Real interview questions from top companies for Java. Includes theoretical concepts and coding problems.
What is the difference between Java and C++?
Java is an object-oriented programming language that is designed to have as few implementation dependencies as possible, while C++ is a high-performance language that is compiled into machine code. Java is known for its platform independence, strong security features, and automatic memory management, whereas C++ is known for its performance, reliability, and flexibility.
What are the key features of Java?
The key features of Java include platform independence, object-oriented design, simple and familiar syntax, strong security features, automatic memory management, and a vast ecosystem of libraries and frameworks.
What is the difference between '==' and '.equals()' in Java?
In Java, '==' is used to compare the memory locations of two objects, while '.equals()' is used to compare the actual values of two objects. For example, when comparing two strings, '==' would return false if the strings are stored in different memory locations, even if they have the same value, while '.equals()' would return true if the strings have the same value, regardless of their memory locations.
What is the purpose of the 'finally' block in Java?
The 'finally' block in Java is used to execute a set of statements regardless of whether an exception is thrown or not. It is typically used to release system resources, such as closing files or database connections, to prevent resource leaks.
What is the difference between 'throw' and 'throws' in Java?
In Java, 'throw' is used to explicitly throw an exception, while 'throws' is used to declare that a method may throw an exception. For example, if a method is declared to 'throws IOException', it means that the method may throw an IOException, but it does not necessarily mean that it will throw one.
What is the purpose of the 'synchronized' keyword in Java?
The 'synchronized' keyword in Java is used to ensure that only one thread can execute a block of code at a time. It is typically used to prevent concurrent access to shared resources, such as variables or data structures, to prevent data corruption or other concurrency-related issues.
What is the difference between a 'HashMap' and a 'HashSet' in Java?
In Java, a 'HashMap' is a data structure that stores key-value pairs, while a 'HashSet' is a data structure that stores unique values. A 'HashMap' is typically used to store and retrieve data by key, while a 'HashSet' is typically used to store and retrieve unique values.
What is the purpose of the 'clone()' method in Java?
The 'clone()' method in Java is used to create a copy of an object. It is typically used to create a deep copy of an object, which means that it creates a new object with the same values as the original object, but with a different memory location.
What is the difference between a 'deep copy' and a 'shallow copy' in Java?
In Java, a 'deep copy' is a copy of an object that creates a new object with the same values as the original object, but with a different memory location. A 'shallow copy', on the other hand, is a copy of an object that creates a new object with the same memory location as the original object. A 'deep copy' is typically used to create a copy of an object that contains mutable objects, such as arrays or lists, while a 'shallow copy' is typically used to create a copy of an object that contains immutable objects, such as strings or integers.
What is the purpose of the 'finalize()' method in Java?
The 'finalize()' method in Java is used to release system resources, such as closing files or database connections, when an object is garbage collected. It is typically used to prevent resource leaks and ensure that system resources are released when they are no longer needed.
What is the difference between a 'checked exception' and an 'unchecked exception' in Java?
In Java, a 'checked exception' is an exception that is checked at compile-time, while an 'unchecked exception' is an exception that is not checked at compile-time. Checked exceptions are typically used to handle exceptions that are expected to occur, such as IOException, while unchecked exceptions are typically used to handle exceptions that are not expected to occur, such as NullPointerException.
What is the purpose of the 'try-with-resources' statement in Java?
The 'try-with-resources' statement in Java is used to automatically close resources, such as files or database connections, when they are no longer needed. It is typically used to prevent resource leaks and ensure that system resources are released when they are no longer needed.
What is the difference between a 'lambda expression' and a 'method reference' in Java?
In Java, a 'lambda expression' is an anonymous function that can be defined inline, while a 'method reference' is a reference to an existing method. Lambda expressions are typically used to define small, one-time use functions, while method references are typically used to reference existing methods.
What is the purpose of the 'Stream API' in Java?
The 'Stream API' in Java is used to process data in a declarative way, using a pipeline of operations. It is typically used to process large datasets, such as collections or arrays, and perform operations such as filtering, mapping, and reducing.
