Comparable vs Comparator

Posted by: Instanceofjava Posted date: 6:37 PM / comment : 0 Collections , Core java Interview Questions

One of the common interview question What are the differences between comparable and comparator and how to sort collections using these interfaces ?
What are the differences between comparable and comparator and how to sort employee object by empId or empName using these interfaces.
In this topic we will clear questions like difference between comparable and comparator in java, comparator vs comparable in java with example and comparable and comparator in java
Comparator and Comparable are two interfaces in Java API.
Before discussing about differences lets see brief description about these two interfaces

Comparable Interface:

Comparable Interface is actually from java.lang package.
It will have a method compareTo(Object obj)to sort objects
public int compareTo(Object obj){ }
Comparable interface used for natural sorting these is the reason all wrapper classes and string class implementing this comparator and overriding compareTo(Object obj) method.
So in String and all wrapper classes compareTo(Object obj) method is implemented in such way that it will sort all objects.

String class:

If we observe String class it is implementing comparable interface.
If compareTo(String str) methods returns 0 : both strings are equal
If compareTo(String str) method returns 1: string is lexicographically greater than the string argument
If compareTo(String str) method returns -1: string is lexicographically less than the string argument

package java.lang;
public final class String
extends Object
implements Serializable, Comparable<String>, CharSequence {
public int compareTo(String anotherString){
//logic
}
}

Comparing String Objects:

Lets see an example java program that will explains how two string objects are compared using compareTo(String str) method in String class.

package com.instanceofjava;
public class StringCompareDemo {
public static void main(String [] args){
String str1="comparable";
String str2="comparator";
int value=str1.compareTo(str2);
if(value==0){
System.out.println("Strings are equal");
}
else{
System.out.println("Strings are not equal");
}
}
}

Output:

Strings are not equal

Wrapper classes:

Wrapper classes is used to convert primitive data values into java objects. for 8 primitive data types java has 8 corresponding wrapper classes. All these classes implementing comparable interface.
Lets see an example on Integer wrapper class

Integer:

package java.lang;
public final class Integer
extends Number
implements Comparable<Integer> {
public int compareTo(Integer i){
//
}
}

Lets see an example program of comparing two integer objects

package instanceofjava;
public class IntegerComparableDemo {
public static void main(String [] args){
// compares two Integer objects numerically
Integer obj1 = new Integer("37");
Integer obj2 = new Integer("37");
int retval = obj1.compareTo(obj2);
if(retval > 0) {
System.out.println("obj1 is greater than obj2");
}
else if(retval < 0) {
System.out.println("obj1 is less than obj2");
}
else {
System.out.println("obj1 is equal to obj2");
}
}
}

Output:

obj1 is equal to obj2;

Sorting Collections using Comparable :

By using Collections.sort(list); method we can sort objects in natural object sorting order
An example program on Collections.sort(list);
Sorting Employee objects by id.

package instanceofjava;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
public class Employee implements Comparable {
String name;
int id;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public Employee(String name, int id) {
this.name=name;
this.id=id;
}
@Override
public int compareTo(Object in) {
return new Integer(this.id).compareTo(((Employee)in).id);
}
public static void main(String[] args) {
Employee e1= new Employee("xyz", 37);
Employee e2= new Employee("abc", 46);
Employee e3= new Employee("sai", 38);
ArrayList al= new ArrayList();
al.add(e1);
al.add(e2);
al.add(e3);
Collections.sort(al);
Iterator itr= al.iterator();
while (itr.hasNext()) {
Employee em = (Employee) itr.next();
System.out.println(em.getId()+" "+em.getName());
}
}
}

Output:

37 xyz
38 sai
46 abc

Comparator:

Comparator Interface is actually from java.util package.
It will have a method compare(Object obj1, Object obj2)to sort objects
public int compare(Object obj1, Object obj2){ }
Comparator interface used for customized sorting.
Will place sorting logic in other class so that its easy to change.
An example program which will sort employee objects by name

1.Employee.java

package instanceofjava;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
public class Employee {
String name;
int id;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public Employee(String name, int id) {
this.name=name;
this.id=id;
}
public static void main(String[] args) {
Employee e1= new Employee("xyz", 37);
Employee e2= new Employee("abc", 46);
Employee e3= new Employee("sai", 38);
ArrayList al= new ArrayList();
al.add(e1);
al.add(e2);
al.add(e3);
Collections.sort(al, new EmpSortByName());
Iterator itr= al.iterator();
while (itr.hasNext()) {
Employee em = (Employee) itr.next();
System.out.println(em.getId()+" "+em.getName());
}
}
}

EmpSortByName.java:

package instanceofjava;
import java.util.Comparator;
public class EmoSortByName implements Comparator<Employee> {
@Override
public int compare(Employee arg0, Employee arg1) {
return arg0.getName().compareTo(arg1.getName());
}
}

Output:

46 abc
38 sai
37 xyz

Differences Between Comparable and Comparator:

