- JDK. It stands for Java Development Kit. It is the tool necessary to compile, document and package Java programs. It contains JRE + development tools.
- JRE. It stands for Java Runtime Environment. JRE refers to a runtime environment in which Java bytecode can be executed. It’s an implementation of the JVM which physically exists.
- JVM. It stands for Java Virtual Machine. It is an abstract machine. It is a specification that provides a run-time environment in which Java bytecode can be executed. JVM follows three notations: Specification, Implementation, and Runtime Instance.
Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.
“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.
| Feature | Stack | Heap |
|---|---|---|
| Memory | Stack memory is used only by one thread of execution. | Heap memory is used by all the parts of the application. |
| Access | Stack memory can’t be accessed by other threads. | Objects stored in the heap are globally accessible. |
| Memory Management | Follows LIFO manner to free memory. | Memory management is based on the generation associated with each object. |
| Lifetime | Exists until the end of execution of the thread. | Heap memory lives from the start till the end of application execution. |
| Usage | Stack memory only contains local primitive and reference variables to objects in heap space. | Whenever an object is created, it’s always stored in the Heap space. |
- final variable. When the final keyword is used with a variable then its value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.
- final method. When a method is declared final then it can’t be overridden by the inheriting class.
- final class. When a class is declared as final in Java, it can’t be extended by any subclass class but it can extend other class.
The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:
- Bootstrap ClassLoader
- Extension ClassLoader
- System/Application ClassLoader
- E for an element
- K for a map key
- V for a map value
- N for a number
- T for a generic data type
- S , U , V , and so forth for multiple generic types
- Driver - Knows how to get a connection to the database
- Connection - Knows how to communicate with the database
- Statement - Knows how to run the SQL
- ResultSet - Knows what was returned by a SELECT query
