Java for Software Testers

Difference between abstract class and interface?

Interfaces define contracts and (since Java 8) default methods; abstract classes can hold state and partial implementation. Use interfaces for capability, abstract classes for shared behaviour.

ArrayList vs LinkedList?

ArrayList is array-backed (fast random access, slower middle insertions); LinkedList is doubly linked (faster middle ops, slower random access). Default to ArrayList.

What is the difference between == and equals?

== compares references for objects (and values for primitives); equals compares logical equality and should be overridden alongside hashCode.

Explain HashMap internals.

Key hashCode buckets, equals resolves collisions in a linked list or balanced tree (Java 8+) past a threshold.

What is autoboxing?

Automatic conversion between primitives (int) and their boxed types (Integer). Beware NullPointerException when unboxing nulls.

Checked vs unchecked exceptions?

Checked exceptions must be declared or caught (IOException); unchecked extend RuntimeException and don't need declaration (NullPointerException).

What are streams used for?

Functional-style processing of collections — map, filter, reduce — typically replacing imperative loops.

Final keyword — three usages?

Final variables (immutable reference), final methods (cannot be overridden), final classes (cannot be subclassed).

What is the synchronized keyword?

Acquires a monitor on the object/class to serialise access to a block of code across threads.

How do you read a JSON in Java?

Use Jackson or Gson — ObjectMapper.readValue or fromJson into POJOs or JsonNode trees.