Import statement usage in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code using import?

Consider the following Java code snippet. What will it print when run?

Java

import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("apple");
        list.add("banana");
        System.out.println(list.get(1));
    }
}

A banana

B apple

C Compilation error due to missing import

D Runtime error: IndexOutOfBoundsException

💡 Hint

Remember that ArrayList is part of java.util package.

The code imports java.util.ArrayList correctly, so it compiles and runs fine. The list has two elements: "apple" at index 0 and "banana" at index 1. The code prints the element at index 1, which is "banana".

❓ Predict Output

intermediate

What error occurs if import is missing?

What error will this Java code produce if the import statement is removed?

Java

import java.util.HashMap;

public class Test {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("one", 1);
        System.out.println(map.get("one"));
    }
}

A Compilation error: missing semicolon

B Runtime error: NullPointerException

C Compilation error: cannot find symbol HashMap

D Prints 1

💡 Hint

Think about what happens if you use a class without importing it or fully qualifying it.

If the import statement is removed, the compiler does not know what HashMap is, causing a compilation error: cannot find symbol HashMap.

🔧 Debug

advanced

Why does this import cause a conflict?

Given the following code, why does it cause a compilation error?

Java

import java.util.Date;
import java.sql.Date;

public class Test {
    public static void main(String[] args) {
        Date d = new Date(0);
        System.out.println(d);
    }
}

A Compilation error due to ambiguous reference to Date

B Prints the current date and time

C Prints 0

D Runtime error: ClassCastException

💡 Hint

Think about what happens when two classes with the same name are imported.

Importing both java.util.Date and java.sql.Date causes ambiguity. The compiler cannot decide which Date class to use, resulting in a compilation error.

📝 Syntax

advanced

Which import statement is syntactically correct?

Which of the following import statements is correct in Java?

A import java.util.*

B import java.util.*;

C import java.util.*;;

D import java.util

💡 Hint

Remember the syntax for import statements requires a semicolon at the end.

Option B is correct because it uses the wildcard * and ends with a semicolon. Option B is missing the semicolon, C has an extra semicolon, and D is missing the wildcard and semicolon.

🚀 Application

expert

How to use fully qualified class name to avoid import conflict?

You have two classes named List in different packages: java.awt.List and java.util.List. You want to use both in the same file without import statements. How do you declare variables of each type?

A import java.awt.List; import java.util.List; List awtList; List utilList;

B List awtList = new List(); List utilList = new List();

C java.awt.List awtList = new List(); java.util.List utilList = new ArrayList();

D java.awt.List awtList = new java.awt.List(); java.util.List<String> utilList = new java.util.ArrayList<>();

💡 Hint

Use the full package name before the class name to avoid ambiguity.

Option D uses fully qualified names to specify which List class is used. This avoids conflicts without import statements. Other options either omit package names or mix imports incorrectly.

Practice

1. What is the main purpose of an import statement in Java?

easy

A. To allow the use of classes from other packages without full package names

B. To declare a new package for the current class

C. To create a new class in the program

D. To start the main method of the program

Solution

1. Step 1: Understand the role of import statements

   Import statements let you use classes from other packages without writing their full package names every time.

2. Step 2: Differentiate from other Java keywords

   Import does not declare packages, create classes, or start methods; it only helps with class usage.

3. Final Answer:

   To allow the use of classes from other packages without full package names -> Option A

4. Quick Check:

   Import = Use classes easily [OK]

Hint: Import lets you use classes without full package names [OK]

Common Mistakes:

• Confusing import with package declaration
• Thinking import creates classes
• Assuming import runs code

2. Which of the following is the correct syntax to import all classes from the package java.util?

easy

A. import java.util.all;

B. import java.util;

C. import * from java.util;

D. import java.util.*;

Solution

1. Step 1: Recall the syntax for importing all classes

   To import all classes from a package, use the asterisk (*) after the package name with a semicolon.

2. Step 2: Check each option's syntax

   Only import java.util.*; is correct. Others miss the asterisk or use wrong keywords.

3. Final Answer:

   import java.util.*; -> Option D

4. Quick Check:

   Use * to import all classes [OK]

Hint: Use * after package name to import all classes [OK]

Common Mistakes:

• Omitting the asterisk (*) when importing all classes
• Missing semicolon at the end
• Using incorrect keywords like 'all' or 'from'

3. What will be the output of the following code?

import java.util.ArrayList;
public class Test {
  public static void main(String[] args) {
    ArrayList<String> list = new ArrayList<>();
    list.add("Hello");
    list.add("World");
    System.out.println(list.get(1));
  }
}

medium

A. ArrayList

B. Hello

C. World

D. Compilation error

Solution

1. Step 1: Understand the import and list usage

   The code imports ArrayList correctly and creates a list with "Hello" at index 0 and "World" at index 1.

2. Step 2: Check the printed element

   list.get(1) accesses the second element, which is "World".

3. Final Answer:

   World -> Option C

4. Quick Check:

   list.get(1) = World [OK]

Hint: Index 1 means second element in list [OK]

Common Mistakes:

• Confusing index 0 and 1
• Assuming import causes error
• Thinking output is class name

4. Identify the error in the following code snippet:

import java.util.List;
public class Demo {
  public static void main(String[] args) {
    List<String> items = new ArrayList<>();
  }
}

medium

A. Missing semicolon after import statement

B. ArrayList is not imported

C. List cannot be used without initialization

D. No error, code is correct

Solution

1. Step 1: Check import statements

   The code imports List but does not import ArrayList, which is used in the code.

2. Step 2: Identify missing import causing error

   ArrayList must be imported from java.util to be used; missing import causes compilation error.

3. Final Answer:

   ArrayList is not imported -> Option B

4. Quick Check:

   Missing import for ArrayList causes error [OK]

Hint: Import all classes you use explicitly [OK]

Common Mistakes:

• Forgetting semicolon after import
• Assuming List needs initialization differently
• Ignoring missing imports for used classes

5. You want to use both java.util.Date and java.sql.Date in the same Java file. How should you import them to avoid confusion?

hard

A. import java.util.Date;
// Use fully qualified name for java.sql.Date in code

B. import java.util.*;
import java.sql.*;

C. import java.util.Date;
import java.sql.Date;

D. import java.util.*;
import java.sql.Date;

Solution

1. Step 1: Understand class name conflict

   Both packages have a class named Date, so importing both directly causes ambiguity.

2. Step 2: Use import for one and full name for the other

   Import one Date class and use the full package name for the other to avoid confusion.

3. Final Answer:

   import java.util.Date; // Use fully qualified name for java.sql.Date in code -> Option A

4. Quick Check:

   Import one Date, fully qualify the other [OK]

Hint: Import one class, fully qualify the other to avoid name clash [OK]

Common Mistakes:

• Importing both classes directly causing ambiguity
• Using wildcard imports causing confusion
• Not using fully qualified names when needed