Parameter	Comparable	Comparator
Package	java.lang.Comparable	java.util.Comparator
Sorting logic	Must be in Same class	Separate class
Method definition	public int compareTo(Object obj)	public int compare(Object obj1, Object obj2)
Method call	Collections.sort(list)	Collections.sort(list, new OtherSortClass())
Purpose	Natural Sorting	Custom Sorting

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10 Interesting Core Java Interview Coding Questions and Answers

Posted by: Instanceofjava Posted date: 8:16 PM / comment : 10 Concept and Interview Questions , Find Output of Program

1. What is the output of following program?

package com.instanceofjava;
public class B{
B b= new B();
public int show(){
return (true ? null : 0);
}
public static void main(String[] args) {
B b= new B();
b.show();
}
}

Exaplanation:

Whenever we create the object of any class constructor will be called first and memory allocated for all non static variables
Here B b= new B(); variable is object and assigned to new object of same class
B b= new B(); statement leads to recursive execution of constructor will create infinite objects so at run time an exception will be raised
Exception in thread "main" java.lang.StackOverflowError
The common cause for a stack overflow exception is a bad recursive call. Typically this is caused when your recursive functions doesn't have the correct termination condition

2. What is the output of following program?

package com.instanceofjava;
public class A{
public static void show(){
System.out.println("Static method called");
}
public static void main(String[] args) {
A obj=null;
obj.show();
}
}

Exaplanation:

We can call static methods using reference variable which is pointing to null because static methods are class level so we can either call using class name and reference variable which is pointing to null.

3. What is the output of following program?

package com.instanceofjava;
public class A{
static int a = 1111;
static
{
a = a-- - --a;
}
{
a = a++ + ++a;
}
public static void main(String[] args) {
System.out.println(a);
}
}

Explanation:

Top 10 Increment and decrement operators interview Questions

4. What is the output of following program?

package com.instanceofjava;
public class A{
int GetValue()
{
return (true ? null : 0);
}
public static void main(String[] args) {
A obj= new A();
obj.GetValue();
}
}

5. What is the output of following program?

package com.instanceofjava;
public class A{
public static void main(String[] args) {
Integer i1 = 128;
Integer i2 = 128;
System.out.println(i1 == i2);
Integer i3 = 127;
Integer i4 = 127;
System.out.println(i3 == i4);
}
}

6. What is the output of following program?

package com.instanceofjava;
class A
{
void method(int i)
{
}
}
class B extends A
{
@Override
void method(Integer i)
{
}
}

7. Which line will throw compile time error? 8 or 9?

package com.instanceofjava;
class A
{
public static void main(String [] args)
{
Integer i = new Integer(null);
String s = new String(null);
}
}

8.What is the output of following program?

package com.instanceofjava;
class A
{
public static void main(String [] args)
{
String s = "ONE"+3+2+"TWO"+"THREE"+5+4+"FOUR"+"FIVE"+5;
System.out.println(s);
}
}

9.What is the output of following program?

package com.instanceofjava;
class A
{
static int method1(int i)
{
return method2(i *= 11);
}
static int method2(int i)
{
return method3(i /= 11);
}
static int method3(int i)
{
return method4(i -= 11);
}
static int method4(int i)
{
return i += 11;
}
public static void main(String [] args)
{
System.out.println(method1(11));
}
}

10.What is the output of following program?

package com.instanceofjava;
class A
{
public static void main(String [] args)
{
System.out.println(null);
}
}

Explanation:

What happens When System.out.println(null)?

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Group Discussion on Java Topic

Posted by: Instanceofjava Posted date: 8:03 PM / comment : 0

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We will conclude the topic at the end.

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Java Interview programs on Strings

Posted by: Instanceofjava Posted date: 10:53 PM / comment : 0 Concept and Interview Questions

1.Reverse a String Without using String API?

package com.instaceofjava;
public class ReverseString {
public static void main(String[] args) {
String str="Hello world";
String revstring="";
for(int i=str.length()-1;i>=0;--i){
revstring +=str.charAt(i);
}
System.out.println(revstring);
}
}

Program:

output: dlrow olleH.

2)Sorting the String without using String API?

package com.Instanceofjava;
public class SortString {
public static void main(String[] args) {
String original = "edcba";
int j=0;
char temp=0;
char[] chars = original.toCharArray();
for (int i = 0; i <chars.length; i++) {
for ( j = 0; j < chars.length; j++) {
if(chars[j]>chars[i]){
temp=chars[i];
chars[i]=chars[j];
chars[j]=temp;
}
}
}
for(int k=0;k<chars.length;k++){
System.out.println(chars[k]);
}
}
}

program:

output:abcde.

