3.StringBuffer, 4.StringBuilder
| String | StringBuffer |
|---|---|
| String class is immutable. | StringBuffer class is mutable. |
| String is slow and consumes more memory when you concat too many strings, because every time it creates new instance. | StringBuffer is fast and consumes less memory when you concat strings. |
| String class overrides the equals() method of Object class. So you can compare the contents of two strings by equals() method. | StringBuffer class doesn’t override the equals() method of Object class. |
| String is synchronized i.e. thread safe | StringBuffer is synchronized i.e. thread safe. It means two threads can’t call the methods of StringBuffer simultaneously. |
Creates an empty StringBuffer object with default initial capacity 16.If it reaches max capacity then a new StringBuffer object will be created with new capacity = (currentcapacity + 1) * 2
StringBuffer sb = new StringBuffer();
1.What is immutable object? Can you write immutable object?
Don’t confuse over SingleTon class
Immutable classes are Java classes whose objects cannot be modified once created.
-
Declare the class as final so it can’t be extended.
-
Make all fields private & final so that direct access is not allowed & it’s values can be assigned only once.
-
Initialize all the fields via a constructor
-
Write getters only, not setters.
// An immutable class
final class Student {
final int sno;
final String name;
public Student(int sno, String name) {
this.name = name;
this.sno = sno;
}
public String getName() {
return name;
}
public int getsno() {
return sno;
}
}
//--------------------------------------
// Driver class
public class Test {
public static void main(String args[]) {
Student s1 = new Student(101, "Satya");
s1.name = "Vijay";
}
}
//--------------------------------------
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The final field Student.name cannot be assigned
All String & Wrapper class objects are Immutable
2.What is Singleton? Can you write critical section code for singleton?
A Singleton class is one which allows us to create only one object for JVM.
Rules:
-
Create Singleton class Object make it as PRIVATE, so no other classes will access it.
-
Create PRIVATE constructor, so no other classes won’t create Object using new Student(), because it is not visible to out the classes.
-
Every Singleton class contains at least one static factory method, make it public so, it is visible other classes
class Student {
private static Student st;
private Student() {
System.out.println("OBJECET Created FIRST TIME");
}
public static Student getObject() {
if (st == null) {
st = new Student();
} else {
System.out.println("OBJECET ALREDAY CREATED");
}
return st;
}
}
//-------------------------------------------
public class Test {
public static void main(String args[]) {
Student s1 = Student.getObject();
System.out.println("s1 : " + s1.hashCode());
Student s2 = Student.getObject();
System.out.println("s2 : " + s2.hashCode());
System.out.println("s1==s2 : " + (s1 == s2));
}
}
//-------------------------------------------
OBJECET Created FIRST TIME
s1 : 1829164700
OBJECET ALREDAY CREATED
s2 : 1829164700
s1==s2 : true
In above code, it will create multiple instances of Singleton class if called by more than one thread parallel
Double checked locking of Singleton is a way to ensure only one instance of Singleton class is created through application life cycle.
This will bring us to double checked locking pattern, where only critical section of code is locked. Programmer call it double checked locking because there are two checks for _instance == null, one without locking and other with locking (inside synchronized) block. Here are how double checked locking looks like in Java
public static Singleton getInstanceDC() {
if (_instance == null) { // Single Checked
synchronized (Singleton.class) {
if (_instance == null) { // Double checked
_instance = new Singleton();
}
}
}
return _instance;
}
How do you reverse a String in Java without using StringBuffer?
The Java library provides String Buffer and StringBuilder class with reverse() method, which can be used to reverse String in Java.
String reverse = "";
String source= "My Name is Khan";
for(int i = source.length() -1; i>=0; i--){
reverse = reverse + source.charAt(i);
}
How to Print duplicate characters from String?
public class RepreatedChar {
public static void main(String[] args) {
String a = "success";
// 1.convert into char array. [‘s’, ‘u’, ‘c’, ‘c’, ‘e’, ‘s’, ‘s’,]
char[] c = a.toCharArray();
// 2.create Hashmap store key as character, count as value
HashMap map = new HashMap<>();
for (char ch : c) {
// 3.Check if Map contains given Char as <key> or not
if (map.containsKey(ch)) {
// if their, get the value & increment it
int i = (int) map.get(ch);
i++;
// add updated value to it
map.put(ch, i);
} else {
// if not their , add key & value as 1
map.put(ch, 1);
}
}
Set set = map.entrySet();
Iterator iterator = set.iterator() ;
while (iterator.hasNext()) {
Map.Entry entry = (Entry) iterator.next();
System.out.println(entry.getKey()+" : "+entry.getValue());
}
}
}
s : 3
c : 2
u : 1
e : 1
Is String contains Number or not
public class RegEx {
public static void main(String[] args) {
// Regular expression in Java to check if String is number or not
Pattern pattern = Pattern.compile(".*[^0-9].*");
String[] inputs = { "123", "-123", "123.12", "abcd123" };
/* Matches m = pattern.match(input);
* boolean ch = m.match(); */
for (String input : inputs) {
System.out.println("does " + input + " is number : " + !pattern.matcher(input).matches());
}
// Regular expression in java to check if String is 6 digit number or not
String[] numbers = { "123", "1234", "123.12", "abcd123", "123456" };
Pattern digitPattern = Pattern.compile("\\d{6}");
// Pattern digitPattern = Pattern.compile("\\d\\d\\d\\d\\d\\d");
for (String number : numbers) {
SOP("does " + number + " is 6 digit number : " + digitPattern.matcher(number).matches());
}
}
}
Reverse Words in a String
public class RevWords {
public static void main(String[] args) {
// using s.split("\\s");
String s = "My name is Satya";
String words[] = s.split("\\s");
String rev = "";
int len = words.length;
for (int i = (len - 1); i >= 0; i--) {
rev = rev + words[i];
}
System.out.println(rev);
// using Collections.reverse(str)
List<String> word = Arrays.asList(s.split("\\s"));
Collections.reverse(word);
System.out.println(word);
}
}