Shallow Copy and Deep Copy

There are two ways to make a copy of an object called shallow copy and deep copy.
Here is what they mean:

Shallow Copy
Shallow copy is a bit-wise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the references are copied. Thus, if the object you are copying contains references to yet other objects, a shallow copy refers to the same subobjects.

Deep Copy
Deep copy is a complete duplicate copy of an object. If an object has references to other objects, complete new copies of those objects are also made. A deep copy generates a copy not only of the primitive values of the original object, but copies of all subobjects as well, all the way to the bottom. If you need a true, complete copy of the original object, then you will need to implement a full deep copy for the object.

Shallow Copy

Deep Copy

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Shallow and deep copy using clone

Stringifying Java Arrays

J2SE 5 provided significant new language features that made Java significantly easier to use and more expressive than it had ever been. Although "big" new J2SE 5 features such as enums, generics, and annotations, there was a plethora of new APIs and methods that have made our lives easier ever since. In this post, I briefly look at two of my favorites when dealing with Java arrays: the Arrays.toString(Object[]) method and the Arrays.deepToString(Object[]) method.

The Arrays class has been around since JDK 1.2, but the methods for conveniently and simply converting arrays to a readable String including relevant array content without using Arrays.asList() were added with J2SE 5. Both Arrays.toString(Object[]) and Arrays.deepToString(Object[]) are static methods that act upon the arrays provided to them. The former, Arrays.toString(Object[]), is intended for single-dimension arrays while the latter, Arrays.deepToString(Object[]), is intended for multi-dimensional arrays. As my example later in this post will demonstrate, the multi-dimension deepToString will produce expected results even for single-dimension arrays. The two methods also provide easy and safe null handling, something that I appreciate.

The next code listing is for a simple demonstration class that demonstrates trying to put the contents of various types of Java arrays into a String format. The types of arrays demonstrated are a single dimension array, a double dimensional array representing multi-dimensional arrays of various sizes, and an array that is really just null. The three methods demonstrated for getting a String out of these three types of arrays are (1) simple Object.toString() on each array (implicitly in case of null array to avoid the dreaded NullPointerException), (2) Arrays.toString(Object[]), and (3) Arrays.deepToString(Object[]).

package com.vaani.arraytest;

import java.util.Arrays;
import static java.lang.System.out;

/**
 * Simple demonstration of Arrays.toString(Object[]) method and the
 * Arrays.deepToString(Object[]) method.
 */
public class Array2String
{
   /**
    * Demonstrate usage and behavior of Arrays.toString(Object[]).
    */
   private static void demonstrateArraysToString()
   {
      out.println(
           "Single Dimension Arrays.toString: "
         + Arrays.toString(prepareSingleDimensionArray()));
      out.println(
           "Double Dimension Arrays.toString: "
         + Arrays.toString(prepareDoubleDimensionArray()));
      out.println(
           "Null Array Arrays.toString: "
         + Arrays.toString(prepareNullArray()));
   }

   /**
    * Demonstrate usage and behavior of Arrays.deepToString(Object[]).
    */
   private static void demonstrateArraysDeepToString()
   {
       out.println(
           "Single Dimension Arrays.deepToString: "
         + Arrays.deepToString(prepareSingleDimensionArray()));
      out.println(
           "Double Dimension Arrays.deepToString: "
         + Arrays.deepToString(prepareDoubleDimensionArray()));
      out.println(
           "Null Array Arrays.deepToString: "
         + Arrays.deepToString(prepareNullArray()));
   }

   /**
    * Demonstrate attempting to get String version of array with simple toString()
    * call (not using Arrays class).
    */
   private static void demonstrateDirectArrayString()
   {
      out.println("Single Dimension toString(): " + prepareSingleDimensionArray().toString());
      out.println("Double Dimension toString(): " + prepareDoubleDimensionArray());
      out.println("Null Array toString(): " + prepareNullArray());
   }

   /**
    * Prepare a single-dimensional array to be used in demonstrations.
    *
    * @return Single-dimensional array.
    */
   private static Object[] prepareSingleDimensionArray()
   {
      final String[] names = {"Aaron", "Bianca", "Charles", "Denise", "Elmer"};
      return names;
   }

   /**
    * Prepare a double-dimension array to be used in demonstrations.
    *
    * @return Double-dimensional array.
    */
   private static Object[] prepareDoubleDimensionArray()
   {
      final Object[][] namesAndAges = {
         {"Aaron", 10}, {"Bianca", 25}, {"Charles", 32}, {"Denise", 29}, {"Elmer", 67}};
      return namesAndAges;
   }

