CSC301: Week 5: Hash Tables (3.4)

Contents [0/9]

Built in Hash Functions [1/9]
Amusing collisions [2/9]
Implement your own Hash Functions [3/9]
Phone numbers [4/9]
Phone numbers Performance [5/9]
Phone numbers with bad equality [6/9]
Phone numbers lost in a table [7/9]
Floating point gone bad [8/9]
Floating point the right way [9/9]

(Click here for one slide per page)


Built in Hash Functions [1/9]

file:algs34/XBuiltInHashcodes.java [source] [doc-public] [doc-private]
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package algs34;

import java.util.Arrays;
import java.util.Objects;

public class XBuiltInHashcodes {
  public static void main (String[] args) {
    System.out.format ("int\n");
    System.out.format ("%x ", Integer.hashCode (0));
    System.out.format ("%x ", Integer.hashCode (1));
    System.out.format ("%x ", Integer.hashCode (2));
    System.out.format ("%x ", Integer.hashCode (Integer.MAX_VALUE));
    System.out.format ("%x ", Integer.hashCode (-0));
    System.out.format ("%x ", Integer.hashCode (-1));
    System.out.format ("%x ", Integer.hashCode (-2));
    System.out.format ("%x ", Integer.hashCode (Integer.MIN_VALUE));
    System.out.println ();

    System.out.format ("\nlong\n");
    System.out.format ("%x ", Long.hashCode (0));
    System.out.format ("%x ", Long.hashCode (1));
    System.out.format ("%x ", Long.hashCode (2));
    System.out.format ("%x ", Long.hashCode (Long.MAX_VALUE));
    System.out.format ("%x ", Long.hashCode (-0));
    System.out.format ("%x ", Long.hashCode (-1));
    System.out.format ("%x ", Long.hashCode (-2));
    System.out.format ("%x ", Long.hashCode (Long.MIN_VALUE));
    System.out.println ();

    System.out.format ("\nfloat\n");
    System.out.format ("%x ", Float.hashCode (0));
    System.out.format ("%x ", Float.hashCode (1));
    System.out.format ("%x ", Float.hashCode (2));
    System.out.format ("%x ", Float.hashCode (Float.MAX_VALUE));
    System.out.format ("%x ", Float.hashCode (-0));
    System.out.format ("%x ", Float.hashCode (-1));
    System.out.format ("%x ", Float.hashCode (-2));
    System.out.format ("%x ", Float.hashCode (Float.MIN_VALUE));
    System.out.println ();

    System.out.format ("\ndouble\n");
    System.out.format ("%x ", Double.hashCode (0));
    System.out.format ("%x ", Double.hashCode (1));
    System.out.format ("%x ", Double.hashCode (2));
    System.out.format ("%x ", Double.hashCode (Double.MAX_VALUE));
    System.out.format ("%x ", Double.hashCode (-0));
    System.out.format ("%x ", Double.hashCode (-1));
    System.out.format ("%x ", Double.hashCode (-2));
    System.out.format ("%x ", Double.hashCode (Double.MIN_VALUE));
    System.out.println ();

    System.out.format ("\nchar\n");
    System.out.format ("%x ", Character.hashCode ('0'));
    System.out.format ("%x ", Character.hashCode ('1'));
    System.out.format ("%x ", Character.hashCode ('2'));
    System.out.println ();

    System.out.format ("\nstring\n");
    System.out.format ("%x ", Objects.hashCode ("0"));
    System.out.format ("%x ", Objects.hashCode ("1"));
    System.out.format ("%x ", Objects.hashCode ("2"));
    System.out.println ();

    System.out.format ("\nstring decimal\n");
    System.out.format ("%d ", Objects.hashCode ("0"));
    System.out.format ("%d ", Objects.hashCode ("1"));
    System.out.format ("%d ", Objects.hashCode ("2"));
    System.out.println ();

    System.out.format ("%d (=%d*31 + %d) ", Objects.hashCode ("00"), (int)'0', (int)'0');
    System.out.format ("%d (=%d*31 + %d) ", Objects.hashCode ("01"), (int)'0', (int)'1');
    System.out.format ("%d (=%d*31 + %d) ", Objects.hashCode ("02"), (int)'0', (int)'2');
    System.out.println ();

