Last Updated on by Sarina Sindurakar
Arrays
An array is a collection of similar elements with the same data type. It’s used to keep multiple values in a single variable.
Java array is an object that contains the elements of similar data types. In Java, arrays are indexed. In the array, the index starts at 0. The first element is indexed as 0, the last element as n-1, and so on.
Arrays are classified into two types:
- Single dimensional Array
- Multidimensional Array
Single Dimensional Array
Syntax to declare an array/Single dimensional array:
Datatype []array_name;
Datatype array_name [];
Instantiation of an Array in Java
variable=new datatype[size];
We can also declare the, instantiate and initialize the java array together by:
Int a[]={1,2,3,4,5};
Example of single dimensional array
Output:
Multidimensional array
In multidimensional array,data is stored in matrix form i.e. row and column form
Syntax to declare multidimensional array
datatype [] [] array_name;
datatype array_name [][];
datatype [] array_name[];
Example:
Output:
For each loop
- For each loop traverse the array in Java.
- It traverse each element one by one.
- It makes the code readable and eliminates the bugs.
- For each loop is declare with for keyword.
- It does not work in index basis and cannot traverse the element in reverse order.
- Instead of declaring and initializing a loop counter variable, you declare a variable of the same type as the array’s base type, followed by a colon, and then the array name.
- Instead of using an indexed array element in the loop body, you can use the loop variable you created.
Syntax:
For(data-type variable:array)
{
//body of for-each loop
}
Example:
Output:
Classes and objects
Class is a blueprint of real world objects that specifies what data and what methods will be included in objects of the class. The class is a description group of objects having similar properties.A class is also called user defined data type or programmers defines data type because we can define new data types according to our needs by using classes.
Syntax:
class <class_name>
{
field;
method;
}
Objects are instances of classes. We can say that objects are variables of class type.Memory for instance variables are not allocated at the time of class declaration rather at the time of object creation.Thus we can say that objects have physical existence and classes are only concepts.
Creating objects
It can be done using the new operator. The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it. This reference is, more or less, the address in memory of the object allocated by new. This reference is then stored in the variable. Thus, in Java, all class objects must be dynamically allocated.
Box mybox = new Box();
Example:
Output:
Access Modifiers
Access Modifiers in java specifies the visibility and accessibility of class,methods and member variables. It also set access levels for classes,variables, methods and constructors. There are two type of access modifiers .They are:
- Access Modifiers :It controls the access levels.
- Non Access Modifiers:It does not control the access level but provide other functionality.
There are four access levels .They are:
Default Access Modifier:
In default access modifier ,no keyword is used to define an access modifier for a class,field,method etc.The access level of default access modifier is in all the class of same package i.e only within the package but not outside of the package.If no access level is specified then,it will be the default.
Private Access Modifier:
The declared private methods, classes, constructors, and variables are only accessible within the declared class. It is not available outside of class. Variables declared private can be accessed outside the class if the class contains public methods. Using private access modifier is the main way that an object encapsulates itself and hides data from the outside world.
Protected Access Modifier:
Variables,methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected class members class It cannot be accessed from outside the package if a child class is not created. The protected access modifier is inapplicable to classes and interfaces.
Public Access Modifier:
A class,variable,methods ,constructor ,interface which are declared public can be accessed from everywhere i.e any other class.It can be accessed within the class and package and ,outside the class and package.
Example:
ACCESS..java file:
public class ACCESS {
private int a;//private access modifiers
int b;//default access modifiers
protected int c;//protected access modifiers
public int d;//public access modifiers
}
Access-modifier.java file:
public class Access_modifier {
public static void main(String args[]) {
ACCESS obj=new ACCESS();
obj.a=5;//generates compiler error as a is private .
obj.b=10;
obj.c=12;
obj.d=15;
}
}
Final Modifier
The final modifier in java can be used with classes,method and member variables. It is used to restrict the user .The final keyword can be used with variables; a final variable with no value is referred to as a blank final variable or an uninitialized final variable. It can only be initialized in the constructor. The final modifier is used for following purposes:
To define constants
It is used with variable to declare them as a constant.once the value is assigned to final variable,it cannot be changed during the runtime.
Example:
public class Circle{
private final float pi=3.14;
private int r;
Circle(int x)
{
Pi=3.1;//cannot be changed cause Pi is defined as final.
r=x;
}
Void area(){
float a=pi*r*r;
System.out.println(“area”,a);
}
}
To prevent Overridiing:
Final access modifier can prevent a method being overridden from subclass by declaring them as a final.
Example:
class Dog{
final void eat(){
System.out.println(“Eating”);}
}
class Animal extends Dog{
void eat()
{
System.out.println(“eating meat”);
}
public static void main(String args[]){
Animal obj= new Animal();
obj.eat();
}
To prevent Inheritance
Final access modifier can be use with class declaration to prevent it from further inheritance since final class cannot be further inherited.
Example
Final class A
{
//code;
}
Class B extends A // throws error
{
//code ;
Method Overloading
Method overloading is a Java feature that allows you to define different methods with the same name either with different number of parameters or a different type of parameter. Polymorphism is supported by Java through method overloading. Method overloading helps to increase the readability of the program. In method overloading the return type of the method does not matter.
