointer variables to make: char* s; (a string) char s[]; (a character array) become equivalent constructs)

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  • However, although a string and an array of char resemble each other, you can decide to treat them differently.

    In Java, the class definition in Java library java.lang.String class defines methods that forces the users to manipulate String typed veriables as specified by the given methods inside the java.lang.String class.

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• String and array of char in Java

  • String and array of char are treated differently in Java

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  • Example of differences:

    In Java: a String is a single object an array of char are a serie (multiple) of variables In Java: a String is immutable (because the instance methods in the String class will not update the characters inside a String an array of char is not protected by a private access modifier (so we can update the characters stored in an array of char)

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• Converting a String into an array of char
  • Because Java treats Strings and arrays of char differently, we need a conversion mechanism to tell the Java compiler to convert one into another.

    (No conversion mechanism is necessary in C/C++ to make strings into arrays of char and vice versa).

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  • (Instance) method in Java that converts a string into an array of char:

    String s; s = "...."; s.toCharArray( ) returns an array of char containing all characters in string s

    Example:

    String s; s = "abc"; char[] A; A = s.toCharArray( ); // Result: A[0] = 'a', A[1] = 'b', A[2] = 'c'

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• The other way:   Converting an array of char into a String
  • To "convert" an array of char into a String object, we use a constructor method in the String class:

    Example:

    char[] A = new char[ 3 ]; A[0] = '?'; A[1] = '>'; A[2] = "a"; String s s = new String( A ); // Make a String using the characters in array A

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  • Example: Java program

    public class ArrayChar1 { public static void main( String[] args ) { String s = "abc"; char[] A; /* ========================================= Convert String s into an array of char ========================================= */ A = s.toCharArray(); System.out.println( s ); for ( int i = 0; i < A.length; i++ ) System.out.println( "A[" + i + "] = " + A[i] ); System.out.println( "\n" ); /* ========================================= Convert an array of char A into string ========================================= */ A = new char[4]; A[0] = '>'; A[1] = '<'; A[2] = '?'; A[3] = 'a'; s = new String( A ); for ( int i = 0; i < A.length; i++ ) System.out.println( "A[" + i + "] = " + A[i] ); System.out.println( s ); } }

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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 ArrayChar1.java To run:          java ArrayChar1

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• Programming note
  • It is not advisable to manipulate strings by converting it into an array of char

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  • That is because:

    the String class contains many instance method, and there are other classes in the Java library that manipulate Strings.

    In other words:

    The chances are very good that the Java library contains instance methods that help you write your program It you use an array of char, you will be manipulating the characters in the string and you will need to write the methods (to manipulate the string yourself.... Nobody likes to write his/her own methods when he/she can use existing ones to do the job !!!