Collections Methods

Question 1

coll.size()

Answer 1

returns an int that gives the number of objects in the collection.

Question 2

coll.isEmpty()

Answer 2

returns a boolean value which is true if the size of the collection is 0.

Question 3

coll.clear()

Answer 3

removes all objects from the collection.

Question 4

coll.add(tobject)

Answer 4

adds tobject to the collection. The parameter must be of type T; if not, a syntax error occurs at compile time.
The add() method returns a boolean value which tells you whether the operation actually modified the collection. For example, adding an object to a Set has no effect if that object was already in the set.

Question 5

coll.contains(object)

Answer 5

returns a boolean value that is true if object is in the collection. Note that object is not required to be of type T, since it makes sense to check whether object is in the collection, no matter what type object has.

Question 6

coll.remove(object)

Answer 6

removes object from the collection, if it occurs in the collection, and returns a boolean value that tells you whether the object was found. Again, object is not required to be of type T.

Question 7

coll.containsAll(coll2)

Answer 7

returns a boolean value that is true if every object in coll2 is also in coll.

Question 8

coll.addAll(coll2)

Answer 8

adds all the objects in coll2 to coll. The parameter, coll2, can be any collection of type Collection. However, it can also be more general. For example, if T is a class and S is a sub-class of T, then coll2 can be of type Collection.

Question 9

coll.removeAll(coll2)

Answer 9

removes every object from coll that also occurs in the collection coll2. coll2 can be any collection.

Question 10

coll.retainAll(coll2)

Answer 10

removes every object from coll that does not occur in the collection coll2. It “retains” only the objects that do occur in coll2. coll2 can be any collection.

Question 11

coll.toArray()

Answer 11

returns an array of type Object[] that contains all the items in the collection. Note that the return type is Object[], not T[]! However, there is another version of this method that takes an array of type T[] as a parameter: the method coll.toArray(tarray) returns an array of type T[] containing all the items in the collection. If the array parameter tarray is large enough to hold the entire collection, then the items are stored in tarray and tarray is also the return value of the collection. If tarray is not large enough, then a new array is created to hold the items; in that case tarray serves only to specify the type of the array. For example, coll.toArray(new String[0]) can be used if coll is a collection of Strings and will return a new array of type String[].

Question 12

coll.iterator()

Answer 12

f coll is a collection, then coll.iterator() returns an iterator that can be used to traverse the collection. Iterators are defined by a parameterized interface named Iterator. If coll implements the interface Collection for some specific type T, then coll.iterator() returns an iterator of type Iterator, with the same type T as its type parameter.

Question 13

What are the three methods that are defined by the interface Iterator?

Answer 13

iter. next()
iter. hasNext()
iter. remove()

Question 14

iter.next()

Answer 14

returns the next item, and advances the iterator. The return value is of type T.

Question 15

iter.hasNext()

Answer 15

returns a boolean value telling you whether there are more items to be processed. In general, you should test this before calling iter.next().

Question 16

iter.remove()

Answer 16

if you call this after calling iter.next(), it will remove the item that you just saw from the collection. Note that this method has no parameter. It removes the item that was most recently returned by iter.next(). This might produce an UnsupportedOperationException, if the collection does not support removal of items.

Question 17

Using iterators, we can write code for printing all the items in any collection.

Answer 17

Iterator iter; // Declare the iterator variable.
iter = coll.iterator(); // Get an iterator for the collection.
while ( iter.hasNext() ) {
String item = iter.next(); // Get the next item.
System.out.println(item);
}

Question 18

What is the syntax of the for-each loop in Java?

Answer 18

for (T x : coll) { // "for each object x, of type T, in coll"
     // process x 
}
x is the loop control variable. Each object in coll will be assigned to x in turn, and the body of the loop will be executed for each object. Since objects in coll are of type T, x is declared to be of type T.

Question 19

The == operator tests whether two objects are identical in the sense that they share the __ __ __ __ .

Answer 19

same location in memory.