MutableString.java


Below is the syntax highlighted version of MutableString.java from §1.2 Data Abstraction.


/*************************************************************************
 *  Compilation:  javac MutableString.java
 *  Execution:    java MutableString
 *
 *  Shows that Strings are mutable if you allow reflection.
 *
 *************************************************************************/

import java.lang.reflect.Field; 

public class MutableString {

    public static void main(String[] args) { 
        String s = "Immutable"; 
        String t = "Notreally"; 

        mutate(s, t);
        StdOut.println(t); 

        // strings are interned so this doesn't even print "Immutable" (!)
        StdOut.println("Immutable");
    } 

    // change the first min(|s|, |t|) characters of s to t
    public static void mutate(String s, String t) {
        try {
            Field val = String.class.getDeclaredField("value"); 
            Field off = String.class.getDeclaredField("offset"); 
            val.setAccessible(true); 
            off.setAccessible(true); 
            int offset   = off.getInt(s); 
            char[] value = (char[]) val.get(s); 
            for (int i = 0; i < Math.min(s.length(), t.length()); i++)
                value[offset + i] = t.charAt(i); 
        } 
        catch (Exception e) { e.printStackTrace(); }
    } 

} 


Copyright © 2002–2010, Robert Sedgewick and Kevin Wayne.
Last updated: Wed Mar 27 18:43:02 EDT 2013.