5 Tricky Java Questions That Reveal Hidden Pitfalls

In the Java community, many seemingly simple code snippets hide subtle traps that can surprise even experienced developers. Below are five classic questions, each with a code example and multiple‑choice answers, designed to test and deepen your understanding of Java fundamentals.

1. Primitive Float Comparison

public class FloatPrimitiveTest {
    public static void main(String[] args) {
        float a = 1.0f - 0.9f;
        float b = 0.9f - 0.8f;
        if (a == b) {
            System.out.println("true");
        } else {
            System.out.println("false");
        }
    }
}

Which output is correct?

A: true

B: false

C: Determined by hardware instructions

2. Wrapper Float Equality

public class FloatWrapperTest {
    public static void main(String[] args) {
        Float a = Float.valueOf(1.0f - 0.9f);
        Float b = Float.valueOf(0.9f - 0.8f);
        if (a.equals(b)) {
            System.out.println("true");
        } else {
            System.out.println("false");
        }
    }
}

Which output is correct?

A: true

B: false

C: Compilation error

3. Switch on a Null String

public class SwitchTest {
    public static void main(String[] args) {
        String param = null;
        switch (param) {
            case "null":
                System.out.println("null");
                break;
            default:
                System.out.println("default");
        }
    }
}

Which statement is correct?

A: null

B: Throws an exception

C: default

4. BigDecimal Construction

public class BigDecimalTest {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(0.1);
        System.out.println(a);
        BigDecimal b = new BigDecimal("0.1");
        System.out.println(b);
    }
}

Which statement is correct?

A: Both assignment methods are identical

B: The first method (using double) is recommended

C: The second method (using String) is recommended

5. ReentrantLock Usage

public class LockTest {
    private final static Lock lock = new ReentrantLock();

    public static void main(String[] args) {
        try {
            lock.tryLock();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

Which statement is false?

A: The lock is a non‑fair lock

B: The finally block will never throw an exception

C: If tryLock fails, execution continues without acquiring the lock

These questions are derived from real‑world discussions among developers and aim to highlight subtle language behaviors that can lead to bugs if misunderstood.