Java Interview Questions for Freshers

The JVM runs compiled bytecode, the JRE is the JVM plus the standard libraries needed to run programs, and the JDK adds the compiler and tools needed to develop them.

== compares references for objects, meaning whether they are the same object. equals() compares content when a class overrides it, as String does. Compare strings with equals().

An abstract class can hold state and partly implemented methods and a class extends only one. An interface describes a contract and a class can implement many. Since Java 8 interfaces can have default methods.

Overloading is the same method name with different parameters in one class, decided at compile time. Overriding is a subclass giving its own version of a parent's method, decided at run time.

Immutability makes strings safe to share, cache and use as keys. The string pool stores one copy of each literal, so equal literals can share memory. Use StringBuilder when you change text a lot.

Checked exceptions must be handled or declared, such as IOException. Unchecked ones extend RuntimeException, such as NullPointerException, and the compiler does not force handling them.

It uses the key's hashCode to pick a bucket and equals to find the key inside it. Collisions go into a list, which becomes a tree when it grows long in Java 8 and later. It resizes as it fills.

ArrayList uses an array, so reading by index is fast and inserting in the middle is slow. LinkedList is fast at inserting and removing at known positions but slow at reading by index.

The JVM automatically frees memory of objects that nothing refers to any more. You cannot force it, only suggest it, and good code avoids keeping references it no longer needs.

final marks a variable, method or class that cannot change or be overridden. finally is a block that always runs after try and catch. finalize was a method called before garbage collection and is now deprecated.