CSC300: Boxing and Equality

Contents [0/23]

Video [1/23]
Objects and equality [2/23]
Equals Operator and Equals Method [3/23]
Null on the Right [4/23]
Null on the Left [5/23]
Object [6/23]
String Literal [7/23]
String Constructor [8/23]
String Operators [9/23]
String Interning [10/23]
Base Type [11/23]
Boxing [12/23]
Unboxing [13/23]
Boxing [14/23]
Integers Close to Zero [15/23]
Integers Far from Zero [16/23]
Integer Boxing with valueOf [17/23]
Integer Constructor (Deprecated) [18/23]
Copying a Reference [19/23]
Arrays of Base Values [20/23]
Arrays of Objects [21/23]
Arrays of Arrays of Base Values [22/23]
Arrays of Arrays of Objects [23/23]

(Click here for one slide per page)


Video [1/23]

This lecture is semi-optional. You should watch it, but it will not be on exams.

Open Playlist

Objects and equality [2/23] 

int[] x = new int[] { 11, 21, 31 };
int[] y = new int[] { 11, 21, 31 };
x=[I@8807e25, y=[I@2a3046da
x=[11, 21, 31], y=[11, 21, 31]
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
    Arrays.equals(x,y) : true
 eqa01

file:Hello.java [source] [doc-public] [doc-private]
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package algs12;
import stdlib.*;
import java.util.*;
public class Hello {
  public static void main (String[] args) {
    Trace.showBuiltInObjects (true);
    Trace.run ();   
    int[] x = new int[] { 11, 21, 31 };
    int[] y = new int[] { 11, 21, 31 };
    Trace.draw ();
    StdOut.println ("x=" + x + ", y=" + y);
    StdOut.println ("x=" + Arrays.toString(x) + ", y=" + Arrays.toString(y));
    StdOut.println ("                  x==y : " + (x == y));
    StdOut.println ("   Objects.equals(x,y) : " + (Objects.equals(x,y)));
    StdOut.println ("           x.equals(y) : " + (x.equals(y)));
    StdOut.println ("    Arrays.equals(x,y) : " + (Arrays.equals(x,y)));
  }
}

Equals Operator and Equals Method [3/23]

In Java, == checks object identity on object types

Unlike other languages (such as C++) this behavior cannot be changed.

Objects all have an equals method.

The java.util.Objects class provides some handy utilities, like Objects.equals

Null on the Right [4/23] 

Integer x = 3000;
Integer y = null;
x=3000, y=null
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
 eqi03

Null on the Left [5/23] 

Integer x = null;
Integer y = 3000;
x=null, y=3000
                  x==y : false
   Objects.equals(x,y) : false
Exception in thread "main" java.lang.NullPointerException
  at algs12.Hello.main(Hello.java:15)
 eqi04

Object [6/23]

Try java.lang.Object 

Object x = new Object();
Object y = new Object();
x=java.lang.Object@8807e25, y=java.lang.Object@2a3046da
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
 eqo01

String Literal [7/23]

Try java.lang.String 

String x = "Hello";
String y = "Hello";
x=Hello, y=Hello
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqs01

String Constructor [8/23] 

String x = new String ("Hello");
String y = new String ("Hello");
x=Hello, y=Hello
                  x==y : false
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqs02

String Operators [9/23] 

String x = "Hel" + "lo";
String y = "Hel" + "lo";
x=Hello, y=Hello
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqs03

String Interning [10/23] 

String a = new String ("Hello");
String b = new String ("Hello");
String x = a.intern();
String y = b.intern();
x=Hello, y=Hello
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqs04

Base Type [11/23] 

int x = 3000;
int y = 3000;
x=3000, y=3000
                  x==y : true
 eqi05

Boxing [12/23] 

Integer x = 3000;
Integer y = 3000;
x=3000, y=3000
                  x==y : false
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi01

Unboxing [13/23] 

Integer a = 3000;
Integer b = 3000;
int x = a.intValue();
int y = b.intValue();
x=3000, y=3000
                  x==y : true
 eqi09