   /**
    * Prepare a null array.
    *
    * @return Array that is really null.
    */
   private static Object[] prepareNullArray()
   {
      return null;
   }

   /**
    * Main executable function for demonstrating Arrays.toString(Object[]) and
    * Arrays.deepToString(Object[]) methods.
    */
   public static void main(final String[] arguments)
   {
       out.println("\n\n\nDemonstrating direct array to string using toString():");
      demonstrateDirectArrayString();
     
      out.println("\n\n\nDemonstrating Arrays.toString() to get arrays as String:");
      demonstrateArraysToString();
      
      out.println("\n\n\nDemonstrating Arrays.deepToString() to get the String:");
      demonstrateArraysDeepToString();
   }
}

The above code exercises the three mentioned approaches for getting a String out of an array on the three different types of arrays: single dimension, multi dimension, and null array. The output from running this code demonstrates the utility of the different approaches. That output is shown next.

Demonstrating direct array to string using toString():
Single Dimension toString(): [Ljava.lang.String;@3e25a5
Double Dimension toString(): [[Ljava.lang.Object;@19821f
Null Array toString(): null
///////////
Demonstrating Arrays.toString() to get arrays as String:
Single Dimension Arrays.toString: [Aaron, Bianca, Charles, Denise, Elmer]
Double Dimension Arrays.toString: [[Ljava.lang.Object;@addbf1, [Ljava.lang.Object;@42e816, [Ljava.lang.Object;@9304b1, [Ljava.lang.Object;@190d11, [Ljava.lang.Object;@a90653]
Null Array Arrays.toString: null


///////////
Demonstrating Arrays.deepToString() to get the String:
Single Dimension Arrays.deepToString: [Aaron, Bianca, Charles, Denise, Elmer]
Double Dimension Arrays.deepToString: [[Aaron, 10], [Bianca, 25], [Charles, 32], [Denise, 29], [Elmer, 67]]
Null Array Arrays.deepToString: null

The code above and its corresponding output lead to several observations:

1. Simple Object.toString() on arrays is seldom what we want as it only prints the String representation of the array itself and not of its contents.
   
2. Arrays.toString(Object[]) will print a String representation for multi-dimensional arrays, but this representation suffers the same drawbacks as Object.toString() after the first dimension. The first dimension (and only dimension for a single dimension array) gets put into an expected String, but deeper dimensions have the same Object.toString() treatment.
   
3. Arrays.deepToString(Object[]), while intended for multi-dimensional arrays, produces the expected results for both single and multi-dimensional arrays.
   
4. Both Arrays.toString(Object[]) and Arrays.deepToString(Object[]) handle null array gracefully, simply returning a String "null".

I tend to use Java Collections far more than I use Java arrays. However, when I do need to work with arrays, it is nice to have the many useful features of the java.util.Arrays class. As this post has demonstrated, Arrays.toString(Object[]) and Arrays.deepToString(Object[]) are particularly valuable in obtaining a useful String representation of an array's contents. The java.util.Arrays class provides similar "deep" methods for performing equals and hashCode functionality on multi-dimensional arrays: Arrays.deepEquals and Arrays.deepHashCode.

How substring() works in java?

substring() is a function to get substring from string. A String class is immutable in java.
A String can be implemented as an object with three fields ( normal string class have even more, but consider 3 for now) -- a character array, an offset into that array, and a length.
Consider the following code

String s1 = "Monday";
String s = s1.substring(0,3);
or
s1.substring(0,3).equals("Mon");

substring() creates a new String and returns it back. But substring is clever. It does not make a deep copy of the substring the way most languages do. It just creates a pointer into the original immutable String, i.e. points to the value char[] of the base string, and tracks the starting offset where the substring starts and count of how long the substring is. So only length and offset are different per string but the character array is shared with the original string class. This can be shown in figure:

The downside of this cleverness is a tiny substring of a giant base String could suppress garbage collection of that big String in memory even if the whole String were no longer needed. (actually its value char[] array is held in RAM; the String object itself could be collected.)
If you know a tiny substring is holding a giant string in RAM, that would otherwise be garbage collected, you can break the bond by using

String s = new String(s1.substring(0,3));

Cloning Interview questions

Q1) What are different types of cloning in Java?

Ans) Java supports two type of cloning: - Deep and shallow cloning. By default shallow copy is used in Java. Object class has a method clone() which does shallow cloning.

Q2) What is Shallow copy?

