Top Java Support Interview Questions

This section covers a range of Java-related topics frequently asked in Java support engineer interviews.

Superclass of All Classes

The Object class is the root of the Java class hierarchy; all other classes (directly or indirectly) inherit from it.

Palindrome Check

import java.util.*;
public class TestString {
    public static void main(String args[]) {
        String a, b = "";
        Scanner s = new Scanner(System.in);
        System.out.print("Enter the string : ");
        a = s.nextLine();
        int n = a.length();
        for (int i = n - 1; i >= 0; i--) {
            b = b + a.charAt(i);
        }
        if (a.equalsIgnoreCase(b)) {
            System.out.println("The string is palindrome");
        } else {
            System.out.println("The string is not a palindrome");
        }
    }
}

String Permutations

import java.util.Scanner;
public class StringArrangements {
    public static String swapString(String a, int i, int j) {
        char[] b = a.toCharArray();
        char ch;
        ch = b[i];
        b[i] = b[j];
        b[j] = ch;
        return String.valueOf(b);
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter String");
        String str = sc.nextLine();
        int len = str.length();
        System.out.println("All the permutations of the string are: ");
        generatePermutation(str, 0, len);
    }
    public static void generatePermutation(String str, int start, int end) {
        if (start == end - 1)
            System.out.println(str);
        else {
            for (int i = start; i < end; i++) {
                str = swapString(str, start, i);
                generatePermutation(str, start + 1, end);
                str = swapString(str, start, i);
            }
        }
    }
}

Real-Life Multithreading Examples

• Playing music while using other applications.
• Video games (concurrent tasks like rendering graphics, handling user input, playing sounds).
• Online banking/transaction systems (concurrent users accessing accounts).

Immutable Classes in Java

Immutable classes (like String, Integer, etc.) cannot be modified after creation. Their state remains constant throughout their lifetime.

Pyramid Star Pattern

public class PyramidStarPattern {
    public static void main(String args[]) {
        int i, j, row = 6;
        for (i = 0; i < row; i++) {
            for (j = row - i - 1; j >= 0; j--) {
                System.out.print(" ");
            }
            for (j = 0; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }
    }
}

Detecting Circular Linked Lists

[Explain an algorithm to detect if a linked list is circular. This would typically involve using two pointers, one moving one node at a time and the other moving two nodes at a time. If the fast pointer catches up to the slow pointer, the list is circular.]

Pass-by-Value in Java

Java uses pass-by-value. When you pass an object to a method, you're passing a copy of the object's reference, not the object itself. Therefore, changes made to the object within the method will be reflected outside the method. Primitives are copied directly.

More on Pass-by-Value

Java: Compiled or Interpreted?

Java source code is compiled into bytecode (.class files), which is then interpreted by the JVM. This combination of compilation and interpretation is a key feature of Java's platform independence.

Simple SQL Query

SELECT * FROM employees;

Pointers in Java

Java does not use explicit pointers for memory management. The JVM handles memory allocation and deallocation automatically, preventing many errors related to direct memory access.

Multiple Inheritance in Java

Java doesn't support multiple inheritance of classes but achieves similar functionality using interfaces (a class can implement multiple interfaces).

Path vs. Classpath

The PATH environment variable tells the operating system where to find executable files (like java.exe, javac.exe). The CLASSPATH environment variable tells the JVM where to find class files (.class files) needed to run a Java program.

Setting JAVA_HOME in Linux

Add this line to your ~/.bashrc or /etc/profile file (depending on your shell and desired scope):

export JAVA_HOME=/path/to/your/jdk
export PATH=$PATH:$JAVA_HOME/bin

Then, source the file (source ~/.bashrc or source /etc/profile) to apply the changes.

Full Form of SQL

SQL stands for Structured Query Language.

Writing an HQL Query

HQL (Hibernate Query Language) is similar to SQL but operates on objects instead of database tables.

FROM Employee WHERE salary > 50000

Learn more about HQL

Heap Dumps in Java

A heap dump is a snapshot of the JVM's heap memory at a specific moment. It's useful for diagnosing memory leaks.

Thread Dumps in Java

A thread dump shows the state of all threads in a Java process, including their stack traces. It helps in troubleshooting concurrency issues.

OutOfMemoryError

This error occurs when the JVM cannot allocate sufficient memory for an object, usually because the heap is full.

InvocationTargetException

This exception is thrown when an exception occurs during a method invocation using reflection (Method.invoke()).

Garbage Collection in Java

Java's garbage collector automatically reclaims memory occupied by objects that are no longer referenced.

JDBC vs. JNDI

JDBC (Java Database Connectivity): Provides APIs for database access. JNDI (Java Naming and Directory Interface): Provides APIs for accessing various naming and directory services, including database connection lookups.

JDBC Connection Pooling

Connection pooling is creating and managing a pool of JDBC connection objects for reuse, improving performance by reducing the overhead of creating new connections for each database request.

TCP/IP vs. UDP

Key Differences Between TCP and UDP

Learn more about TCP vs. UDP

How TCP/IP Works

The TCP/IP model breaks messages into packets for transmission. If a packet is lost, it's retransmitted. Packets can travel different routes to reach their destination, making it a robust communication protocol.

Learn more about the TCP/IP protocol

Checking Remote Server Status

The telnet command can check if a server is running and listening on a specific port. If successful, you'll get a connection; otherwise, you might get a connection timeout.

Learn more about the telnet command

Deadlocks in Java

A deadlock in multithreaded Java occurs when two or more threads are blocked indefinitely, each waiting for the other to release a resource that it needs.

Learn more about deadlocks in Java

Deadlock Detection

Deadlock detection involves analyzing thread dumps (using tools like jstack). Deadlocks can be challenging to resolve, sometimes requiring restarting the application.

Collections in Java

The Java Collections Framework provides classes and interfaces for storing and manipulating groups of objects. It simplifies working with collections of data efficiently.

Removing Duplicates from an ArrayList

Two common methods for removing duplicates from an ArrayList:

1. Using HashSet: Fast but doesn't preserve insertion order.
2. Using LinkedHashSet: Preserves insertion order.

The process generally involves copying the ArrayList to a HashSet (or LinkedHashSet) to remove duplicates, then clearing the original ArrayList and copying the elements back from the set.

Core Java Concepts for Java Support Engineers

A strong foundation in Core Java, including OOP concepts, multithreading, exception handling, and database interactions (JDBC), is essential for Java support engineers.