The Object and Objects Classes - Beginning Java 8 Fundamentals - page 306

#1 value = 101
#2 value = 207
The output shows that the IntHolderWrapper class is mutable. Two calls to its getValue() method return
different values. The culprit is its getIntHolder() method. It returns the instance variable valueHolder , which is a
reference variable. Note that the valueHolder instance variable represents an object of the IntHolder class, which
makes up the state of an IntHolderWrapper object. If the object that valueHolder reference variable references is
changed, the state of IntHolderWrapper is changed, too. Since the IntHolder object is mutable, you should not return
its reference to the client from the getIntHolder() method. The following two statements change the state of the
object from the client code:
IntHolder holder = ihw.getIntHolder(); /* Got hold of instance variable */
holder.setValue(207); /* Change the state by changing the instance variable's state */
Note that the designer of the IntHolderWrapper class missed the point when he returned the valueHolder
reference, that even though there is no direct way to change the state of the IntHolderWrapper class, it can be
changed indirectly.
How do you correct the problem? The solution is easy. In the getIntHolder() method, make a copy of the
valueHolder object and return the reference of the copy instead of the instance variable itself. This way, if the client
changes the value, it will be changed only in client's copy, not in the copy held by IntHolderWrapper object.
Listing 7-20 has the correct immutable version of the IntHolderWrapper class, which you call IntHolderWrapper2 .
Listing 7-20. A Modified, Immutable Version of the IntHolderWrapper Class
// IntHolderWrapper2.java
package com.jdojo.object;
public class IntHolderWrapper2 {
private final IntHolder valueHolder;
public IntHolderWrapper2(int value) {
this.valueHolder = new IntHolder(value);
}
public IntHolder getIntHolder() {
// Make a copy of valueHolder
int v = this.valueHolder.getValue();
IntHolder copy = new IntHolder(v);
// Return the copy instead of the original
return copy;
}
public int getValue() {
return this.valueHolder.getValue();
}
}