What is the difference between a 'serializable' object and a 'non-serializable' object in Java?
In Java, a 'serializable' object is an object that can be converted to a byte stream and written to a file or sent over a network, while a 'non-serializable' object is an object that cannot be converted to a byte stream. Serializable objects are typically used to store or transmit data, while non-serializable objects are typically used to represent complex data structures or algorithms.
What is the purpose of the 'Java Reflection API'?
The 'Java Reflection API' is used to inspect and modify the behavior of classes, methods, and fields at runtime. It is typically used to implement frameworks, libraries, and tools that require dynamic access to classes and methods.
What is the difference between a 'singleton' class and a 'multiton' class in Java?
In Java, a 'singleton' class is a class that can only be instantiated once, while a 'multiton' class is a class that can be instantiated multiple times. Singleton classes are typically used to implement global variables or caches, while multiton classes are typically used to implement pools or factories.
What is the purpose of the 'Java Collections Framework'?
The 'Java Collections Framework' is a set of classes and interfaces that provide a unified way to work with collections, such as lists, sets, and maps. It is typically used to implement data structures and algorithms that require efficient and flexible data storage and retrieval.
What is the difference between a 'thread-safe' class and a 'non-thread-safe' class in Java?
In Java, a 'thread-safe' class is a class that can be safely accessed and modified by multiple threads, while a 'non-thread-safe' class is a class that cannot be safely accessed and modified by multiple threads. Thread-safe classes are typically used to implement concurrent data structures and algorithms, while non-thread-safe classes are typically used to implement sequential data structures and algorithms.
What is the purpose of the 'Java Concurrency API'?
The 'Java Concurrency API' is a set of classes and interfaces that provide a unified way to work with concurrent programming, such as threads, locks, and atomic variables. It is typically used to implement concurrent data structures and algorithms that require efficient and flexible synchronization and communication between threads.
What is the difference between a 'daemon thread' and a 'non-daemon thread' in Java?
In Java, a 'daemon thread' is a thread that runs in the background and is terminated when the JVM exits, while a 'non-daemon thread' is a thread that runs in the foreground and is not terminated when the JVM exits. Daemon threads are typically used to implement background tasks, such as garbage collection or logging, while non-daemon threads are typically used to implement foreground tasks, such as user interface updates or network communication.
What is the purpose of the 'Java Security API'?
The 'Java Security API' is a set of classes and interfaces that provide a unified way to work with security, such as encryption, authentication, and access control. It is typically used to implement secure data storage and transmission, as well as secure authentication and authorization.
What is the difference between a 'signed' integer and an 'unsigned' integer in Java?
In Java, a 'signed' integer is an integer that can represent both positive and negative values, while an 'unsigned' integer is an integer that can only represent positive values. Signed integers are typically used to represent numeric values that can be positive or negative, while unsigned integers are typically used to represent numeric values that are always positive, such as indices or counts.
What is the purpose of the 'Java Math API'?
The 'Java Math API' is a set of classes and interfaces that provide a unified way to work with mathematical functions, such as trigonometry, statistics, and random number generation. It is typically used to implement mathematical algorithms and data analysis.
What is the difference between a 'big integer' and a 'long integer' in Java?
In Java, a 'big integer' is an integer that can represent arbitrarily large values, while a 'long integer' is an integer that can represent values up to a certain maximum size. Big integers are typically used to represent very large numeric values, while long integers are typically used to represent large numeric values that fit within a certain range.
What is the purpose of the 'Java Util API'?
The 'Java Util API' is a set of classes and interfaces that provide a unified way to work with utility functions, such as date and time manipulation, string manipulation, and collections. It is typically used to implement utility functions and data processing algorithms.
What is the difference between a 'synchronized block' and a 'synchronized method' in Java?
In Java, a 'synchronized block' is a block of code that is synchronized on a specific object, while a 'synchronized method' is a method that is synchronized on the current object. Synchronized blocks are typically used to synchronize access to specific resources, while synchronized methods are typically used to synchronize access to the current object.
