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If a class does not specify its superclass, it implicitly inherits from java.lang.Object class. Thus all classes in Java are subclasses of Object class. If the superclass does not have a constructor without parameters the subclass must specify in its constructors what constructor of the superclass to use. For example:
public class Fruit {} // parent class public class Apple extends Fruit {} // child class public static void main (String [] args) {// The following is an implicit upcast: Fruit parent = new Apple (); // The following is a downcast. Here, it works since the variable `parent` is // holding an instance of Apple: Apple child = (Apple) parent;}
Collection implementations in pre-JDK 1.2 versions of the Java platform included few data structure classes, but did not contain a collections framework. [4] The standard methods for grouping Java objects were via the array, the Vector, and the Hashtable classes, which unfortunately were not easy to extend, and did not implement a standard member interface.
Integer are reference objects, on the surface no different from List, Object, and so forth. To convert from an int to an Integer , one had to "manually" instantiate the Integer object. As of J2SE 5.0, the compiler will accept the last line, and automatically transform it so that an Integer object is created to store the value 9 . [ 2 ]
Primitive wrapper classes are not the same thing as primitive types. Whereas variables, for example, can be declared in Java as data types double, short, int, etc., the primitive wrapper classes create instantiated objects and methods that inherit but hide the primitive data types, not like variables that are assigned the data type values.
In object-oriented programming, a class defines the shared aspects of objects created from the class. The capabilities of a class differ between programming languages , but generally the shared aspects consist of state ( variables ) and behavior ( methods ) that are each either associated with a particular object or with all objects of that class.
If the superclass is not explicitly specified, the superclass is implicitly Object. The class keyword can also be used in the form Class.class to get a Class object without needing an instance of that class. For example, String.class can be used instead of doing new String().getClass(). continue
In object-oriented languages, string functions are often implemented as properties and methods of string objects. In functional and list-based languages a string is represented as a list (of character codes), therefore all list-manipulation procedures could be considered string functions.