What is the difference between HashSet and HashMap?
The differences between the HashSet and HashMap are listed below.
HashSet contains only values whereas HashMap includes the entry (key, value). HashSet can be iterated, but HashMap needs to convert into Set to be iterated.
HashSet implements Set interface whereas HashMap implements the Map interface
HashSet cannot have any duplicate value whereas HashMap can contain duplicate values with unique keys.
HashSet contains the only single number of null value whereas HashMap can hold a single null key with n number of null values.

What is the difference between HashMap and TreeMap?
The differences between the HashMap and TreeMap are given below.
HashMap maintains no order, but TreeMap maintains ascending order.
HashMap is implemented by hash table whereas TreeMap is implemented by a Tree structure.
HashMap can be sorted by Key or value whereas TreeMap can be sorted by Key.
HashMap may contain a null key with multiple null values whereas TreeMap cannot hold a null key but can have multiple null values.

What is the difference between Collection and Collections?
The differences between the Collection and Collections are given below.
The Collection is an interface whereas Collections is a class.
The Collection interface provides the standard functionality of data structure to List, Set, and Queue. However, Collections class is to sort and synchronize the collection elements.
The Collection interface provides the methods that can be used for data structure whereas Collections class provides the static methods which can be used for various operation on a collection.

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HAS-A Relationship
1.HAS-A Relationship is also knows as composition(or) aggregation.
2.There is no specific keyword to implement HAS-A relationship but mostly
we can use new operator.
3.The main advantage of reusability.

Example:-
class Engine
{
// engine functionality
}
class Car
{
Engine e=new Engine();
//------------------------------;
//------------------------------;
//------------------------------;
}
NOTE:-
class car has-a engine reference.
The main dis-advantage of has-a relationship increases dependency between the
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Core Java Inetrview Questions java Opps Concept What is method overloading in OOP or Java? (answer)
It's one of the oldest OOPS concept questions, I have seen it 10 years ago and still sees it now. When we have multiple methods with the same name but different functionality then it's called method overloading. For example. System.out.println() is overloaded as we have 6 or 7 println() method each accepting a different type of parameter.


What is method overriding in OOP or Java? (answer)
It's one of the magic of object oriented programming where the method is chose based upon an object at runtime. In order for method overriding, we need Inheritance and Polymorphism, as we need a method with the same signature in both superclass and subclass. A call to such method is resolved at runtime depending upon the actual object and not the type o variable. See the answer for more detailed discussion. more Questions Visit https://cjavapoint.blogspot.com/👌👌👌👈👈👈👈

What is method overloading?

Method overloading is a programming technique that allows developers to use the same method name multiple times in the same class, but with different parameters. In this case, we say that the method is overloaded. Listing 1 shows a single method whose parameters differ in number, type, and order.
Listing 1. Three types of method overloading


Number of parameters:
public class Calculator {
void calculate(int number1, int number2) { }
void calculate(int number1, int number2, int number3) { }
}

Type of parameters:
public class Calculator {
void calculate(int number1, int number2) { }
void calculate(double number1, double number2) { }
}

Order of parameters:
public class Calculator {
void calculate(double number1, int number2) { }
void calculate(int number1, double number2) { }
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Strings in Java
Strings in Java are Objects that are backed internally by a char array. Since arrays are immutable(cannot grow), Strings are immutable as well. Whenever a change to a String is made, an entirely new String is created.

Below is the basic syntax for declaring a string in Java programming language.
Syntax: String str = "learnJava";
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8.1 What is the difference between a Choice and a List ?

A Choice is displayed in a compact form that must be pulled down, in order for a user to be able to see the list of all available
choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible.
A List supports the selection of one or more List items.
8.2
What is a layout manager ?

A layout manager is the used to organize the components in a container.
8.3
What is the difference between a Scrollbar and a JScrollPane ?

A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs
its own scrolling.
8.4
Which Swing methods are thread-safe ?

There are only three thread-safe methods: repaint, revalidate, and invalidate.
8.5
Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
8.6
What is clipping ?

Clipping is defined as the process of confining paint operations to a limited area or shape.
8.7
What is the difference between a MenuItem and a CheckboxMenuItem ?

The CheckboxMenuItem class extends the MenuItem class and supports a menu item that may be either checked or unchecked

public class RemoveNonASCIIString{
public static void main(String [] args){
String str = "Instance¡of¥java";
System.out.println(str);
str = str.replaceAll("[^\\p{ASCII}]", "");
System.out.println("After removing non ASCII chars:");
System.out.println(str);
}
}

import java.util.Arrays;
import java.util.LinkedHashSet;

public class ArrayExample
{
    public static void main(String[] args) throws CloneNotSupportedException
    {
        //Array with duplicate elements
        Integer[] numbers = new Integer[] {1,2,3,4,5,1,3,5};
        
        //This array has duplicate elements
        System.out.println( Arrays.toString(numbers) );
        
        //Create set from array elements
        LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<>( Arrays.asList(numbers) );
        
        //Get back the array without duplicates
        Integer[] numbersWithoutDuplicates = linkedHashSet.toArray(new Integer[] {});
        
        //Verify the array content
        System.out.println( Arrays.toString(numbersWithoutDuplicates) );
    }
}