What is the difference between a deep copy and a shallow copy?

Answer 1

Shallow copy: a bitwise copy of the object...

If X = Y then they both point to the same object.

If x changes, so does Y. If Y changes, so does X.

Deep copy: The entire object INCLUDING the fields, dynamically allocated

data members, and anything else referenced by the object

are copied into a separate memory location

In this case if X is deep copied into Y, then if X changes Y is NOT changed.

If Y changes, X stays the same. If one disappears, the other is still valid in memory.

The proper term is CLONING. This is typically done with a COPY constructor.

Here is some sample code:

class DataRec

{

private String strCode;

private long longIntNum;

private double flAmt;

//constructor

DataRec( String str, long L, double flX)

{

if (str != null)

{

strCode = new String(str);

}

else

{

strCode = null;

}

longIntNum = L;

flAmt = flX;

}

//public getters and setters

public String getStr() { return(strCode); }

public long getLong() { return(longIntNum); }

public double getAmt() { return(flAmt); }

public void setStr( String newStr) { strCode = new String(newStr); }

public void setLong( long L) { longIntNum = L; }

public void setAmt( double flX) { flAmt = flX; }

public DataRec ShallowCopy( DataRec X)

{

DataRec Y = X; //just copies the object reference

return(Y);

} //shallowCOpy

//copy constructor

DataRec(DataRec X)

{

//copies ALL of the data into this instance

String str = X.getStr();

if (str!=null)

{

this.strCode = new String(str);

}

else

{

this.strCode = null;

}

this.longIntNum = X.getLong();

this.flAmt = X.getAmt();

} //copy constructor

public DataRec DeepCopy( DataRec X)

{

DataRec Y = new DataRec(X);

return(Y);

} //deepCopy

public void Output(String debugMsg)

{

System.out.println("***********************************");

System.out.println(debugMsg);

System.out.println("***********************************");

System.out.println(" StrCode = >" + this.getStr() +"<");

System.out.println(" longIntNum = " + this.getLong() );

System.out.println(" Amt = " + this.getAmt() );

} //output

public static void main(String args[])

{

DataRec copyDummy = new DataRec(null,0,0);

DataRec dataRec1 = new DataRec("DataRec1",12345,3.45f);

DataRec dataRec2 = copyDummy.ShallowCopy(dataRec1);

dataRec1.Output( "DataRec1 before the change");

dataRec2.Output( "DataRec2 before the change");

dataRec2.setAmt(3.33f);

dataRec1.setLong(54321);

dataRec1.Output( "DataRec1 after the change");

dataRec2.Output( "DataRec2 after the change"); //they both shall be DataRec1, 54321, 3.33

DataRec dataRecA = new DataRec("data Rec A",98765,35.42f);

DataRec dataRecB = copyDummy.DeepCopy(dataRecA);

dataRecA.Output(" DataRec A before the change");

dataRecB.Output(" DataRec B before the change");

dataRecB.setAmt(42.35f);

dataRecA.setLong(711333);

dataRecA.Output(" DataRec A after the change"); //data Rec A, 711333. 35.42

dataRecB.Output(" DataRec B after the change"); //data Rec A, 98765, 42.35

} //main

} //class