Java platform and JVM overview - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of JVM Bytecode Execution

What is the output of the following Java program when run on the JVM?

Java

public class Main {
    public static void main(String[] args) {
        int x = 5;
        int y = 10;
        int z = x + y;
        System.out.println(z);
    }
}

A 5 + 10

B 15

C Compilation error

D 510

💡 Hint

Remember that the + operator adds numbers in Java.

The program adds two integers 5 and 10, resulting in 15, which is printed.

🧠 Conceptual

intermediate

Role of JVM in Java Platform

Which of the following best describes the role of the JVM in the Java platform?

A It interprets compiled Java bytecode and manages memory and execution on any platform.

B It compiles Java source code directly into machine code specific to the operating system.

C It converts Java bytecode into Python code for cross-language compatibility.

D It only checks syntax errors in Java source code before compilation.

💡 Hint

Think about how Java achieves platform independence.

The JVM runs Java bytecode on any platform by interpreting it and managing execution and memory.

🔧 Debug

advanced

Identify the JVM Error

What error will the JVM throw when running this code?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = new int[3];
        System.out.println(arr[5]);
    }
}

A ArrayIndexOutOfBoundsException

B ClassCastException

C NullPointerException

D No error, prints 0

💡 Hint

Check the array index used for access.

Accessing index 5 in an array of size 3 causes ArrayIndexOutOfBoundsException at runtime.

📝 Syntax

advanced

Identify JVM Bytecode Syntax Error

Which option contains a syntax error that prevents JVM from running the code?

Java

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello JVM");
    }
}

A

ublic class Main {
    public static void main(String[] args) {
        System.out.println("Hello JVM");
    }
}

B

}
}    
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C

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello JVM");
    }
}

D

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello JVM");
    }

💡 Hint

Look carefully at punctuation in the code.

Option C is missing a semicolon after the print statement, causing a syntax error.

🚀 Application

expert

JVM Memory Management Behavior

Given this Java code snippet, what will be the state of the heap after execution?

Java

public class Main {
    public static void main(String[] args) {
        String a = new String("test");
        String b = a;
        a = null;
        System.gc();
    }
}

A The string object "test" is deleted from heap before System.gc() is called.

B The string object "test" is eligible for garbage collection immediately after a = null.

C The string object "test" is moved to stack memory after System.gc().

D The string object "test" remains in heap because b still references it.

💡 Hint

Consider which variables still reference the object.

Since b still references the string object, it is not eligible for garbage collection.

Practice

1. What is the main role of the Java Virtual Machine (JVM) in the Java platform?

easy

A. To convert Java bytecode back to source code

B. To write Java source code

C. To run compiled Java bytecode on any device

D. To manage Java libraries only

Solution

1. Step 1: Understand JVM's purpose

   The JVM executes Java bytecode, which is the compiled form of Java source code.

2. Step 2: Identify JVM's role in portability

   Because JVM runs bytecode on any device, it makes Java programs portable and platform-independent.

3. Final Answer:

   To run compiled Java bytecode on any device -> Option C

4. Quick Check:

   JVM runs bytecode = C [OK]

Hint: JVM runs bytecode anywhere, enabling portability [OK]

Common Mistakes:

• Confusing JVM with Java compiler
• Thinking JVM writes source code
• Believing JVM manages only libraries

2. Which of the following is the correct way to compile a Java source file named HelloWorld.java using the command line?

easy

A. java HelloWorld.java

B. run HelloWorld.java

C. compile HelloWorld.java

D. javac HelloWorld.java

Solution

1. Step 1: Identify Java compilation command

   The Java compiler command is javac, used to compile source files into bytecode.

2. Step 2: Match command to file

   Using javac HelloWorld.java compiles the source file correctly.

3. Final Answer:

   javac HelloWorld.java -> Option D

4. Quick Check:

   Compile with javac = B [OK]

Hint: Use 'javac' to compile Java files, not 'java' [OK]

Common Mistakes:

• Using 'java' to compile instead of run
• Typing 'compile' which is not a Java command
• Confusing 'run' with compile command

3. You want to write a Java program that runs on multiple devices without changing code. Which Java platform feature ensures this portability?

easy

A. Java Virtual Machine (JVM) running bytecode

B. Java source code compatibility

C. Java compiler generating machine code

D. Java libraries only

Solution

1. Step 1: Understand portability in Java

   Java programs are portable because compiled bytecode runs on any device with a JVM.

2. Step 2: Identify the feature enabling portability

   The JVM interprets bytecode on different devices, making the same program run everywhere without changes.

3. Final Answer:

   Java Virtual Machine (JVM) running bytecode -> Option A

4. Quick Check:

   JVM enables portability = A [OK]

Hint: JVM runs bytecode anywhere for portability [OK]

Common Mistakes:

• Confusing source code compatibility with portability
• Thinking compiler generates machine-specific code
• Believing libraries alone ensure portability

4. What will be the output when the following Java code is compiled and run?

public class Test {
    public static void main(String[] args) {
        System.out.println("Java runs on " + System.getProperty("os.name"));
    }
}

medium

A. Java runs on [Your OS name]

B. Java runs on Linux

C. Java runs on Windows

D. Compilation error

Solution

1. Step 1: Understand System.getProperty("os.name")

   This method returns the name of the operating system where the program runs.

2. Step 2: Analyze output concatenation

   The program prints "Java runs on " plus the OS name dynamically, so output depends on the actual OS.

3. Final Answer:

   Java runs on [Your OS name] -> Option A

4. Quick Check:

   Output shows actual OS name = A [OK]

Hint: System.getProperty("os.name") shows your OS name [OK]

Common Mistakes:

• Assuming output is fixed to Windows or Linux
• Expecting a compilation error
• Not understanding dynamic OS detection

5. Identify the error in the following Java code snippet related to JVM and platform:

public class Demo {
    public static void main(String[] args) {
        bytecode.run();
    }
}

medium

A. No error, code runs fine

B. bytecode is not a valid object or method

C. Syntax error in class declaration

D. Missing main method

Solution

1. Step 1: Check usage of 'bytecode.run()'

   The code calls bytecode.run(), but 'bytecode' is not defined as an object or class.

2. Step 2: Verify class and method correctness

   The class and main method are correctly declared, so no syntax error there.

3. Final Answer:

   bytecode is not a valid object or method -> Option B

4. Quick Check:

   Undefined 'bytecode' causes error = D [OK]

Hint: Undefined variables cause runtime errors [OK]

Common Mistakes:

• Thinking main method is missing
• Assuming class declaration is wrong
• Believing code runs without error