Core Java mock test: free online practice

What does the Core Java mock test cover?

The test covers 12 topics across three levels. Each attempt picks 15 to 30 questions at random, by the level you choose, so every attempt is different.

• Collections
• Concurrency
• Exceptions
• Inheritance and Interfaces
• Streams and Lambdas
• OOP and Classes
• Strings
• Data Types and Operators
• Generics
• JVM and Memory
• equals and hashCode
• Modern Java

Sample Core Java mock test questions

One question from each level, the way they appear in the test. A new set comes on the 1st of every month. Try it, then open the answer.

1. FresherStreams and Lambdas

   Which of these is a terminal operation on a Stream?

   • filter()
   • count()
   • map()
   • sorted()
   Show the answer

   Answer: count(). Intermediate operations such as filter, map and sorted return a new Stream and do nothing until a terminal operation runs. Terminal operations like count, collect and forEach consume the stream and produce a result or a side effect.

2. Intermediate (1-5 years)Collections

   What does this code print?

   List<Integer> ids = new ArrayList<>(List.of(10, 20, 30, 40));
   ids.remove(1);
   ids.remove(Integer.valueOf(40));
   System.out.println(ids);

   • [30]
   • [20, 30]
   • [10, 20, 30]
   • [10, 30]
   Show the answer

   Answer: [10, 30]. remove(1) with an int argument calls remove(int index) and removes the element at index 1, which is 20. remove(Integer.valueOf(40)) calls remove(Object) and removes the value 40. Mixing these up is a classic List<Integer> bug.

3. Experienced (5+ years)Strings

   Why do security guidelines suggest holding a password in a char[] rather than a String?

   • The array can be wiped right after use, while a String stays in memory until collected
   • The JVM encrypts the contents of char arrays, while Strings are always stored as plain text
   • A char[] never appears in heap dumps, because arrays are stored outside the heap
   • Strings cannot hold special characters such as @ or #, which passwords often contain
   Show the answer

   Answer: The array can be wiped right after use, while a String stays in memory until collected. Strings are immutable, so you cannot wipe one; it may linger in the heap and show up in a heap dump. A char[] can be cleared with Arrays.fill as soon as it has been checked, which shortens the exposure. Console.readPassword returns char[].

Take the full Core Java mock test: 15 to 30 questions and 15 to 30 minutes, by level.

How is the Core Java mock test scored?

A question counts as right only when you pick exactly the right option, or all the right options where it says so; a question left blank counts as wrong. Your score is the share of the test's questions you got right (15 to 30, by level). The result also scores every topic, so you know what to revise before the interview.

• 75% or more in a topica strength
• 50% to 74%worth a revision
• Under 50%a topic to work on