Ans) In shallow copy the object is copied without its contained objects.
Shallow clone only copies the top level structure of the object not the lower levels.
It is an exact bit copy of all the attributes.

      

Figure 1: Original java object obj

The shallow copy is done for obj and new object obj1 is created but contained objects of obj are not copied.

Figure 2: Shallow copy object obj1

It can be seen that no new objects are created for obj1 and it is referring to the same old contained objects. If either of the containedObj contain any other object no new reference is created

Q3) What is deep copy and how it can be acheived?

Ans) In deep copy the object is copied along with the objects it refers to. Deep clone copies all the levels of the object from top to the bottom recursively.

Figure 3 : Original Object obj

When a deep copy of the object is done new references are created.

Figure 4: obj2 is deep copy of obj1

One solution is to simply implement your own custom method (e.g., deepCopy()) that returns a deep copy of an instance of one of your classes. This may be the best solution if you need a complex mixture of deep and shallow copies for different fields, but has a few significant drawbacks:

You must be able to modify the class (i.e., have the source code) or implement a subclass. If you have a third-party class for which you do not have the source and which is marked final, you are out of luck.
You must be able to access all of the fields of the class’s superclasses. If significant parts of the object’s state are contained in private fields of a superclass, you will not be able to access them.
You must have a way to make copies of instances of all of the other kinds of objects that the object references. This is particularly problematic if the exact classes of referenced objects cannot be known until runtime.
Custom deep copy methods are tedious to implement, easy to get wrong, and difficult to maintain. The method must be revisited any time a change is made to the class or to any of its superclasses.
Other common solution to the deep copy problem is to use Java Object Serialization (JOS). The idea is simple: Write the object to an array using JOS’s ObjectOutputStream and then use ObjectInputStream to reconsistute a copy of the object. The result will be a completely distinct object, with completely distinct referenced objects. JOS takes care of all of the details: superclass fields, following object graphs, and handling repeated references to the same object within the graph.

It will only work when the object being copied, as well as all of the other objects references directly or indirectly by the object, are serializable. (In other words, they must implement java.io.Serializable.) Fortunately it is often sufficient to simply declare that a given class implements java.io.Serializable and let Java’s default serialization mechanisms do their thing. Java Object Serialization is slow, and using it to make a deep copy requires both serializing and deserializing.
There are ways to speed it up (e.g., by pre-computing serial version ids and defining custom readObject() and writeObject() methods), but this will usually be the primary bottleneck. The byte array stream implementations included in the java.io package are designed to be general enough to perform reasonable well for data of different sizes and to be safe to use in a multi-threaded environment. These characteristics, however, slow down ByteArrayOutputStream and (to a lesser extent) ByteArrayInputStream .

Q4) What is difference between deep and shallow cloning?

Ans) The differences are as follows:

Consider the class:

public class MyData{
String id;
Map myData;
}

The shallow copying of this object will have new id object and values as “” but will point to the myData of the original object. So a change in myData by either original or cloned object will be reflected in other also. But in deep copying there will be new id object and also new myData object and independent of original object but with same values.

Shallow copying is default cloning in Java which can be achieved using clone() method of Object class. For deep copying some extra logic need to be provided.

Q5) What are the characteristics of a shallow clone?

Ans) If we do a = clone(b)
1) Then b.equals(a)
2) No method of a can modify the value of b.

Q6) What are the disadvantages of deep cloning?

Ans) Disadvantages of using Serialization to achieve deep cloning –

Serialization is more expensive than using object.clone().
Not all objects are serializable.
Serialization is not simple to implement for deep cloned object..

Java Clone, Shallow Copy and Deep Copy

Java provides a mechanism for creating copies of objects called cloning. There are two ways to make a copy of an object called shallow copy and deep copy. See here to understand that.

Java Clone
Java supports shallow and deep copy with the Cloneable interface to create copies of objects. To make a clone of a Java object, you declare that an object implements Cloneable, and then provide an override of the clone method of the standard Java Object base class. Implementing Cloneable tells the java compiler that your object is Cloneable. The cloning is actually done by the clone method.

In java, though clone is ‘intended’ to produce a copy of the same object, though it is not guaranteed. Clone comes with lots of ifs and buts. So my first advice is to not depend on clones. If you want to provide a handle / method to deliver a copy of the current instance write a kind of factory method and provide it with a good documentation. When you are in a situation to use a third party component and produce copies of it using the clone method, then investigate that implementation carefully and get to know what is underlying.

