Java platform and JVM overview - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print "Hello, JVM!" in Java.

Java

public class Main {
    public static void main(String[] args) {
        System.[1]("Hello, JVM!");
    }
}

Drag options to blanks, or click blank then click option'

Aprintln

Becho

Cwrite

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'print' which does not add a new line.

Using 'write' or 'echo' which are not valid methods here.

The println method prints the text and moves to a new line, which is the standard way to output in Java.

2fill in blank

medium

Complete the code to declare a variable of type int with value 10.

Java

public class Main {
    public static void main(String[] args) {
        [1] number = 10;
    }
}

Drag options to blanks, or click blank then click option'

Adouble

Bint

CString

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'String' which is for text.

Using 'double' which is for decimal numbers.

The int type is used to store whole numbers like 10.

3fill in blank

hard

Fix the error in the code to correctly create a new String object.

Java

public class Main {
    public static void main(String[] args) {
        String text = new [1]("Java");
    }
}

Drag options to blanks, or click blank then click option'

Astring

BStringBuilder

CString

DText

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'string' which is not a class in Java.

Using 'StringBuilder' which is a different class.

The class name String must be capitalized exactly to create a new String object.

4fill in blank

hard

Fill both blanks to create a simple for loop that prints numbers 1 to 5.

Java

public class Main {
    public static void main(String[] args) {
        for (int i = [1]; i [2] 5; i++) {
            System.out.println(i);
        }
    }
}

Drag options to blanks, or click blank then click option'

A1

B0

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from 0 which prints 0 to 4.

Using '<' which excludes 5.

The loop starts at 1 and continues while i is less than or equal to 5 to print numbers 1 through 5.

5fill in blank

hard

Fill all three blanks to create a method that returns the square of a number.

Java

public class Main {
    public static int [1](int [2]) {
        return [2] [3] [2];
    }
}

Drag options to blanks, or click blank then click option'

Asquare

Bnum

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' which adds instead of multiplies.

Using unclear variable names.

The method named 'square' takes an integer 'num' and returns its multiplication by itself.

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