    System.out.format ("\nstring\n");
    System.out.format ("%d ", Objects.hashCode ("0"));
    System.out.format ("%d ", Objects.hashCode ("00"));
    System.out.format ("%d ", Objects.hashCode ("000"));
    System.out.format ("%d ", Objects.hashCode ("0000"));
    System.out.format ("%d ", Objects.hashCode ("00000"));
    System.out.format ("%d ", Objects.hashCode ("000000"));
    System.out.format ("%d ", Objects.hashCode ("0000000"));
    System.out.format ("%d ", Objects.hashCode ("00000000"));
    System.out.format ("%d ", Objects.hashCode ("000000000"));
    System.out.println ();

    {
      System.out.format ("\nfloat 0's\n");
      float x = Float.intBitsToFloat (0);
      float y = Float.intBitsToFloat (0x80000000);
      System.out.format ("(x==y)=%b Float.compare(x,y)=%d\n", x==y, Float.compare (x, y));
      System.out.format ("x=%s Float.hashCode(x)=%x\n", x, Float.hashCode (x));
      System.out.format ("y=%s Float.hashCode(y)=%x\n", y, Float.hashCode (y));
    }
    {
      System.out.format ("\nfloat NaN's\n");
      float x = Float.NaN;
      float y = Float.NaN;
      System.out.format ("(x==y)=%b Float.compare(x,y)=%d\n", x==y, Float.compare (x, y));
      System.out.format ("x=%s Float.hashCode(x)=%x\n", x, Float.hashCode (x));
      System.out.format ("y=%s Float.hashCode(y)=%x\n", y, Float.hashCode (y));
    }
    {
      System.out.format ("\ndouble 0's\n");
      double x = Double.longBitsToDouble (0L);
      double y = Double.longBitsToDouble (0x8000000000000000L);
      System.out.format ("(x==y)=%b Double.compare(x,y)=%d\n", x==y, Double.compare (x, y));
      System.out.format ("x=%s Double.hashCode(x)=%x\n", x, Double.hashCode (x));
      System.out.format ("y=%s Double.hashCode(y)=%x\n", y, Double.hashCode (y));
    }
    {
      System.out.format ("\ndouble NaN's\n");
      double x = Double.NaN;
      double y = Double.NaN;
      System.out.format ("(x==y)=%b Double.compare(x,y)=%d\n", x==y, Double.compare (x, y));
      System.out.format ("x=%s Double.hashCode(x)=%x\n", x, Double.hashCode (x));
      System.out.format ("y=%s Double.hashCode(y)=%x\n", y, Double.hashCode (y));
    }

