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object cloning in java
If you would like copy to be a new object that begins its life being identical to original but
whose state can diverge over time, use the clone method.
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Employee copy = original.clone();
copy.raiseSalary(10); // OK--original unchanged
But it isn’t quite so simple. The clone method is a protected method of Object, which means
that your code cannot simply call it. Only the Employee class can clone Employee objects. There is a
reason for this restriction. Think about the way in which the Object class can implement clone.
it knows nothing about the object at all, so it can make only a field-by-field copy. If all data fields in the
object are numbers or other basic types, copying the fields is just fine. But if the object contains
references to subobjects, then copying the field gives you another reference to the same subobject, so
the original and the cloned objects still share some information.
To visualize that phenomenon, let’s consider the Employee class that was introduced in Chapter 4.
Figure 6.2 shows what happens when you use the clone method of the Object class to clone such an
Employee object. As you can see, the default cloning operation is “shallow”—it doesn’t clone
objects that are referenced inside other objects.
Does it matter if the copy is shallow? It depends. If the subobject shared between the original and
the shallow clone is immutable, then the sharing is safe. This certainly happens if the subobject
belongs to an immutable class, such as String. Alternatively, the subobject may simply remain
constant throughout the lifetime of the object, with no mutators touching it and no methods yielding a
reference to it.
Quite frequently, however, subobjects are mutable, and you must redefine the clone method to
make a deep copy that clones the subobjects as well. In our example, the hireDay field is a Date,
which is mutable.
For every class, you need to decide whether
1. The default clone method is good enough;
2. The default clone method can be patched up by calling clone on the mutable subobjects