User-defined functions (UDFs) in Hadoop - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple UDF that returns the length of a string.

Hadoop

public class StringLengthUDF extends UDF {
    public int evaluate(String input) {
        return input[1];
    }
}

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

A.length()

B.size()

C.count()

D.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length without parentheses, which is a property in arrays but not in String.

Using .size() or .count(), which are not valid String methods.

The correct method to get the length of a string in Java is .length() with parentheses.

2fill in blank

medium

Complete the code to register the UDF in Hive using the correct syntax.

Hadoop

CREATE TEMPORARY FUNCTION string_len AS '[1]';

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

Astring_len

BStringLengthUDF

Ccom.example.StringLengthUDF

Dcom.example.udf.StringLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using only the class name without package.

Using the function name instead of the class path.

The full class path including package is needed to register the UDF in Hive.

3fill in blank

hard

Fix the error in the UDF evaluate method to handle null input safely.

Hadoop

public Integer evaluate(String input) {
    if (input == null) {
        return [1];
    }
    return input.length();
}

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

Anull

B0

C-1

Dthrow new IllegalArgumentException()

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 or -1 which may be misleading as length.

Throwing an exception which breaks query execution.

Returning null for null input is a common safe practice in UDFs to avoid errors.

4fill in blank

hard

Fill both blanks to create a UDF that converts input string to uppercase safely.

Hadoop

public class ToUpperUDF extends UDF {
    public String evaluate(String input) {
        if (input == null) {
            return [1];
        }
        return input[2];
    }
}

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

Anull

B""

C.toUpperCase()

D.toLowerCase()

Attempts:

3 left

💡 Hint

Common Mistakes

Returning empty string instead of null.

Using .toLowerCase() instead of .toUpperCase().

Return null if input is null, and use .toUpperCase() to convert string to uppercase.

5fill in blank

hard

Fill all three blanks to create a UDF that returns the first character of a string or null if empty.

Hadoop

public class FirstCharUDF extends UDF {
    public String evaluate(String input) {
        if (input == null || input[1] 0) {
            return [2];
        }
        return input[3];
    }
}

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

A.length() <=

Bnull

C.substring(0, 1)

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == 0 instead of .length() <= 0.

Returning empty string instead of null.

Using substring with wrong indexes.

Check if length is 0 or less, return null, else return substring of first character.

Practice

1. What is the main purpose of a User-defined Function (UDF) in Hadoop?

easy

A. To add custom data processing logic not available in built-in functions

B. To store data in Hadoop Distributed File System (HDFS)

C. To manage cluster resources automatically

D. To monitor network traffic between nodes

Solution

1. Step 1: Understand what UDFs do in Hadoop

   UDFs allow users to write their own functions to process data beyond what built-in functions offer.

2. Step 2: Compare options with UDF purpose

   Only To add custom data processing logic not available in built-in functions describes adding custom data processing logic, which matches UDF use.

3. Final Answer:

   To add custom data processing logic not available in built-in functions -> Option A

4. Quick Check:

   UDF purpose = custom processing [OK]

Hint: UDFs add your own data steps in Hadoop [OK]

Common Mistakes:

• Confusing UDFs with storage or resource management
• Thinking UDFs handle system monitoring
• Assuming UDFs replace built-in functions entirely

2. Which method must you implement when creating a Hadoop UDF class?

easy

A. initialize()

B. execute()

C. evaluate()

D. process()

Solution

1. Step 1: Recall UDF class requirements

   Hadoop UDFs must extend the UDF class and implement the evaluate method to process input.

2. Step 2: Match method names

   Only evaluate() is the required method for UDF logic; others are incorrect or do not exist.

3. Final Answer:

   evaluate() -> Option C

4. Quick Check:

   UDF method = evaluate() [OK]

Hint: UDFs always implement evaluate() method [OK]

Common Mistakes:

• Using initialize() or execute() instead of evaluate()
• Forgetting to override evaluate()
• Confusing with other Hadoop APIs

3. Given this UDF code snippet, what will be the output when input is null?

public class MyUDF extends UDF {
  public String evaluate(String input) {
    if (input == null) return "empty";
    return input.toUpperCase();
  }
}

medium

A. null

B. "empty"

C. "EMPTY"

D. Runtime error

Solution

1. Step 1: Analyze input null check in evaluate()

   The code checks if input is null and returns the string "empty" exactly as is.

2. Step 2: Determine output for null input

   Since input is null, the method returns "empty" (lowercase), not uppercase or error.

3. Final Answer:

   "empty" -> Option B

4. Quick Check:

   null input returns "empty" [OK]

Hint: Check null input handling in evaluate() [OK]

Common Mistakes:

• Assuming output is uppercase "EMPTY"
• Expecting null to be returned
• Thinking code throws an error on null

4. Identify the error in this UDF code:

public class SampleUDF extends UDF {
  public Integer evaluate(Integer num) {
    return num + 10;
  }
}

medium

A. Method name should be process, not evaluate

B. evaluate method must return String, not Integer

C. Class must implement interface, not extend UDF

D. Missing null check for input parameter

Solution

1. Step 1: Review evaluate method logic

   The method adds 10 to input num but does not check if num is null, which can cause NullPointerException.

2. Step 2: Validate other options

   Returning Integer is allowed; extending UDF is correct; method name evaluate is required.

3. Final Answer:

   Missing null check for input parameter -> Option D

4. Quick Check:

   Always check for null inputs in UDFs [OK]

Hint: Always check for null inputs in evaluate() [OK]

Common Mistakes:

• Ignoring null input checks
• Confusing return types for evaluate()
• Misunderstanding class inheritance rules

5. You want to create a UDF that returns the length of a string, but returns 0 if input is null or empty. Which evaluate method implementation is correct?

public Integer evaluate(String input) {
  // Choose the correct code
}

hard

A. if (input == null || input.isEmpty()) return 0; else return input.length();

B. return input.length();

C. if (input == null) return null; else return input.length();

D. return input == null ? 0 : input.length() + 1;

Solution

1. Step 1: Understand requirements for null or empty input

   The method must return 0 if input is null or empty string, otherwise return string length.

2. Step 2: Evaluate each option

   if (input == null || input.isEmpty()) return 0; else return input.length(); correctly checks null or empty and returns 0; others either miss empty check or return wrong values.

3. Final Answer:

   if (input == null || input.isEmpty()) return 0; else return input.length(); -> Option A

4. Quick Check:

   Null or empty returns 0, else length [OK]

Hint: Check both null and empty before length [OK]

Common Mistakes:

• Not checking for empty string
• Returning null instead of 0
• Adding extra 1 to length incorrectly