    /*
     * Objects and Arrays act like this:
     *
     * Objects.equals(a, b) { return (a == b) || (a != null && a.equals(b)); }
     *   if (a==b) return true;
     *   if (a==null || b==null || (! a.equals(b)) return false;
     *   return true;
     * }
     * Objects.hashCode(a)  {
     *   if (a==null) return 0;
     *   return a.hashCode();
     * }
     *
     * Arrays.equals(a, b) {
     *   if (a==b) return true;
     *   if (a==null || b==null || (a.length != b.length)) return false;
     *   for (int i=0; i<a.length; i++) if (! Objects.equals(a[i],b[i])) return false;
     *   return true;
     * }
     * Arrays.hashCode(a) {
     *   if (a == null) return 0;
     *   int result = 1;
     *   for (Object element : a) result = 31 * result + Objects.hashCode(element);
     *   return result;
     * }
     *
     * Arrays.deepEquals is similar, but recursively calls Arrays.equals(a[i],b[i]) for nested arrays
     * Arrays.deepHashCode is similar, but recursively calls Arrays.hashCode(element) for nested arrays
     */
    {
      System.out.format ("\nObject\n");
      Object x = new Object ();
      Object y = new Object ();
      System.out.format ("(x==y)=%b Objects.equals(x,y)=%b\n", x==y, Objects.equals (x,y));
      System.out.format ("x=%s Objects.hashCode(x)=%x\n", x, Objects.hashCode (x));
      System.out.format ("y=%s Objects.hashCode(y)=%x\n", y, Objects.hashCode (y));
    }
    {
      System.out.format ("\nObject null\n");
      Object x = null;
      Object y = null;
      System.out.format ("(x==y)=%b Objects.equals(x,y)=%b\n", x==y, Objects.equals (x,y));
      System.out.format ("x=%s Objects.hashCode(x)=%x\n", x, Objects.hashCode (x));
      System.out.format ("y=%s Objects.hashCode(y)=%x\n", y, Objects.hashCode (y));
    }
    {
      System.out.format ("\nint[]\n");
      int a[] = new int[4];
      int b[] = new int[4];
      System.out.format ("(a==b)=%b Objects.equals(a,b)=%b Arrays.equals(a,b)=%b\n", a==b, Objects.equals (a,b), Arrays.equals(a,b));
      System.out.format ("a=%s Objects.hashCode(a)=%x Arrays.hashCode(a)=%x\n", a, Objects.hashCode (a), Arrays.hashCode (a));
      System.out.format ("b=%s Objects.hashCode(b)=%x Arrays.hashCode(b)=%x\n", b, Objects.hashCode (b), Arrays.hashCode (b));
    }
    {
      System.out.format ("\nint[] null\n");
      int a[] = null;
      int b[] = null;
      System.out.format ("(a==b)=%b Objects.equals(a,b)=%b Arrays.equals(a,b)=%b\n", a==b, Objects.equals (a,b), Arrays.equals(a,b));
      System.out.format ("a=%s Objects.hashCode(a)=%x Arrays.hashCode(a)=%x\n", a, Objects.hashCode (a), Arrays.hashCode (a));
      System.out.format ("b=%s Objects.hashCode(b)=%x Arrays.hashCode(b)=%x\n", b, Objects.hashCode (b), Arrays.hashCode (b));
    }
    {
      System.out.format ("\nObject[] (for most object arrays, Arrays.equals is enough)\n");
      String a[] = new String[] { new String("hi") };
      String b[] = new String[] { new String("hi") };
      System.out.format ("(a==b)=%b Objects.equals(a,b)=%b Arrays.equals(a,b)=%b Arrays.deepEquals(a,b)=%b\n", a==b, Objects.equals (a,b), Arrays.equals(a,b), Arrays.deepEquals(a,b));
      System.out.format ("a=%s Objects.hashCode(a)=%x Arrays.hashCode(a)=%x Arrays.deepHashCode(a)=%x\n", a, Objects.hashCode (a), Arrays.hashCode (a), Arrays.deepHashCode (a));
      System.out.format ("b=%s Objects.hashCode(b)=%x Arrays.hashCode(b)=%x Arrays.deepHashCode(b)=%x\n", b, Objects.hashCode (b), Arrays.hashCode (b), Arrays.deepHashCode (b));
    }
    {
      System.out.format ("\nint[][] (multidimensional arrays need Arrays.deepEquals)\n");
      int a[][] = new int[4][4];
      int b[][] = new int[4][4];
      System.out.format ("(a==b)=%b Objects.equals(a,b)=%b Arrays.equals(a,b)=%b Arrays.deepEquals(a,b)=%b\n", a==b, Objects.equals (a,b), Arrays.equals(a,b), Arrays.deepEquals(a,b));
      System.out.format ("a=%s Objects.hashCode(a)=%x Arrays.hashCode(a)=%x Arrays.deepHashCode(a)=%x\n", a, Objects.hashCode (a), Arrays.hashCode (a), Arrays.deepHashCode (a));
      System.out.format ("b=%s Objects.hashCode(b)=%x Arrays.hashCode(b)=%x Arrays.deepHashCode(b)=%x\n", b, Objects.hashCode (b), Arrays.hashCode (b), Arrays.deepHashCode (b));
    }
  }
}

