Core Java Interview Questions & Programs - Yash-777/LearnJava GitHub Wiki

Programs
Input:
int array[] = { 0, 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 18, 20, 21, 23 };

Output:
Missing number(s): 5, 16, 17, 19, 22.

Example:

List list = new ArrayList();
// To get least and highest value sort array
Arrays.sort( arr );
int least = array[0], highest = arrar[ array.length - 1 ]

list.add( ... ); // For each 0 - 23

list.remove( array[0] );  // For each of array and remove from list.

list.removeAll(Arrays.asList( array ));
//Java 8
list.removeIf(s -> s.equals( 1 )); // removes all instances, not just the 1st one
int[] numbers = { 1, 5, 23, 2, 1, 6, 3, 1, 8, 12, 3 };
Arrays.sort(numbers);

for(int i = 1; i < numbers.length; i++) {
  if(numbers[i] == numbers[i - 1]) {
    System.out.println("Duplicate: " + numbers[i]);
  }
}
Map<Character, Integer> map = new LinkedHashMap<Character, Integer>();
for (char c : str.toCharArray()) {
  map.merge(c, 1, (prevValue, newValue) -> prevValue + newValue );
}
int maxValueInMap = Collections.max( map.values() );
for (Entry<Character, Integer> entry : map.entrySet()) {
  if ( entry.getValue() == maxValueInMap ) {
   System.out.format("Char ['%s'] repeated %d times\n", entry.getKey(), maxValueInMap);
  }
}
  • Factorial Function [n! = n × (n−1)!], Print 1-50 numbers with out using any loops.

Example: 5! = 5 * 4 * 3 * 2 * 1

public static int factorial(int n) {
  if( n==0 || n==1 ) {
    return 1;
  } else {
    return n * factorial(n-1);
  }
}
public static void recurciveMethod( int val ) {
  System.out.println( val );
  if( val < 100 ) {
    recurciveMethod( ++val );
  }
}
  • Fibonacci series Sum of the two preceding numbers. The simplest is the series 0, 1, 1, 2, 3, 5, 8, etc.
int x = 0, y = 1;
for(int i=0; i<10; i++) {
  z = x + y;
    System.out.println(z);
  x = y;
  y = x;
}
  • Perfect Number and Armstrong Number. perfect number, a positive integer that is equal to the sum of its proper divisors.
    The smallest perfect number is 6, which is the sum of 1, 2, and 3
Core Java

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behavior.

Method signature: It consists of method name and parameter list (number/type/order of the parameters). methodName(parametersList y). An instance method in a subclass with the same signature and return type as an instance method in the super-class overrides the super-class's method.

Java OOP concepts
Class  - Collection of a common features of a group of object [static/instance Fields, blocks and Methods]
Object - Instance of a class (instance fields)
Abstraction - Process of hiding complex info and providing required info like API, Marker Interfaces ...
Encapsulation(Security) - Class Binding up with data members(fields) and member functions.
Inheritance  (Reusability by placing common code in single class)
 1. Multilevel - {A -> B -> C} 2. Multiple - Diamond problem {A <- (B) -> C} [Java not supports] 3. Cyclic {A <-> B} [Java not supports]
 * Is-A   Relation - Class A extends B
 * Hash-A Relation - Class A { B obj = new B(); } - (Composition/Aggregation)
Polymorphism (Flexibility) 1. Compile-Time Overloading    2. Runtime Overriding  [Greek - "many forms"]


int[] arr = {1,2,3}; int arrLength = arr.length;   // Fixed length of sequential blocks to hold same data type
String str = "Yash"; int strLength = str.length(); // Immutable Object value can't be changed.

List<?> collections = new ArrayList<String>(); int collectionGroupSize = collections.size();
Map<?, ?> mapEntry = new HashMap<String, String>();
Set<?> keySet = mapEntry.keySet(); // Set of Key's
Set<?> entrySet = mapEntry.entrySet(); // Set of Entries [Key, Value]

// Immutable Objects once created they can't be modified. final class Integer/String/Employee
Integer val = Integer.valueOf("100"); String str2 = String.valueOf(100); // Immutable classes
final class Employee { // All Wrapper classes, java.util.UUID, java.io.File ...
    private final String empName; // Field as Final(values can be assigned only once) Only getter functions.
    public Employee(String name) { this.empName = name; }
}

Native Java Code for Hashtable.h, Hashtable.cpp SQL API.

You can check your current JDK and JRE versions on your command prompt respectively,

  • JDK javac -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/p: javac 1.8.0_121
  • JRE java -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/P: java version "1.8.0_102"

JAVA_HOME - Must be set to JDK otherwise maven projects leads to compilation error. [ERROR] No compiler is provided in this environment. Perhaps you are running on a JRE rather than a JDK? C:\Softwares\OpenJDK\, 7-zip

Fatal error compiling: invalid target release: JRE and JDK must be of same version 1.8.0.XXX

Disable TLS 1.0 and 1.1security-libs/javax.net.ssl: TLS 1.0 and 1.1 are versions of the TLS protocol that are no longer considered secure and have been superseded by more secure and modern versions (TLS 1.2 and 1.3).

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