Java compilation and execution flow - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Java compilation and execution steps

What will be the output when the following Java program is compiled and run?

Java

public class Test {
    public static void main(String[] args) {
        System.out.println("Step 1");
        int x = 5;
        System.out.println("Value: " + x);
    }
}

A Runtime error

B Step 1 Value: 5

C Compilation error

D

Step 1
Value: 5

💡 Hint

Remember that System.out.println prints with a newline after the message.

The program prints "Step 1" followed by a newline, then prints "Value: 5" on the next line. So the output has two lines.

🧠 Conceptual

intermediate

Java compilation produces which file?

When you compile a Java source file named Example.java, what file is generated?

A Example.class

B Example.exe

C Example.java

D Example.jar

💡 Hint

Think about the file that the Java Virtual Machine runs.

The Java compiler creates a .class file containing bytecode that the JVM executes.

🔧 Debug

advanced

Identify the error in Java compilation

What error will the Java compiler report for this code?

Java

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

A Runtime exception

B Class not found error

C Missing semicolon error

D No error, runs fine

💡 Hint

Check the end of the System.out.println statement.

The statement is missing a semicolon at the end, causing a compilation error.

❓ Predict Output

advanced

Output of Java program with static block and main

What is the output when this Java program is compiled and run?

Java

public class StaticDemo {
    static {
        System.out.println("Static block executed");
    }
    public static void main(String[] args) {
        System.out.println("Main method executed");
    }
}

A

Main method executed
Static block executed

B

Static block executed
Main method executed

C Static block executed

D Main method executed

💡 Hint

Static blocks run before main method when class loads.

The static block runs first when the class loads, then main runs.

🧠 Conceptual

expert

Java execution flow with multiple classes

Given two Java classes, Main and Helper, where Main calls Helper's static method, what is the correct order of compilation and execution?

A Compile Helper.java and Main.java, then run Main; Helper's static method runs when called.

B Run Main.java directly without compiling; Helper.java compiles automatically.

C Compile only Main.java; Helper.java is ignored.

D Compile Helper.java only; Main.java runs Helper's method automatically.

💡 Hint

Java requires all classes to be compiled before running the main class.

Both classes must be compiled before running Main. Helper's static method runs only when called.

Practice

1. What is the first step when running a Java program?

easy

A. Compile the Java source code into bytecode using javac

B. Run the bytecode directly using the java command

C. Convert bytecode into machine code manually

D. Write the Java source code in a text editor

Solution

1. Step 1: Understand Java program execution

   Java source code must first be compiled into bytecode before running.

2. Step 2: Identify the compilation command

   The javac command compiles source code into bytecode.

3. Final Answer:

   Compile the Java source code into bytecode using javac -> Option A

4. Quick Check:

   Compilation = javac [OK]

Hint: Remember: javac compiles, java runs bytecode [OK]

Common Mistakes:

• Thinking java command compiles code
• Confusing source code with bytecode
• Skipping compilation step

2. Which command correctly runs a compiled Java program named HelloWorld?

easy

A. javac HelloWorld.java

B. java HelloWorld

C. java HelloWorld.java

D. javac HelloWorld

Solution

1. Step 1: Identify the command to run bytecode

   The java command runs compiled bytecode, not source files.

2. Step 2: Match the correct syntax

   Use java HelloWorld without the .java extension to run the program.

3. Final Answer:

   java HelloWorld -> Option B

4. Quick Check:

   Run bytecode = java ClassName [OK]

Hint: Run with java ClassName, no .java extension [OK]

Common Mistakes:

• Trying to run source file with java command
• Using javac to run program
• Including .java extension when running

3. Given the commands below, what will be the output?

javac Test.java
java Test

Assuming Test.java contains:

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

medium

A. Compilation error

B. Runtime error

C. Hello Java

D. No output

Solution

1. Step 1: Compile the source code

   javac Test.java compiles the code into bytecode without errors.

2. Step 2: Run the compiled bytecode

   java Test runs the program and prints "Hello Java".

3. Final Answer:

   Hello Java -> Option C

4. Quick Check:

   javac + java = program output [OK]

Hint: Compile first, then run to see output [OK]

Common Mistakes:

• Expecting output without compiling
• Confusing compilation and runtime errors
• Not including main method

4. What is wrong with this sequence of commands?

java HelloWorld
javac HelloWorld.java

medium

A. Running before compiling causes an error

B. javac should be run after java

C. Commands are correct and will work

D. HelloWorld.java file is missing

Solution

1. Step 1: Understand command order

   You must compile source code before running it.

2. Step 2: Identify error cause

   Running java HelloWorld before compiling causes a runtime error because bytecode is missing.

3. Final Answer:

   Running before compiling causes an error -> Option A

4. Quick Check:

   Compile first, then run [OK]

Hint: Always compile before running Java programs [OK]

Common Mistakes:

• Running java command before javac
• Assuming java compiles automatically
• Ignoring missing .class files

5. You have a Java source file App.java that depends on another class Helper.java. Which sequence correctly compiles and runs the program?

hard

A. javac App.java
java Helper
java App

B. java App
javac Helper.java
javac App.java

C. javac App.java Helper.java
java Helper

D. javac Helper.java
javac App.java
java App

Solution

1. Step 1: Compile dependencies first

   Compile Helper.java before App.java because App depends on Helper.

2. Step 2: Run the main program

   After compiling both, run java App to execute the program.

3. Final Answer:

   javac Helper.java javac App.java java App -> Option D

4. Quick Check:

   Compile dependencies first, then run main [OK]

Hint: Compile dependencies before main class, then run main [OK]

Common Mistakes:

• Running before compiling
• Compiling only main class
• Running wrong class with java command