Amusing collisions [2/9]

file:algs34/XStringHashcodes.java [source] [doc-public] [doc-private]
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package algs34;
public class XStringHashcodes {
  public static void main (String[] args) {
    // from http://stackoverflow.com/questions/2310498/why-doesnt-strings-hashcode-cache-0
    String[] strings = new String[] {
        "pollinating sandboxes",
        "amusement & hemophilias",
        "schoolworks = perversive",
        "electrolysissweeteners.net",
        "constitutionalunstableness.net",
        "grinnerslaphappier.org",
        "BLEACHINGFEMININELY.NET",
        "WWW.BUMRACEGOERS.ORG",
        "WWW.RACCOONPRUDENTIALS.NET",
        "Microcomputers: the unredeemed lollipop...",
        "Incentively, my dear, I don't tessellate a derangement.",
        "A person who never yodelled an apology, never preened vocalizing transsexuals.",
        "polygenelubricants",
        "And so my fellow mismanagements: ask not what your newsdealer can sugarcoat for you -- ask what you can sugarcoat for your newsdealer."
    };
    for (String s : strings) {
      System.out.format ("%x %s\n", s.hashCode (), s);
    }
  }
}

Implement your own Hash Functions [3/9]

file:algs34/XStudent.java [source] [doc-public] [doc-private]
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package algs34;

import java.util.Arrays;
import java.util.Objects;
import stdlib.*;

public class XStudent {
  int i;
  double d;
  int[] a;
  Object o;

  public boolean equals(Object y) {
    //Trace.draw ();
    if (y == this) return true;
    if (y == null) return false;
    if (y.getClass() != this.getClass()) return false;
    XStudent that = (XStudent) y;
    if (Integer.compare (this.i, that.i) != 0) return false;
    if (Double.compare (this.d, that.d) != 0) return false;
    if (! Arrays.equals (this.a, that.a)) return false;
    if (! Objects.equals (this.o, that.o)) return false;
    // don't use ((this.o).equals(that.o)) unless you know that (this.o!=null)
    return true;
  }
  public int hashCode() {
    int h = 17;
    h = 31 * h + Integer.hashCode (i);
    h = 31 * h + Double.hashCode (d);
    h = 31 * h + Arrays.hashCode (a);
    h = 31 * h + Objects.hashCode (o);
    // don't use (o.hashCode()) unless you know that (o!=null)
    return h;
  }


  public XStudent (double d, int[] a, Object o) {
    this.d = d;
    this.a = a;
    this.o = o;
  }
  public static void main (String[] args) {
    //Trace.run ();
    Object p = new Object ();
    XStudent x = new XStudent (0.1, new int[] { 1,2,3 }, p);
    XStudent y = new XStudent (0.1, new int[] { 1,2,3 }, p);
    StdOut.format ("(x==y)=%b x.equals(y)=%b\n", x==y, Objects.equals(x,y));
    StdOut.format ("x=%s Objects.hashCode(x)=%x\n", x, Objects.hashCode (x));
    StdOut.format ("y=%s Objects.hashCode(y)=%x\n", y, Objects.hashCode (y));
  }
}

Phone numbers [4/9]

file:algs34/XPhoneNumber.java [source] [doc-public] [doc-private]
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package algs34;
import stdlib.*;
import java.util.HashSet;
/* ***********************************************************************
 *  Compilation:  javac PhoneNumber.java
 *  Execution:    java PhoneNumber
 *  Dependencies:
 *
 *  Immutable data type for US phone numbers.
 *
 *************************************************************************/

public final class XPhoneNumber {
  private final int area;   // area code (3 digits)
  private final int exch;   // exchange  (3 digits)
  private final int ext;    // extension (4 digits)

  public XPhoneNumber(int area, int exch, int ext) {
    this.area = area;
    this.exch = exch;
    this.ext  = ext;
  }

  // how you're supposed to implement equals
  public boolean equals(Object y) {
    if (y == this) return true;
    if (y == null) return false;
    if (y.getClass() != this.getClass()) return false;
    XPhoneNumber that = (XPhoneNumber) y;
    if (!((this.area == that.area) && (this.exch == that.exch) && (this.ext == that.ext))) return false;
    return true;
  }

  // satisfies the hashCode contract
  public int hashCode() {
    int h = 17;
    h = ext + 31 * h;
    h = exch + 31 * h;
    h = area + 31 * h;
    return h;
  }

