What is Java - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is Java primarily used for?

Java is a popular programming language. What is its main use?

A Creating mobile apps, web applications, and large systems

B Only for designing websites with HTML and CSS

C Writing scripts to automate Excel spreadsheets

D Building hardware devices and microchips

💡 Hint

Think about where you see Java programs running, like apps or websites.

Java is widely used to build mobile apps (especially Android), web applications, and large software systems because it is versatile and runs on many devices.

❓ Predict Output

intermediate

What is the output of this Java code?

Look at this Java program and choose the output it produces when run.

Java

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

A Sum is: 15

B Sum is: 510

C Sum is: x + y

D Sum is: 5 + 10

💡 Hint

Remember that parentheses change how expressions are calculated in Java.

The parentheses around (x + y) make Java add the numbers first, then convert to string. So it prints "Sum is: 15".

❓ Predict Output

advanced

What does this Java code print?

Analyze the code and select the exact output it produces.

Java

public class Main {
    public static void main(String[] args) {
        String s = "Java";
        s += 123;
        System.out.println(s);
    }
}

A Java 123

B Java123

C Java+123

D Error: cannot add int to String

💡 Hint

In Java, adding a number to a string converts the number to text.

Using += with a string and a number converts the number to string and joins them. So "Java" + 123 becomes "Java123".

🔧 Debug

advanced

What error does this Java code cause?

Look at this Java code and choose the error it produces when compiled.

Java

public class Main {
    public static void main(String[] args) {
        int number = "100";
        System.out.println(number);
    }
}

A Syntax error: missing semicolon

B Variable number might not have been initialized

C Type mismatch: cannot convert from String to int

D No error, prints 100

💡 Hint

Check the type of the value assigned to an int variable.

You cannot assign a text (String) value to an int variable directly. This causes a type mismatch error.

🧠 Conceptual

expert

Which feature makes Java platform-independent?

Java is called platform-independent. What feature allows this?

A Java programs must be rewritten for each operating system

B Java programs are written in machine code for each platform

C Java uses HTML to run in browsers

D Java Virtual Machine (JVM) that runs bytecode on any device

💡 Hint

Think about how Java runs the same program on different computers without changes.

Java compiles code into bytecode, which the JVM interprets on any device, making it platform-independent.

Practice

1. What is Java primarily used for?

easy

A. Writing programs that can run on many devices

B. Designing websites with HTML and CSS

C. Creating spreadsheets and documents

D. Managing databases with SQL

Solution

1. Step 1: Understand Java's purpose

   Java is a programming language designed to write software that works on different devices.

2. Step 2: Compare with other options

   Options A, B, and D relate to other technologies, not Java's main use.

3. Final Answer:

   Writing programs that can run on many devices -> Option A

4. Quick Check:

   Java = cross-device programming language [OK]

Hint: Java runs programs on many devices, not just one [OK]

Common Mistakes:

• Confusing Java with web design languages
• Thinking Java is for document editing
• Mixing Java with database query languages

2. Which of the following is the correct way to start a Java program?

easy

A. def main():

B. function main() {}

C. public static void main(String[] args) {}

D. start() {}

Solution

1. Step 1: Identify Java's main method syntax

   Java programs start with the method: public static void main(String[] args) {}.

2. Step 2: Recognize other languages' syntax

   Options B, C, and D are syntax from JavaScript, Python, or invalid.

3. Final Answer:

   public static void main(String[] args) {} -> Option C

4. Quick Check:

   Java main method = public static void main(String[] args) [OK]

Hint: Java main method always uses 'public static void main(String[] args)' [OK]

Common Mistakes:

• Using Python or JavaScript syntax instead
• Omitting 'static' or 'public' keywords
• Writing main without parameters

3. What will the following Java code print?

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

medium

A. Hello Java

B. Hello, Java!

C. System.out.println("Hello, Java!");

D. Error: main method missing

Solution

1. Step 1: Read the code output statement

   The code calls System.out.println with the string "Hello, Java!" which prints exactly that.

2. Step 2: Confirm no syntax errors

   The main method and class are correctly defined, so it runs without error.

3. Final Answer:

   Hello, Java! -> Option B

4. Quick Check:

   Print statement outputs string exactly [OK]

Hint: System.out.println prints exactly the text inside quotes [OK]

Common Mistakes:

• Ignoring punctuation in output
• Confusing code with output
• Assuming missing main method error

4. Identify the error in this Java program:

public class Sample {
  public static void main(String args) {
    System.out.println("Test");
  }
}

medium

A. System.out.println is misspelled

B. Missing semicolon after println

C. Class name should be lowercase

D. Missing square brackets [] in main method parameter

Solution

1. Step 1: Check main method parameter

   The main method must have String[] args, but here it is String args without brackets.

2. Step 2: Verify other parts

   Semicolon is present, class name capitalization is allowed, and println is spelled correctly.

3. Final Answer:

   Missing square brackets [] in main method parameter -> Option D

4. Quick Check:

   Main method parameter must be String[] args [OK]

Hint: Main method parameter needs brackets: String[] args [OK]

Common Mistakes:

• Omitting [] in main method parameter
• Thinking class name must be lowercase
• Assuming missing semicolon error

5. You want to write a Java program that can run on any device without changes. Which feature of Java makes this possible?

hard

A. Java programs are compiled into bytecode run by the JVM

B. Java programs are interpreted directly from source code

C. Java programs require device-specific compilation

D. Java programs use HTML for compatibility

Solution

1. Step 1: Understand Java compilation

   Java code is compiled into bytecode, which is not tied to any device.

2. Step 2: Role of JVM

   The JVM (Java Virtual Machine) runs this bytecode on any device, enabling portability.

3. Final Answer:

   Java programs are compiled into bytecode run by the JVM -> Option A

4. Quick Check:

   Bytecode + JVM = cross-device Java programs [OK]

Hint: Java bytecode runs on JVM anywhere, no device changes needed [OK]

Common Mistakes:

• Thinking Java runs source code directly
• Assuming device-specific compilation is needed
• Confusing Java with web languages like HTML