Supporting Shallow Copy by clone

Generally clone method of an object, creates a new instance of the same class and copies all the fields to the new instance and returns it. This is nothing but shallow copy. Object class provides a clone method and provides support for the shallow copy. It returns ‘Object’ as type and you need to explicitly cast back to your original object.

Since the Object class has the clone method (protected) you cannot use it in all your classes. The class which you want to be cloned should implement clone method and overwrite it. It should provide its own meaning for copy or to the least it should invoke the super.clone(). Also you have to implement Cloneable marker interface or else you will get CloneNotSupportedException. When you invoke the super.clone() then you are dependent on the Object class’s implementation and what you get is a shallow copy.

Note that for primitive types, shallow copy works fine, but for classes having other class or user defined types, it may be a problem. So if there is no problem with shallow copies, its fine, otherwise we need deep copies.

Deep Copy

When you need a deep copy then you need to implement it yourself. When the copied object contains some other object its references are copied recursively in deep copy. When you implement deep copy be careful as you might fall for cyclic dependencies. If you don’t want to implement deep copy yourselves then you can go for serialization. It does implements deep copy implicitly and gracefully handling cyclic dependencies.

One more disadvantage with this clone system is that, most of the interface / abstract class writers in java forget to put a public clone method. For example you can take List. So when you want to clone their implementations you have to ignore the abstract type and use actual implementations like ArrayList by name. This completely removes the advantage and goodness of abstractness.

When implementing a singleton pattern, if its superclass implements a public clone() method, to prevent your subclass from using this class’s clone() method to obtain a copy overwrite it and throw a CloneNotSupportedException.

Note that clone is not for instantiation and initialization. It should not be synonymously used as creating a new object. Because the constructor of the cloned objects may never get invoked in the process. It is about copying the object in discussion and not creating new. It completely depends on the clone implementation. One more disadvantage (what to do there are so many), clone prevents the use of final fields. We have to find roundabout ways to copy the final fields into the copied object.

Clone is an agreement between you, compiler and implementer. If you are confident that you all three have good knowledge of java, then go ahead and use clone. If you have a slightest of doubt better copy the object manually.

Example - Shallow and Deep Copy :

import java.util.List;
import java.util.LinkedList;
import java.util.Iterator;

public class CloningExample implements Cloneable {

    private LinkedList names = new LinkedList();
    public CloningExample() {
        names.add(“Alex”);
        names.add(“Melody”);
        names.add(“Jeff”);
    }
    public String toString() {
        StringBuffer sb = new StringBuffer();
        Iterator i = names.iterator();
        while (i.hasNext()) {
            sb.append(“\n\t” + i.next());
        }
        return sb.toString();
    }
    public Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            throw new Error(“This should not occur since we implement Cloneable”);
        }
    }
    public Object deepClone() {
        try {
            CloningExample copy = (CloningExample)super.clone();
            copy.names = (LinkedList)names.clone();
            return copy;
        } catch (CloneNotSupportedException e) {
            throw new Error(“This should not occur since we implement Cloneable”);
        }
    }

    public boolean equals(Object obj) {

        /* is obj reference this object being compared */
        if (obj == this) {
            return true;
        }

        /* is obj reference null */
        if (obj == null) {
            return false;
        }

        /* Make sure references are of same type */
        if (!(this.getClass() == obj.getClass())) {
            return false;
        } else {
            CloningExample tmp = (CloningExample)obj;
            if (this.names == tmp.names) {
                return true;
            } else {
                return false;
            }
        }
       
    }
   

    public static void main(String[] args) {

        CloningExample ce1 = new CloningExample();
        System.out.println(“\nCloningExample[1]\n” +
                           “—————–” + ce1);

        CloningExample ce2 = (CloningExample)ce1.clone();
        System.out.println(“\nCloningExample[2]\n” +
                           “—————–” + ce2);

        System.out.println(“\nCompare Shallow Copy\n” +
                           “——————–\n” +
                           “    ce1 == ce2      : ” + (ce1 == ce2) + “\n” +
                           “    ce1.equals(ce2) : ” + ce1.equals(ce2));

        CloningExample ce3 = (CloningExample)ce1.deepClone();
        System.out.println(“\nCompare Deep Copy\n” +
                           “——————–\n” +
                           “    ce1 == ce3      : ” + (ce1 == ce3) + “\n” +
                           “    ce1.equals(ce3) : ” + ce1.equals(ce3));

        System.out.println();

    }

}

So as we see in deepClone, lists are explicitly copied.
In this case the shallow copy has the same reference ( == ) and the deep copy only has an equivalent reference ( .equals() ).