  // 0 for padding with leading 0s
  public String toString() {
    return String.format("(%03d) %03d-%04d", area, exch, ext);
  }

  public static void main(String[] args) {
    XPhoneNumber a = new XPhoneNumber(609, 258, 4455);
    XPhoneNumber b = new XPhoneNumber(609, 876, 5309);
    XPhoneNumber c = new XPhoneNumber(609, 003, 5309);
    XPhoneNumber d = new XPhoneNumber(215, 876, 5309);
    XPhoneNumber e = new XPhoneNumber(609, 876, 5309);
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("c = %s [hashcode=%d]\n", c, c.hashCode ());
    StdOut.format("d = %s [hashcode=%d]\n", d, d.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    HashSet<XPhoneNumber> set = new HashSet<>();
    set.add(a);
    set.add(b);
    set.add(c);
    StdOut.println("Added a, b, and c");
    StdOut.println("contains a:  " + set.contains(a));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("contains c:  " + set.contains(c));
    StdOut.println("contains d:  " + set.contains(d));
    StdOut.println("contains e:  " + set.contains(e));
    StdOut.println("b == e:      " + (b == e));
    StdOut.println("b.equals(e): " + (b.equals(e)));
  }



}

Phone numbers Performance [5/9]

file:algs34/XPhoneNumberPerformanceTest.java [source] [doc-public] [doc-private]
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package algs34;

import java.util.HashSet;
import stdlib.*;

public class XPhoneNumberPerformanceTest {
  private static int NUM_SIZES = 6;
  private static int START_SIZE = 25000;
  public static void main(String[] args) {
    int size = START_SIZE;
    for (int count=1; count<NUM_SIZES; count++) {
      size += size;
      HashSet<XPhoneNumber> set = new HashSet<> ();
      Stopwatch sw1 = new Stopwatch ();
      for (int i=size-1; i>=0; i--) {
        XPhoneNumber x = new XPhoneNumber
          (StdRandom.uniform (1000), StdRandom.uniform (1000), StdRandom.uniform (10000));
        set.add(x);
      }
      double time1 = sw1.elapsedTime ();
      Stopwatch sw2 = new Stopwatch ();
      for (int i=size-1; i>=0; i--) {
        XPhoneNumber x = new XPhoneNumber
          (StdRandom.uniform (1000), StdRandom.uniform (1000), StdRandom.uniform (10000));
        set.contains (x);
      }
      double time2 = sw1.elapsedTime ();
      System.out.format ("%9d: add=%f contains=%f\n", size, time1, time2);
    }
  }
}

Phone numbers with bad equality [6/9]

file:algs34/XPhoneNumberOverload.java [source] [doc-public] [doc-private]
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package algs34;
import stdlib.*;
import java.util.HashSet;
/* ***********************************************************************
 *  Compilation:  javac PhoneNumber.java
 *  Execution:    java PhoneNumber
 *  Dependencies:
 *
 *  Immutable data type for US phone numbers, but with a broken
 *  overloaded implementation of equals(). By implementing equals()
 *  with the signature
 *
 *       public boolean equals(PhoneNumber that)
 *
 *  we do not override the equals() method inherited from Object.
 *  This causes unexpected behavior when used with java.util.HashSet.
 *
 *  DO NOT USE THIS CLASS
 *
 *************************************************************************/

public final class XPhoneNumberOverload {
  private final int area;   // area code (3 digits)
  private final int exch;   // exchange  (3 digits)
  private final int ext;    // extension (4 digits)

  public XPhoneNumberOverload(int area, int exch, int ext) {
    this.area = area;
    this.exch = exch;
    this.ext  = ext;
  }

  // overloaded equals - don't do this
  public boolean equals(XPhoneNumberOverload that) {
    return (this.area == that.area) && (this.exch == that.exch) && (this.ext == that.ext);
  }

  // satisfies the hashCode contract
  public int hashCode() {
    int h = 17;
    h = ext + 31 * h;
    h = exch + 31 * h;
    h = area + 31 * h;
    return h;
  }

