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Core Java Interview Questions & Programs

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Yash edited this page Dec 14, 2022 · 6 revisions
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. Basic, Refactoring Techniques

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).

Core Java

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