SCJP 6 study guide: error in book?

843789

Hi,

In the example on page 607 in the box "exam watch"; The pre-java 5 example says:

List test = new ArrayList();
test.add(43);
...

But on page 568 in the section "Autoboxing with Collections" it says in the pre-java 5 example:

List myInts = new ArrayList();
myInts.add(new Integer(42));
...

The issue is the fact that collections can't contain primitives. Java 5 introduced the autoboxing preventing the explicit creation of a wrapper.

So I think the example on page 607 contains an error and it should state

...
test.add(new Integer(43));
...

Correct?

JosAH

Balleke wrote:
So I think the example on page 607 contains an error and it should state

...
test.add(new Integer(43));
...

Correct?

In pre-Java 5 the original example is indeed incorrect; later Javas do apply autoboxing in the example code. You do get a few warnings about "raw types" though.

kind regards,

Jos

user12283397

Perhaps this has been changed in a newer edition of this book, but I think I found a problem as well. On page 583 of my copy there is an example of a class called MapTest.
The text says that a class called Dog() finds the Map entry because Dog overrides equals() and hashCode(). The problem is that Dog's equals method uses '==' rather than .equals() to compare
two strings. I thought that perhaps this was a clever use of a String feature discussed earlier. The JVM putting strings on the heap in case they were referenced again.
This equals method would work, therefore, if I was comparing two string references, both pointing to "baloney". But, if I construct a new String object using "baloney" in the constructor, the '--' comparison
fails.

For example:
import java.util.*;

class Dog
{
public Dog(String n) { name = n; }
public String name;

public boolean equals( Object o )
{
if (( o instanceof Dog ) && (((Dog) o).name == name ))
{
return true;
}
return false;
}
public int hashCode() { return name.length(); }
}

class Cat {}

enum Pets {DOG, CAT, HORSE}

class MapTest
{
public static void main( String argv[] )
{
Map<Object, Object> m = new HashMap<Object, Object> ();

Dog d1 = new Dog( "aiko" );

m.put( d1, new Dog("aiko"));

System.err.println( "found aiko??? " + m.get("k1"));
System.err.println( "found aiko??? " + m.get(d1));
System.err.println( "found aiko??? " + m.get( new Dog( "aiko" )));
System.err.println( "found aiko??? " + m.get( new Dog( new String("aiko") )));
}
}

The last get fails, even though I'd expect it to succeed. It seems to me a better implementation of the
Dog equals method would be

public boolean equals( Object o )
{
if (( o instanceof Dog ) && (((Dog) o).name.equals(name) ))
{
return true;
}
return false;
}

Is this nitpicking. Is this a problem? I am enjoying the book very much and hope to take the exam soon.

796440

user12283397 wrote:
Perhaps this has been changed in a newer edition of this book, but I think I found a problem as well. On page 583 of my copy there is an example of a class called MapTest.
The text says that a class called Dog() finds the Map entry because Dog overrides equals() and hashCode(). The problem is that Dog's equals method uses '==' rather than .equals() to compare
two strings.

That is indeed an error. It should be equals(). If the author's example works, it's because he happened to use the same string literal in both cases. It will not work in the general case.

I thought that perhaps this was a clever use of a String feature discussed earlier. The JVM putting strings on the heap in case they were referenced again.

All Strings, just like all other objects, are on the heap. What you're thinking of is the constant pool, which is a special area of the heap. All interned Strings goes there (as does some other stuff).