3.Sort the String with using String API?

program:

package com.instanceofjava;
public class SortString {
public static void main(String[] args) {
String original = "edcba";
char[] chars = original.toCharArray();
Arrays.sort(chars);
String sorted = new String(chars);
System.out.println(sorted);
}
}

OutPut:abcde

4.Check String is palindrome or not?

program:

Solution #1:

package com.instaceofjava;
public class PalindromeDemo{
public static void main(String[] args) {
String str="MADAM";
String revstring="";
for(int i=str.length()-1;i>=0;--i){
revstring +=str.charAt(i);
}
System.out.println(revstring);
if(revstring.equalsIgnoreCase(str)){
System.out.println("The string is Palindrome");
}
else{
System.out.println("Not Palindrome");
}
}
}

Output:

The string is Palindrome

Solution #2:

package com.instaceofjava;
import java.util.Scanner;
public class Palindrome {
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.println("Enter a string");
String str=in.nextLine();
StringBuffer strone=new StringBuffer(str);
StringBuffer strtwo=new StringBuffer(strone);
strone.reverse();
System.out.println("Orginal String ="+strtwo);
System.out.println("After Reverse ="+strone);
if(String.valueOf(strone).compareTo(String.valueOf(strtwo))==0)
System.out.println("Result:Palindrome");
else
System.out.println("Result:Not Palindrome");
}
}

Output:

Enter a string
MOOM
Orginal String =MOOM
After Reverse =MOOM

Result:Palindrome

5.Program to Check given number is palindrome or not?

Program:

package com.instaceofjava;
import java.util.Scanner;
public class Palindrome {
public static void main(String[] args)
{
System.out.println("Please Enter a number : ");
int givennumber = new Scanner(System.in).nextInt();
int number=givennumber;
int reverse=0;
while (number != 0) {
int remainder = number % 10;
reverse = reverse * 10 + remainder;
number = number / 10;
}
if(givennumber == reverse)
System.out.println("Result:Palindrome");
else
System.out.println("Result:Not Palindrome");
}
}

Output:

Please Enter a number :
535
Result:Palindrome.

Java experience interview programming questions on Strings

Static methods in java example

Posted by: Instanceofjava Posted date: 2:37 PM / comment : 0 concept and program

A method which has static keyword in its definition is called static method.

static void print(){
}

JVM will not execute static methods by default. They are executed only if they are called explicitly by developer either from main method or from static variable as its assignment statement or from static block.

Static method called from main method:

package com.instanceofjava;
class StaticDemo
{
static void show(){
System.out.println("static method"):
}
public static void main(String[] args)
{
show();
}
}

Output:

static method

Static method called from variable assignment:

package com.instanceofjava;
class StaticDemo
{
static int x=show();
static int show(){
System.out.println("static method called"):
return 10;
}
public static void main(String[] args)
{
System.our.println(x);
}
}

Output:

static method called
10

Static method called from static block:

package com.instanceofjava;
class StaticDemo
{
static{
show();
}
static void show(){
System.out.println("static method called"):
}
}

Output:

static method called

Order of Execution:

Static methods are executed in the order of they are called, not in the order of they are defined.
All static methods are executed in java stack area by creating separate stack frame.
When a method is called from main method, JVM creates stack frame in main thread for that method execution.
The stack frame is destroyed immediately after method execution is completed.

Example program on order of execution of static methods:

package com.instanceofjava;
class StaticMehodDemo
{
static void show(){
System.out.println("show method called");
}
static void print(){
System.out.println("print method called");
}
static void display(){
System.out.println("display method called");
}
public static void main(String[] args)
{
System.our.println("main method called");
show();
print();
display();
}
}

Output:

main method called
print method called
show method called
display method called

Variable initialization with same variable:

We can initialize variable with same variable name. this assignment is valid. In this case the variable value is replaced with same value.

int x=20;
x=x; // valid statement

Example program:

package com.instanceofjava;
class StaticDemo
{
static int a=10;
public static void main(String[] args){
int a=20;
a=a;
System.out.println("a="+a);
System.out.println("StaticDemo.a="+StaticDemo.a);
}
}

Output:

Local preference with parameters:

Parameters are also treated as local variables.
Hence if parameter declared with same static variable name and if we want to access static variable in presence of parameter or if we want to initialize static variable with parameter name we must refer variable with class name.

Example program:

package com.instanceofjava;
class StaticDemo
{
static int x=10;
static void m1(int x){
System.out.println(x);
System.out.println(StaticDemo.x);
}
public static void main(String[] args){
m1(37);
System.out.println(x);
}
}

Output:

Static Members in java

Programming Questions on Static

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Comparable Interface:

String class:

Comparing String Objects:

Wrapper classes:

Integer:

Sorting Collections using Comparable :

Comparator:

1.Employee.java

EmpSortByName.java:

Differences Between Comparable and Comparator:

1. What is the output of following program?

Exaplanation:

2. What is the output of following program?

Exaplanation:

3. What is the output of following program?

Explanation:

4. What is the output of following program?

5. What is the output of following program?

6. What is the output of following program?

7. Which line will throw compile time error? 8 or 9?

8.What is the output of following program?

9.What is the output of following program?

10.What is the output of following program?

Also Read:

Static method called from main method:

Static method called from variable assignment:

Static method called from static block:

Order of Execution:

Example program on order of execution of static methods:

Variable initialization with same variable:

Local preference with parameters:

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