Chapter 50: Methods Overview for Collections

In the previous few chapters, we have covered details about the different types of collections in great detail. The purpose of this article is to highlight the other methods that these collections have which we havent covered in our previous chapters.

So, lets get started!!!

Important Methods for List, Set and Map

Method Name	Exists in			Description/Purpose
	List	Set	Map
boolean add(element)	Y	Y	-	Add an element.
boolean add(index, element)	Y	-	-	Add the element at an index point.
boolean contains(object)	Y	Y	Y	Search a collection for an object
boolean containsKey(object key)	-	-	Y	Search a Collection for a Key
boolean containsValue(object value)	-	-	Y	Search a Collection for a Value
object get(index)	Y	-	-	Get an Object from a collection using the index position
object get(key)	-	-	Y	Get an object from a Map using the Key
int indexOf(object)	Y	-	-	Get the location of an object in a List.
Iterator iterator()	Y	Y	-	Get an Iterator for a List or a Set.
Set keySet()	-	-	Y	Return a Set containing a Map’s keys.
put(key, value)	-	-	Y	Add a key/value pair to a Map.
remove(index)	Y	-	-	Remove a value at the index
remove(object)	Y	Y	Y	Remove an object from the collection
remove(key)	-	-	Y	Remove an object corresponding to the Key
int size()	Y	Y	Y	Return the number of elements in a collection.
Object[] toArray()	Y	Y	-	Return an array containing the elements of the collection.
A[] toArray(A[])	Y	Y	-	Return an array containing the elements of the collection.

Important Methods for Arrays:

For the purpose of explanation in the table that follows, A[] refers to an Array that contains objects in it.

Method Name	Description/Purpose
static List asList(A[])	Convert an array to a List
static int binarySearch(ObjecA[], key) static int binarySearch(primitive[], key)	Search a sorted array for a given value, return an index or insertion point.
static int binarySearch(A[], key, Comparator)	Search a Comparator-sorted array for a value.
static boolean equals(ObjecA[], ObjecA[]) static boolean equals(primitive[], primitive[])	Compare two arrays to determine if their contents are equal.
public static void sort(Object[ ] ) public static void sort(primitive[ ] )	Sort the elements of an array by natural order.
public static void sort(A[], Comparator)	Sort the elements of an array using a Comparator
public static String toString(ObjecA[]) public static String toString(primitive[])	Create a String containing the contents of an array.

Important Methods for Collections:

Method Name	Description/Purpose
static int binarySearch(List, key) static int binarySearch(List, key, Comparator)	Search a “sorted” List for a given value, return an index or insertion point.
static int binarySearch(ObjecA[], key) static int binarySearch(primitive[], key)	Search a sorted array for a given value, return an index or insertion point.
static void reverse(List)	Reverse the order of elements in a List..
static Comparator reverseOrder() static Comparator reverseOrder(Comparator)	Return a Comparator that sorts the reverse of the collection’s current sort sequence.
static void sort(List) static void sort(List, Comparator)	Sort a List either by natural order or by a Comparator.

Previous Chapter: Chapter 49 - Priority Queue

Next Chapter: Chapter 31 - Generics

Quick Review – Chapters 15 to 21

We have had a few long or rather very long topics in the previous chapters. Below is a quick recap of what we learnt in the previous chapters:

Stack and Heap:

• Local variables live in the stack
• Objects and their instance variables live on the heap

Literals and Primitive Casting:

• Integer literals can be decimal, octal or hexadecimal
• Literals for Longs end with either l or L
• Float literals end with F or f
• Double literals end with D or d
• The Boolean literals are true and false
• Literals for chars are a single character inside single quotes ‘’

Scope of Variables:

• Scope refers to the lifetime of a variable
• There are four basic scopes:
     o Static variables live as long as the class lives
     o Instance variables live as long as the object lives
     o Local variables live as long as their method is on the stack
     o Block variables live until the code block completes

Variable Assignments:

• Literal integers are implicitly ints
• Integer expressions always result in an int-sized result
• Floating point numbers are implicitly doubles
• Narrowing a primitive truncates the higher order bits
• Compound assignments (Ex: +=) perform an automatic cast
• Reference variables can refer to the subclass of the declared type but never a super class
• When a new object is created three things happen:
     o A new reference variable gets created
     o A new object gets created
     o The new created object gets assigned to the reference variable

Using Variables – Initialization and Unassigned Variables:

• When an array of objects is instantiated, objects within the array are not instantiated automatically, but all the references get the default value of null.
• When an array of primitives is instantiated, elements get default values.
• Instance variables are always initialized with a default value.
• Local/automatic/method variables are never given a default value. If you attempt to use one before initializing it, you’ll get a compiler error.