What is the purpose of the 'Java IO API'?
The 'Java IO API' is a set of classes and interfaces that provide a unified way to work with input/output operations, such as reading and writing files, sockets, and other data streams. It is typically used to implement data storage and transmission algorithms.
What is the difference between a 'buffered stream' and an 'unbuffered stream' in Java?
In Java, a 'buffered stream' is a stream that uses a buffer to improve performance, while an 'unbuffered stream' is a stream that does not use a buffer. Buffered streams are typically used to improve performance when reading or writing large amounts of data, while unbuffered streams are typically used to improve performance when reading or writing small amounts of data.
What is the purpose of the 'Java Networking API'?
The 'Java Networking API' is a set of classes and interfaces that provide a unified way to work with network communication, such as sockets, URL connections, and datagrams. It is typically used to implement network communication algorithms and data transmission protocols.
What is the difference between a 'TCP socket' and a 'UDP socket' in Java?
In Java, a 'TCP socket' is a socket that uses the Transmission Control Protocol (TCP) to establish a connection-oriented communication channel, while a 'UDP socket' is a socket that uses the User Datagram Protocol (UDP) to establish a connectionless communication channel. TCP sockets are typically used to implement reliable data transmission, while UDP sockets are typically used to implement fast data transmission.
What is the purpose of the 'Java Database API'?
The 'Java Database API' is a set of classes and interfaces that provide a unified way to work with databases, such as connecting to databases, executing queries, and retrieving results. It is typically used to implement database access and data storage algorithms.
What is the difference between a 'prepared statement' and a 'statement' in Java?
In Java, a 'prepared statement' is a statement that is pre-compiled and can be executed multiple times with different parameters, while a 'statement' is a statement that is compiled and executed each time it is used. Prepared statements are typically used to improve performance when executing the same query multiple times, while statements are typically used to execute queries that are only executed once.
What is the purpose of the 'Java XML API'?
The 'Java XML API' is a set of classes and interfaces that provide a unified way to work with XML data, such as parsing, generating, and manipulating XML documents. It is typically used to implement XML data processing and validation algorithms.
What is the difference between a 'DOM parser' and a 'SAX parser' in Java?
In Java, a 'DOM parser' is a parser that builds a Document Object Model (DOM) tree from an XML document, while a 'SAX parser' is a parser that uses the Simple API for XML (SAX) to parse an XML document. DOM parsers are typically used to parse and manipulate XML documents, while SAX parsers are typically used to parse and process large XML documents.
Write a Java program to find the maximum value in an array of integers.
publicclassMain {
publicstaticvoidmain(String[] args) {
int[] array = {
1, 2, 3, 4, 5}
;
intmax= array[0];
for (inti=1;
i < array.length;
i++) {
if (array[i] > max) {
max = array[i];
}
}
System.out.println(max);
}
}
Write a Java program to find the minimum value in an array of integers.
publicclassMain {
publicstaticvoidmain(String[] args) {
int[] array = {
1, 2, 3, 4, 5}
;
intmin= array[0];
for (inti=1;
i < array.length;
i++) {
if (array[i] < min) {
min = array[i];
}
}
System.out.println(min);
}
}
Write a Java program to find the sum of all elements in an array of integers.
publicclassMain {
publicstaticvoidmain(String[] args) {
int[] array = {
1, 2, 3, 4, 5}
;
intsum=0;
for (inti=0;
i < array.length;
i++) {
sum += array[i];
}
System.out.println(sum);
}
}
Write a Java program to find the average value of all elements in an array of integers.
publicclassMain {
publicstaticvoidmain(String[] args) {
int[] array = {
1, 2, 3, 4, 5}
;
intsum=0;
for (inti=0;
i < array.length;
i++) {
sum += array[i];
}
doubleaverage= (double) sum / array.length;
System.out.println(average);
}
}
Write a Java program to find the maximum value in a binary search tree.
