When is it required to use a Copy constructor & What necessitates overriding of Object Methods

[zoheb hassan]

Hi,

I am aware that a copy constructor is meant for creating a copy of an object but when and why do we have to use it, why not rather just create a new object, Basically i am trying to understand why we make that choice of using a copy constructor??

also

What necessitates overriding of hashCode(), toString() & equals() methods of Object class??


Please help me understand these questions

[Junilu Lacar]

A copy constructor is a convenient way to create an object that has already been initialized. With a copy constructor, you can instantiate and initialize an object with one statement. A typical usage would be with a class that has one or more immutable attributes whose values can only be set via a constructor.

[Campbell Ritchie]

I always though a copy constructor is used to make an identical copy of an instance. And I thought it was necessary for a class with mutable attributes.
Example

public class Water
{
   private int temperature;
   private int volume;

   public Water(Water w)
   {
      this.temperature = w.temperature;
      this.volume = w.volume;
   }
   // other constructors and all the usual methods.
}

Water w = new Water(75, 10000);
Water w2 = new Water(w);
w.cool(35);
System.out.println(w); // Water @ 40° vol=10000
System.out.println(w2); // Water @ 75° vol=10000

You never actually require a copy constructor, but it is often a much nicer way to copy instances than clone();

[Campbell Ritchie]

If a class is immutable you never need copy its instances. Look what the API says about String. It can be shared.

[zoheb hassan]

Hi guys, thanks for the replies, My Curiosity for copy constructor began when i took a look at the following example and never having used a copy constructor i got curious, I am posting the code below

I understand that a copy constructor helps in creating a identical copy of instance but why not rather create a new instance itself, What necessitates its use below is the code, I understand its a bean meant for setting and getting attributes but whats with the overriding of all those methods. This list basically gets put into a ArrayList

package com.model;

public final class Item {

   private long itemId;
   private long endTime;
   private String picUrl;
   private String smallPicUrl;
   private String pic175Url;
   private String title;
   private String desc;
   private String dealUrl;
   private String convertedCurrentPrice;
   private String primaryCategoryName;
   private String location;
   private int quantity;
   private int quantitySold;
   private String msrp;
   private String savingsRate;
   private boolean hot;

   // favor "copy constructor/getInstance" over clone, clone is tricky and error prone
   // (better yet use immutable objects, but sort of overkill for this example)
   public static Item getInstance(Item item) {
      Item copy = new Item();
      copy.convertedCurrentPrice = item.convertedCurrentPrice;
      copy.dealUrl = item.dealUrl;
      copy.desc = item.desc;
      copy.endTime = item.endTime;
      copy.hot = item.hot;
      copy.itemId = item.itemId;
      copy.location = item.location;
      copy.msrp = item.msrp;
      copy.picUrl = item.picUrl;
      copy.primaryCategoryName = item.primaryCategoryName;
      copy.quantity = item.quantity;
      copy.quantitySold = item.quantitySold;
      copy.savingsRate = item.savingsRate;
      copy.smallPicUrl = item.smallPicUrl;
      copy.title = item.title;
      copy.pic175Url = item.pic175Url;
      return copy;
   }

   public long getItemId() {
      return this.itemId;
   }

   public void setItemId(long itemId) {
      this.itemId = itemId;
   }

   public long getEndTime() {
      return this.endTime;
   }

   public void setEndTime(long endTime) {
      this.endTime = endTime;
   }

   public String getPicUrl() {
      return this.picUrl;
   }

   public void setPicUrl(String picUrl) {
      this.picUrl = picUrl;
   }

   public String getSmallPicUrl() {
      return this.smallPicUrl;
   }

   public void setSmallPicUrl(String smallPicUrl) {
      this.smallPicUrl = smallPicUrl;
   }

   public String getPic175Url() {
      return this.pic175Url;
   }

   public void setPic175Url(String pic175Url) {
      this.pic175Url = pic175Url;
   }

   public String getTitle() {
      return this.title;
   }

   public void setTitle(String title) {
      this.title = title;
   }

   public String getDesc() {
      return this.desc;
   }

   public void setDesc(String desc) {
      this.desc = desc;
   }

   public String getDealUrl() {
      return this.dealUrl;
   }

   public void setDealUrl(String dealUrl) {
      this.dealUrl = dealUrl;
   }

   public String getConvertedCurrentPrice() {
      return this.convertedCurrentPrice;
   }

   public void setConvertedCurrentPrice(String convertedCurrentPrice) {
      this.convertedCurrentPrice = convertedCurrentPrice;
   }

   public String getPrimaryCategoryName() {
      return this.primaryCategoryName;
   }