Passing Variables to Methods:

• Methods can take primitives and/or object references as arguments.
• Method arguments are always copies.
• Method arguments are never actual objects (they can be references to objects).
• A primitive argument is an unattached copy of the original primitive.
• A reference argument is another copy of a reference to the original object.
• Shadowing occurs when two variables with different scopes share the same name. This leads to hard-to-find bugs, and hard-to-answer exam questions.

Array Declaration, Construction, and Initialization

• Arrays can hold primitives or objects, but the array itself is always an object.
• When you declare an array, the brackets can be left or right of the name.
• It is never legal to include the size of an array in the declaration.
• You must include the size of an array when you construct it (using new) unless you are creating an anonymous array.
• Elements in an array of objects are not automatically created, although primitive array elements are given default values.
• You’ll get a NullPointerException if you try to use an array element in an object array, if that element does not refer to a real object.
• Arrays are indexed beginning with zero. An ArrayIndexOutOfBoundsException occurs if you use a wrong index value.
• Arrays have a length variable whose value is the number of array elements.
• The last index you can access is always one less than the length of the array.
• Multidimensional arrays are just arrays of arrays.
• The dimensions in a multidimensional array can have different lengths.
• An array of primitives can accept any value that can be promoted implicitly to the array’s declared type;. e.g., a byte variable can go in an int array.
• An array of objects can hold any object that passes the IS-A (or instanceof) test for the declared type of the array. For example, if Horse extends Animal, then a Horse object can go into an Animal array.
• If you assign an array to a previously declared array reference, the array you’re assigning must be the same dimension as the reference you’re assigning it to.
• You can assign an array of one type to a previously declared array reference of one of its supertypes. For example, a Honda array can be assigned to an array declared as type Car (assuming Honda extends Car).

Initialization Blocks

• Static initialization blocks run once, when the class is first loaded.
• Instance initialization blocks run every time a new instance is created. They run after all super-constructors and before the constructor’s code has run.
• If multiple init blocks exist in a class, they follow the rules stated above, AND they run in the order in which they appear in the source file.
Using Wrappers
• The wrapper classes correlate to the primitive types.
• Wrappers have two main functions:
     o To wrap primitives so that they can be handled like objects
     o To provide utility methods for primitives (usually conversions)
• The three most important method families are
     o xxxValue() Takes no arguments, returns a primitive
     o parseXxx() Takes a String, returns a primitive, throws Number Format Exception
     o valueOf() Takes a String, returns a wrapped object, throws NFE
• Wrapper constructors can take a String or a primitive, except for Character, which can only take a char.
• Radix refers to bases (typically) other than 10; octal is radix = 8, hex = 16.

Boxing

• As of Java 5, boxing allows you to convert primitives to wrappers or to convert wrappers to primitives automatically.
• Using == with wrappers created through boxing is tricky; those with the same small values (typically lower than 127), will be ==, larger values will not be ==.
Advanced Overloading
• Primitive widening uses the “smallest” method argument possible.
• Used individually, boxing and var-args are compatible with overloading.
• You CANNOT widen from one wrapper type to another. (IS-A fails.)
• You CANNOT widen and then box. (An int can’t become a Long.)
• You can box and then widen. (An int can become an Object, via an Integer.)
• You can combine var-args with either widening or boxing.

Garbage Collection

• In Java, garbage collection (GC) provides automated memory management.
• The purpose of GC is to delete objects that can’t be reached.
• Only the JVM decides when to run the GC, you can only suggest it.
• You can’t know the GC algorithm for sure.
• Objects must be considered eligible before they can be garbage collected.
• An object is eligible when no live thread can reach it.
• To reach an object, you must have a live, reachable reference to that object.
• Java applications can run out of memory.
• Islands of objects can be GCed, even though they refer to each other.
• Request garbage collection with System.gc();
• Class Object has a finalize() method.
• The finalize() method is guaranteed to run once and only once before the garbage collector deletes an object.
• The garbage collector makes no guarantees, finalize() may never run.
• You can uneligibilize an object for GC from within finalize().

Previous Chapter: Chapter 21 - Garbage Collection

Next Chapter: Self Test - Chapters 15 to 21