  // 0 for padding with leading 0s
  public String toString() {
    return String.format("(%03d) %03d-%04d", area, exch, ext);
  }

  public static void main(String[] args) {
    XPhoneNumberOverload a = new XPhoneNumberOverload(609, 258, 4455);
    XPhoneNumberOverload b = new XPhoneNumberOverload(609, 876, 5309);
    XPhoneNumberOverload c = new XPhoneNumberOverload(609, 003, 5309);
    XPhoneNumberOverload d = new XPhoneNumberOverload(215, 876, 5309);
    XPhoneNumberOverload e = new XPhoneNumberOverload(609, 876, 5309);
    Object o = e;
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("c = %s [hashcode=%d]\n", c, c.hashCode ());
    StdOut.format("d = %s [hashcode=%d]\n", d, d.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    // show broken behavior when you use covariant equals with a Set
    HashSet<XPhoneNumberOverload> set = new HashSet<>();
    set.add(a);
    set.add(b);
    set.add(c);
    StdOut.println("Added a, b, and c");
    StdOut.println("contains a:  " + set.contains(a));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("contains c:  " + set.contains(c));
    StdOut.println("contains d:  " + set.contains(d));
    StdOut.println("***contains e:  " + set.contains(e));  // not in set, but it should be!
    StdOut.println("b == e:      " + (b == e));
    StdOut.println("b.equals(e): " + (b.equals(e)));
    StdOut.println("o == e:      " + (o == e));
    StdOut.println("b.equals(o): " + (b.equals(o)));
  }

}

Phone numbers lost in a table [7/9]

file:algs34/XPhoneNumberMutable.java [source] [doc-public] [doc-private]
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package algs34;
import stdlib.*;
import java.util.HashSet;
/* ***********************************************************************
 *  Compilation:  javac PhoneNumber.java
 *  Execution:    java PhoneNumber
 *  Dependencies:
 *
 *  Mutable data type for US phone numbers, with overridden versions
 *  equals and hashcode.  This can cause data to get lost in java.util.HashSet.
 *
 *  DO NOT USE THIS CLASS
 *************************************************************************/

public final class XPhoneNumberMutable {
  private int area;   // area code (3 digits)
  private int exch;   // exchange  (3 digits)
  private int ext;    // extension (4 digits)

  public XPhoneNumberMutable(int area, int exch, int ext) {
    this.area = area;
    this.exch = exch;
    this.ext  = ext;
  }

  public void set(int area, int exch, int ext) {
    this.area = area;
    this.exch = exch;
    this.ext  = ext;
  }

  // how you're supposed to implement equals
  public boolean equals(Object y) {
    if (y == this) return true;
    if (y == null) return false;
    if (y.getClass() != this.getClass()) return false;
    XPhoneNumberMutable that = (XPhoneNumberMutable) y;
    return (this.area == that.area) && (this.exch == that.exch) && (this.ext == that.ext);
  }

  // 0 for padding with leading 0s
  public String toString() {
    return String.format("(%03d) %03d-%04d", area, exch, ext);
  }

  // satisfies the hashCode contract
  public int hashCode() {
    int h = 17;
    h = ext + 31 * h;
    h = exch + 31 * h;
    h = area + 31 * h;
    return h;
  }


  public static void main(String[] args) {
    XPhoneNumberMutable a = new XPhoneNumberMutable(609, 258, 4455);
    XPhoneNumberMutable b = new XPhoneNumberMutable(609, 876, 5309);
    XPhoneNumberMutable c = new XPhoneNumberMutable(609, 003, 5309);
    XPhoneNumberMutable d = new XPhoneNumberMutable(215, 876, 5309);
    XPhoneNumberMutable e = new XPhoneNumberMutable(609, 876, 5309);
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("c = %s [hashcode=%d]\n", c, c.hashCode ());
    StdOut.format("d = %s [hashcode=%d]\n", d, d.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    HashSet<XPhoneNumberMutable> set = new HashSet<>();
    set.add(a);
    set.add(b);
    set.add(c);
    StdOut.println("\nAdded a, b, and c: " + set);
    StdOut.println("contains a:  " + set.contains(a));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("contains c:  " + set.contains(c));
    StdOut.println("contains d:  " + set.contains(d));
    StdOut.println("contains e:  " + set.contains(e));
    StdOut.println("b == e:      " + (b == e));
    StdOut.println("b.equals(e): " + (b.equals(e)));