The two way to overload the method in java are:
- By changing the number of parameters.
- By Changing the type of parameter.
Example:
public class method_overloading
{
static int sum(int a, int b)
{
return a+b;
}
static int sum(int a, int b,int c)//changing the number of parameter
{
return a+b+c;
}
static float sub(float x, float y)
{
return x-y;
}
static double sub(double x,double y)//changing the type of parameter
{
return x-y;
}
public static void main(String args[]) {
System.out.println(sum(4,5));
System.out.println(sum(7,10,11));
System.out.println(sub(10.4f,2.5));
System.out.println(sub(8.9,4.5));
}
}
Output:
Method Overriding
The process of redefining the inherited method of the super class in the derived class is called method overriding.In other word if the subclass has the same method as declared in the parent class then it is called method overriding.The overridden method in the subclass must have same access modifier as the super class.It is used to provide the runtime polymorphism.The overridden method must have same name,and parameter as in the parent class.
Example:
class Plant{
void color(){
System.out.println(“It is green color”);}
}
class Flower extends Plant{
void color(){
System.out.println(“It is yellow color flower”);}
public static void main(String args[]){
Flower obj = new Flower();
obj.color();
}
}
Output:
Difference between Method overloading and method overriding
| Method Overloading | Method Overriding |
| It refers to having the same method but different signatures. | It refers to having same method ,same signature but different class connected through inheritance. |
| It is used to increase readability of program. | It is used to provide specific implementation of method. |
| Parameter must be different. | Parameter must be same |
| It is example of compile time polymorphism. | It is example of runtime polymorphism. |
| It is performed within class. | It occurs in two classes that have IS-A relationship. |
Inner class
Java inner class or nested class is a class that is declared inside the class or interface.Inner classes represent a particular type of relationship that can access all the members of the outer class, including private.The purpose of inner classes is to group classes that belong together,which makes your code more readable and maintainable.
Syntax :
class Java_Outer_class{
//code
class Java_Inner_class{
//code
}
}
Example:
Output:
Packages
Packages are containers for classes that are used to keep the class name space
compartmentalized. Packages are stored in a hierarchical manner and are explicitly imported into new class definitions. The package is both a naming and a visibility control mechanism.It is used in order to prevent naming conflicts,to control access, to make searching/locating and usage of classes,interface,enumerations and annotations easier. Some of the existing packages in Java are:
- java.lang
- java.io
Using Package
There are two ways of accessing classes in another package.
One way is to add the full package name in front of every class name.
Example:
jav.util.Date today=new java.util.Date()
Another is using the import keyword to import a specific class or whole package.The import statement will be given at the top of the file just below the package statement.
Example:
Import java.util.*;
Date today= new Date();
Creating a Package
When creating a package ,you should chose a name food the package and put a package statement with that name at the top of every source file that contains the classes,interfaces,enumerations,and annotations types that you want to include in the package.the package statement should be in the first line of the source file.
Example:
Output:
Inheritance
Inheritance is a mechanism in which one class inherits the properties and features of another class.Inheritance represents IS-A relationship.In inheritance, a new class is created based on existing classes in order to inherit the existing classes’ features and properties. In the new class, new methods and fields can be added.with the use of inheritance the information is made manageable in a hierarchical order. A class that is derived from another class is Subclass.The class from where a subclass inherits the features is called superclass/base class/parent class.
Inheritance uses the concept of the code resuability.It allows to use the field and method of the existing class in the new class. Reusing the existing code saves time and money and increases the program’s reliability.
Syntax:
class Subclass-name extends Superclass-name
{
//methods and fields
}
The extends keyword is used to inherit the properties of class.The meaning of “extends” is to increase the functionality.
Types of inheritance
There are four type of inheritance.They are:
- Single inheritance
- Multiple Inheritance
- Hierarchical inheritance
- Multilevel inheritance
Single inheritance:
In single inheritance, a class is derived from only one existing class.
Example:
Output:
Multiple inheritance
A class derives from more than one existing classes.Java does not support
The multiple inheritance.Multiple inheritance can be implemented by interface.
Hierarchical Inheritance
In hierarchical inheritance,two or more classes inherit the properties of one existing class.
Example:
Output:
Multilevel Inheritance:
The mechanism of deriving a class from another subclass is known as multilevel inheritance.The process can be extended to an arbitrary number of levels.
Example:
Output:
Interface
Interfaces are syntactically similar to classes, but they lack instance variables, and
their methods are declared without any body.In practice, this means that you can define interfaces which don’t make assumptions about how they are implemented. Once it is defined, any number of classes can implement an interface. Also, one class can implement any number of interfaces.
To use an interface,we must write a class that implements the interface.When an instantiable class implements an interface,it should either provide method body for each of the methods declared in the interface or the class should be declared abstract.
Defining an interface
[access] interface InterfaceName[extends Iname1,Iname2]
{
Constant declarations;
Method declarations;
}
Example
Output:
