Import statement usage in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an import statement in Java?

An import statement allows you to use classes and interfaces from other packages without writing their full package names every time.

beginner

How do you import all classes from a package named <code>java.util</code>?

You use <code>import java.util.*;</code> to import all classes from the <code>java.util</code> package.

beginner

True or False: You can import a single class using its full package name.

True. For example, <code>import java.util.ArrayList;</code> imports only the <code>ArrayList</code> class.

beginner

What happens if you don't use an import statement but want to use a class from another package?

You must use the full package name every time you refer to the class, like java.util.ArrayList instead of just ArrayList.

beginner

Can you import classes from the same package using an import statement?

No, classes in the same package are accessible without import statements.

Which import statement imports all classes from the java.io package?

A import java.io;

B import java.io.*;

C import * from java.io;

D import java.io.all;

The correct syntax to import all classes from a package is import packageName.*;.

What is the correct way to import only the Scanner class from java.util?

A import java.util.Scanner;

B import java.util.*;

C import Scanner from java.util;

D import java.util.Scanner.*;

To import a single class, use import packageName.ClassName;.

If you do not import a class, how can you still use it?

A By using a wildcard import

B By guessing the class name

C By writing its full package name every time

D By declaring it again

Without import, you must use the full package path to refer to the class.

Can you import a class from the same package?

A Only if the class is private

B Yes, always required

C Only if the class is public

D No, it is not necessary

Classes in the same package are accessible without import statements.

What does the import statement import java.util.*; do?

A Imports all classes in java.util package

B Imports only the java.util package itself

C Imports all sub-packages of java.util

D Imports only the first class in java.util

The wildcard * imports all classes in the specified package, but not sub-packages.

Explain why import statements are useful in Java and how they simplify code.

Think about how you would write code without import statements.

Describe the difference between importing a single class and using a wildcard import.

Consider how many classes each import brings into your code.

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