    c.set(609, 876, 5309);
    b.set(609, 003, 5309);
    StdOut.format("\na = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("c = %s [hashcode=%d]\n", c, c.hashCode ());
    StdOut.format("d = %s [hashcode=%d]\n", d, d.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());
    StdOut.println("\nSwapped values of b and c: " + set);
    StdOut.println("contains a:  " + set.contains(a));
    StdOut.println("***contains b:  " + set.contains(b));
    StdOut.println("***contains c:  " + set.contains(c));
    StdOut.println("contains d:  " + set.contains(d));
    StdOut.println("***contains e:  " + set.contains(e));
    StdOut.println("c == e:      " + (c == e));
    StdOut.println("c.equals(e): " + (c.equals(e)));

  }



}

Floating point gone bad [8/9]

file:algs34/XBadPoint.java [source] [doc-public] [doc-private]
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package algs34;
import stdlib.*;
import java.util.HashSet;
/* ***********************************************************************
 * Floating point is a pain!
 *************************************************************************/

public final class XBadPoint {
  static class BadPoint {
    private final double x;
    private final double y;
    public BadPoint(double x, double y) { this.x = x; this.y = y; }
    public String toString() { return "(" + x + ", " + y + ")";  }

    public boolean equals(Object other) {
      if (other == this) return true;
      if (other == null || other.getClass() != this.getClass()) return false;
      BadPoint that = (BadPoint) other;
      if (this.x != that.x) return false;
      if (this.y != that.y) return false;
      return true;
    }
    public int hashCode() {
      int h = 17;
      h = 31*h + Double.hashCode(x);
      h = 31*h + Double.hashCode(y);
      return h;
    }
  }

  public static void main(String[] args) {
    BadPoint a = new BadPoint(0.0, 0.0);
    BadPoint b = new BadPoint(0.0, 0.0);
    BadPoint e = new BadPoint(0.0,-0.0);
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    HashSet<BadPoint> set = new HashSet<>();
    set.add(a);
    StdOut.println("Added a");
    StdOut.println("a == b:      " + (a == b));
    StdOut.println("a.equals(b): " + (a.equals(b)));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("a == e:      " + (a == e));
    StdOut.println("a.equals(e): " + (a.equals(e)));
    StdOut.println("contains e:  " + set.contains(e));
  }
}

Floating point the right way [9/9]

file:algs34/XGoodPoint.java [source] [doc-public] [doc-private]
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package algs34;
import stdlib.*;
import java.util.HashSet;

public final class XGoodPoint {
  static class GoodPoint {
    private final double x;
    private final double y;
    public GoodPoint(double x, double y) { this.x = x; this.y = y; }
    public String toString() { return "(" + x + ", " + y + ")";  }

    public boolean equals(Object other) {
      if (other == this) return true;
      if (other == null || other.getClass() != this.getClass()) return false;
      GoodPoint that = (GoodPoint) other;
      if (Double.compare(this.x,that.x) != 0) return false;
      if (Double.compare(this.y,that.y) != 0) return false;
      return true;
    }
    public int hashCode() {
      int h = 17;
      h = 31*h + Double.hashCode(x);
      h = 31*h + Double.hashCode(y);
      return h;
    }
  }


  public static void main(String[] args) {
    GoodPoint a = new GoodPoint(0.0, 0.0);
    GoodPoint b = new GoodPoint(0.0, 0.0);
    GoodPoint e = new GoodPoint(0.0,-0.0);
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    HashSet<GoodPoint> set = new HashSet<>();
    set.add(a);
    StdOut.println("Added a");
    StdOut.println("a == b:      " + (a == b));
    StdOut.println("a.equals(b): " + (a.equals(b)));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("a == e:      " + (a == e));
    StdOut.println("a.equals(e): " + (a.equals(e)));
    StdOut.println("contains e:  " + set.contains(e));
  }
}


Revised: 2008/03/17 13:01