Boxing [14/23]

Conversion from base type to object type is called boxing

Conversion from object type to base type is called unboxing

For int and double, the operations are as follows

Base type Object type Boxing (base to object) Unboxing (object to base)
int base = 0; Integer object = null; object = Integer.valueOf(base); base = object.intValue();
double base = 0.0; Double object = null; object = Double.valueOf(base); base = object.doubleValue();

Java has six additional base types:

Base type Object type Boxing (base to object) Unboxing (object to base)
boolean base = false; Boolean object = null; object = Boolean.valueOf(base); base = object.booleanValue();
float base = 0.0F; Float object = null; object = Float.valueOf(base); base = object.floatValue();
byte base = 0; Byte object = null; object = Byte.valueOf(base); base = object.byteValue();
char base = 0; Character object = null; object = Character.valueOf(base); base = object.charValue();
short base = 0; Short object = null; object = Short.valueOf(base); base = object.shortValue();
long base = 0L; Long object = null; object = Long.valueOf(base); base = object.longValue();

Many languages make an explicit distinction between object and base types

Integers Close to Zero [15/23] 

Integer x = 30;
Integer y = 30;
x=30, y=30
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi08

Integers Far from Zero [16/23] 

Integer x = 3000;
Integer y = 3000;
x=3000, y=3000
                  x==y : false
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi01

Integer Boxing with valueOf [17/23] 

Integer x = Integer.valueOf(30);
Integer y = Integer.valueOf(30);
x=30, y=30
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi08

Integer Constructor (Deprecated) [18/23] 

Integer x = new Integer(30);
Integer y = new Integer(30);
x=30, y=30
                  x==y : false
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi07

Copying a Reference [19/23] 

Integer x = 3000;
Integer y = x;
x=3000, y=3000
                  x==y : true
   Objects.equals(x,y) : true
           x.equals(y) : true
 eqi02

Arrays of Base Values [20/23]

Try arrays, with utility functions in java.util.Arrays 

int[] x = new int[] { 11, 21, 31 };
int[] y = new int[] { 11, 21, 31 };
x=[I@8807e25, y=[I@2a3046da
x=[11, 21, 31], y=[11, 21, 31]
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
    Arrays.equals(x,y) : true
 eqa01

Arrays of Objects [21/23] 

Integer[] x = new Integer[] { 11, 21, 31 };
Integer[] y = new Integer[] { 11, 21, 31 };
x=[Ljava.lang.Integer;@8807e25, y=[Ljava.lang.Integer;@2a3046da
x=[11, 21, 31], y=[11, 21, 31]
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
    Arrays.equals(x,y) : true
 eqa03

Arrays of Arrays of Base Values [22/23] 

int[][] x = new int[][] { { 11, 21, 31 }, { 12, 22, 32 } };
int[][] y = new int[][] { { 11, 21, 31 }, { 12, 22, 32 } };
x=[[I@8807e25, y=[[I@2a3046da
x=[[I@12f40c25, [I@3ada9e37], y=[[I@5cbc508c, [I@3419866c]
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
    Arrays.equals(x,y) : false
Arrays.deepEquals(x,y) : true
 eqa04

Arrays of Arrays of Objects [23/23] 

Integer[][] x = new Integer[][] { { 11, 21, 31 }, { 12, 22, 32 } };
Integer[][] y = new Integer[][] { { 11, 21, 31 }, { 12, 22, 32 } };
x=[[Ljava.lang.Integer;@8807e25, y=[[Ljava.lang.Integer;@2a3046da
x=[[Ljava.lang.Integer;@12f40c25, [Ljava.lang.Integer;@3ada9e37], y=[[Ljava.lang.Integer;@5cbc508c, [Ljava.lang.Integer;@3419866c]
                  x==y : false
   Objects.equals(x,y) : false
           x.equals(y) : false
    Arrays.equals(x,y) : false
Arrays.deepEquals(x,y) : true
eqa05

Revised: 2008/03/17 13:01