   public void setPrimaryCategoryName(String primaryCategoryName) {
      this.primaryCategoryName = primaryCategoryName;
   }

   public String getLocation() {
      return this.location;
   }

   public void setLocation(String location) {
      this.location = location;
   }

   public int getQuantity() {
      return this.quantity;
   }

   public void setQuantity(int quantity) {
      this.quantity = quantity;
   }

   public int getQuantitySold() {
      return this.quantitySold;
   }

   public void setQuantitySold(int quantitySold) {
      this.quantitySold = quantitySold;
   }

   public String getMsrp() {
      return this.msrp;
   }

   public void setMsrp(String msrp) {
      this.msrp = msrp;
   }

   public String getSavingsRate() {
      return this.savingsRate;
   }

   public void setSavingsRate(String savingsRate) {
      this.savingsRate = savingsRate;
   }

   public boolean isHot() {
      return this.hot;
   }

   public void setHot(boolean hot) {
      this.hot = hot;
   }

   @Override
   public String toString() {
      return "Item [convertedCurrentPrice=" + this.convertedCurrentPrice + ", dealUrl=" + this.dealUrl + ", desc="
               + this.desc + ", endTime=" + this.endTime + ", hot=" + this.hot + ", itemId=" + this.itemId
               + ", location=" + this.location + ", msrp=" + this.msrp + ", pic175Url=" + this.pic175Url + ", picUrl="
               + this.picUrl + ", primaryCategoryName=" + this.primaryCategoryName + ", quantity=" + this.quantity
               + ", quantitySold=" + this.quantitySold + ", savingsRate=" + this.savingsRate + ", smallPicUrl="
               + this.smallPicUrl + ", title=" + this.title + "]";
   }

   @Override
   public int hashCode() {
      final int prime = 31;
      int result = 1;
      result = prime * result + ((this.convertedCurrentPrice == null) ? 0 : this.convertedCurrentPrice.hashCode());
      result = prime * result + ((this.dealUrl == null) ? 0 : this.dealUrl.hashCode());
      result = prime * result + ((this.desc == null) ? 0 : this.desc.hashCode());
      // end time and hot not part of hashCode
      result = prime * result + (int) (this.itemId ^ (this.itemId >>> 32));
      result = prime * result + ((this.location == null) ? 0 : this.location.hashCode());
      result = prime * result + ((this.msrp == null) ? 0 : this.msrp.hashCode());
      result = prime * result + ((this.pic175Url == null) ? 0 : this.pic175Url.hashCode());
      result = prime * result + ((this.picUrl == null) ? 0 : this.picUrl.hashCode());
      result = prime * result + ((this.primaryCategoryName == null) ? 0 : this.primaryCategoryName.hashCode());
      // quantity sold and quantity not part of hashCode
      result = prime * result + ((this.savingsRate == null) ? 0 : this.savingsRate.hashCode());
      result = prime * result + ((this.smallPicUrl == null) ? 0 : this.smallPicUrl.hashCode());
      result = prime * result + ((this.title == null) ? 0 : this.title.hashCode());
      return result;
   }

   @Override
   public boolean equals(Object obj) {
      if (this == obj) {
         return true;
      }
      if (obj == null) {
         return false;
      }
      if (getClass() != obj.getClass()) {
         return false;
      }
      Item other = (Item) obj;
      if (this.convertedCurrentPrice == null) {
         if (other.convertedCurrentPrice != null) {
            return false;
         }
      } else if (!this.convertedCurrentPrice.equals(other.convertedCurrentPrice)) {
         return false;
      }
      if (this.dealUrl == null) {
         if (other.dealUrl != null) {
            return false;
         }
      } else if (!this.dealUrl.equals(other.dealUrl)) {
         return false;
      }
      if (this.desc == null) {
         if (other.desc != null) {
            return false;
         }
      } else if (!this.desc.equals(other.desc)) {
         return false;
      }
      // end time and hot not part of equals
      if (this.itemId != other.itemId) {
         return false;
      }
      if (this.location == null) {
         if (other.location != null) {
            return false;
         }
      } else if (!this.location.equals(other.location)) {
         return false;
      }
      if (this.msrp == null) {
         if (other.msrp != null) {
            return false;
         }
      } else if (!this.msrp.equals(other.msrp)) {
         return false;
      }
      if (this.pic175Url == null) {
         if (other.pic175Url != null) {
            return false;
         }
      } else if (!this.pic175Url.equals(other.pic175Url)) {
         return false;
      }
      if (this.picUrl == null) {
         if (other.picUrl != null) {
            return false;
         }
      } else if (!this.picUrl.equals(other.picUrl)) {
         return false;
      }
      if (this.primaryCategoryName == null) {
         if (other.primaryCategoryName != null) {
            return false;
         }
      } else if (!this.primaryCategoryName.equals(other.primaryCategoryName)) {
         return false;
      }
      // quanity sold and quanity not part of equals
      if (this.savingsRate == null) {
         if (other.savingsRate != null) {
            return false;
         }
      } else if (!this.savingsRate.equals(other.savingsRate)) {
         return false;
      }
      if (this.smallPicUrl == null) {
         if (other.smallPicUrl != null) {
            return false;
         }
      } else if (!this.smallPicUrl.equals(other.smallPicUrl)) {
         return false;
      }
      if (this.title == null) {
         if (other.title != null) {
            return false;
         }
      } else if (!this.title.equals(other.title)) {
         return false;
      }
      return true;
   }
}

