Setter ignored on collection

[LT]

I remember reading somthing about the behaviour exhibetet by the
program below (where a private collection is altered without going
through its accessor). But cannot seam to find the info anymore - can
someone enlightenme as to why this works:

Class for test
#################
public class TestMe {
private Set<String> values = new HashSet<String>();
private int called = 0;
public Set<String> getValues() {
return values;
}
public void setValues(Set<String> values) {
for (String v : values) {
if (v.equals("invalid"))
throw new IllegalArgumentException("invalid value");
}
this.values = values;
called++;
}
public int getCalled() {
return this.called;
}
}
#################

Test code
#################
TestMe test = new TestMe();
String valid = "valid";
String anotherValid = "anotherValid";
String invalid = "invalid";

// sets values without calling setter
test.getValues().add(valid);
if (test.getValues().contains(valid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());
}

// sets values by calling setter
Set<String> values = new HashSet<String>();
values.add(anotherValid);
test.setValues(values);
if (test.getValues().contains(anotherValid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());
}

// does not work as expected
test.getValues().add(invalid);
if (test.getValues().contains(invalid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());
}

// works as expected
test.getValues().remove(invalid);
values.add(invalid);
boolean caughtException = false;
try {
test.setValues(values);
} catch (IllegalArgumentException e) {
caughtException = true;
}
System.out.println("caught exception: " + caughtException);
#################


The output becomes
#################
setter is called: 0 times, values are: [valid]
setter is called: 1 times, values are: [anotherValid]
setter is called: 1 times, values are: [invalid, anotherValid]
caught exception: true
#################

I can see why this might work given the reference to the collection
returned from the getter, but is there a way to stop this from
happening (besides declaring the collection as unmodifiable ?)

 

[LT]

I remember reading somthing about the behaviour exhibetet by the
program below (where a private collection is altered without going
through its accessor). But cannot seam to find the info anymore - can
someone enlightenme as to why this works:

Class for test
#################
public class TestMe {
private Set<String> values = new HashSet<String>();
private int called = 0;
public Set<String> getValues() {
return values;
}
public void setValues(Set<String> values) {
for (String v : values) {
if (v.equals("invalid"))
throw new IllegalArgumentException("invalid value");
}
this.values = values;
called++;
}
public int getCalled() {
return this.called;
}}

#################

Test code
#################
TestMe test = new TestMe();
String valid = "valid";
String anotherValid = "anotherValid";
String invalid = "invalid";

// sets values without calling setter
test.getValues().add(valid);
if (test.getValues().contains(valid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());

}

// sets values by calling setter
Set<String> values = new HashSet<String>();
values.add(anotherValid);
test.setValues(values);
if (test.getValues().contains(anotherValid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());

}

// does not work as expected
test.getValues().add(invalid);
if (test.getValues().contains(invalid)) {
System.out.println("setter is called: " + test.getCalled()
+ " times, values are: " + test.getValues().toString());

}

// works as expected
test.getValues().remove(invalid);
values.add(invalid);
boolean caughtException = false;
try {
test.setValues(values);} catch (IllegalArgumentException e) {

caughtException = true;}

System.out.println("caught exception: " + caughtException);
#################

The output becomes
#################
setter is called: 0 times, values are: [valid]
setter is called: 1 times, values are: [anotherValid]
setter is called: 1 times, values are: [invalid, anotherValid]
caught exception: true
#################

I can see why this might work given the reference to the collection
returned from the getter, but is there a way to stop this from
happening (besides declaring the collection as unmodifiable ?)

This
http://groups.google.com/group/comp...q=exposing+collection&rnum=1#b454ab63085a4fa4
discussion was enlightening, so I went with the

#################
public Set<String> getValues() {
return Collections.unmodifiableSet(values);
}
#################

which seamed like the better solution.

 

[Patricia Shanahan]

I remember reading somthing about the behaviour exhibetet by the
program below (where a private collection is altered without going
through its accessor). But cannot seam to find the info anymore - can
someone enlightenme as to why this works:

Class for test
#################
public class TestMe {
private Set<String> values = new HashSet<String>();
private int called = 0;
public Set<String> getValues() {
return values;
}

....

Making values private but providing this implementation of getValues
means the caller cannot change which Set values references, but can
change the membership.

If you want the caller to be able to view, but not change, the set:

public Set<String> getValues() {
return Collections.unmodifiableSet(values);
}

This gives the caller a view of the set that will reflect changes you
make to it, but does not give the caller any power to add or remove
elements.

Patricia

 

[Daniel Pitts]

...

Making values private but providing this implementation of getValues
means the caller cannot change which Set values references, but can
change the membership.

If you want the caller to be able to view, but not change, the set:

public Set<String> getValues() {
return Collections.unmodifiableSet(values);

}

This gives the caller a view of the set that will reflect changes you
make to it, but does not give the caller any power to add or remove
elements.

Patricia

Alternatively, if you want the caller to have their own version of the
set to play with

public Set<String> getValues() {
return new HashSet<String>(values);
}