[Winston Gutkowski]

zoheb hassan wrote:I am aware that a copy constructor is meant for creating a copy of an object but when and why do we have to use it, why not rather just create a new object, Basically i am trying to understand why we make that choice of using a copy constructor??

I think that Junilu and Campbell have covered most of the 'what' and 'when', but more formally:

A copy constructor normally creates a new (ie, different) object that is "equal" to its source according to equals().

And it's often used as an alternative to implementing Cloneable, which has some issues of its own that you'll probably find out about later.

What necessitates overriding of hashCode(), toString() & equals() methods of Object class??

Simply put: the need to make an object behave properly. Simplest is probably to create a class of your own that doesn't override them and then create a couple of objects from it and print out the results of calling those methods for each one.

Strictly speaking, hashCode() is only required if you're likely to need to put objects in a "hashed" collection; but, since that's pretty well impossible to know when you're creating a class, the general rule of thumb is:
"override equals(), override hashCode(); don't override equals(), leave hashCode() out as well."

HIH

Winston

[Campbell Ritchie]

I hope those methods were written by a machine
If I had to write that hash method I would try this
return Objects.hash(itemId, endTime, picUrl, smallPicUrl,
pic175Url, title, desc, dealUrl,
convertedCurrentPrice, primaryCategoryName,
location, quantity, quantitySold,
msrp, savingsRate, hot);
There is some poor design in that class, using Strings for a savings rate for example.

[zoheb hassan]

Thanks Campbell Ritchie for pointing out the flaws in the code posted,

I am aware of basics of core Java but the higher specifics about design Thread management, Cache Management e.t.c., are all still a mystery to me, What can i do to get a good hold on these topics, I mean the static getInstance() method was not clear to me took some googling to find that it was a copy constructor.

I Mean i wish these topics were clear to me so it would help me in designing decent Android Applications

Thanks
Zoheb

[Campbell Ritchie]

zoheb hassan wrote:Thanks Campbell Ritchie . . .

You're welcome

. . . the static getInstance() method was not clear to me took some googling to find that it was a copy constructor.
. . .

Please tell us where you read that.

A static getInstance method is not a copy constructor. It is usually a factory method.

[zoheb hassan]

My bad Sheriff, misread it actually anyways apparently all these design patterns, factory methods e.t.c., are alien to me still have to learn a good deal about them,

Question is are there any good light resources for getting hold of these concepts

[Winston Gutkowski]

zoheb hassan wrote:Question is are there any good light resources for getting hold of these concepts

Not all of them, but this book covers most of what you've asked about; and it's excellent (the best "why to" book I've ever read about any language).

As for design patterns, there are several books out there, of which this is the "original". However, if you're still relatively new to programming, it might be a bit soon to be tackling design patterns. For starters, they are not Java-specific, so you may have a bit of "translation" to do; for seconds, you'll have plenty of stuff to learn in the next few months without having to worry about patterns.

HIH

Winston

[Junilu Lacar]

Campbell Ritchie wrote:I always though a copy constructor is used to make an identical copy of an instance. And I thought it was necessary for a class with mutable attributes.

I suppose there may have been other ways but I've used copy constructors as a convenient way to partially initialize other objects from something that's already in memory. The case that comes to mind was a job configuration template. I had common job configuration parameters defined in the database, read those into memory, then just stamped out other job configurations from the "base" configuration using a copy constructor. The base configuration contained values for properties that were global and kept in the database and were immutable, while mutable properties were set on the fly and provided the means to customize each new job configuration. This allowed me to have code like so:

    Job customJob = new Job(baseConfig);  // sets all required properties
    customJob.setFoo(customFoo);          // sets optional properties

[Campbell Ritchie]

Yes, that seems a good alternative use for it. Not seen that before.