Common wrapper methods in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of wrapper classes in Java?

Wrapper classes provide a way to use primitive data types (like int, double) as objects. They allow primitives to be treated like objects, which is useful for collections and methods that require objects.

beginner

How do you convert a String to an int using a wrapper method?

Use Integer.parseInt(String s) to convert a String to an int. For example, int num = Integer.parseInt("123"); converts the string "123" to the integer 123.

intermediate

What does the method Integer.valueOf(String s) do?

It converts a String to an Integer object. Unlike parseInt which returns a primitive int, valueOf returns an Integer object.

beginner

How can you convert a primitive int to a String using wrapper methods?

You can use String.valueOf(int i) or Integer.toString(int i). Both return the String representation of the int.

intermediate

What is the difference between parseXxx() and valueOf() methods in wrapper classes?

parseXxx() methods return primitive types (like int, double), while valueOf() methods return wrapper objects (like Integer, Double).

Which method converts a String to a primitive int?

A Integer.parseInt(String s)

B Integer.valueOf(String s)

C String.valueOf(int i)

D Integer.toString(int i)

Integer.parseInt(String s) returns a primitive int, while valueOf returns an Integer object.

Which method returns an Integer object from a String?

A Integer.parseInt(String s)

B String.valueOf(int i)

C Double.parseDouble(String s)

D Integer.valueOf(String s)

Integer.valueOf(String s) returns an Integer object representing the value of the String.

How do you convert a primitive int to a String?

A String.valueOf(int i)

B Integer.valueOf(String s)

C Integer.parseInt(String s)

D Double.valueOf(String s)

String.valueOf(int i) converts a primitive int to its String representation.

Which of these is NOT a wrapper class in Java?

A Integer

B Double

C String

D Character

String is not a wrapper class; it is a class for text data. Wrapper classes wrap primitive types.

What does Double.parseDouble("3.14") return?

A A Double object

B A primitive double

C A String

D An Integer object

Double.parseDouble returns a primitive double value.

Explain how to convert between Strings and primitive types using common wrapper methods in Java.

Think about methods that convert String to int and int to String.

Describe the difference between primitive types and wrapper classes and why wrapper classes are useful.

Consider why Java needs objects for primitives sometimes.

Practice

1. Which of the following methods converts a String to a primitive int in Java?

easy

A. Integer.valueOf(String s)

B. Integer.toString(int i)

C. Integer.parseInt(String s)

D. Integer.intValue()

Solution

1. Step 1: Understand method purpose

   parseInt converts a string directly to a primitive int.

2. Step 2: Differentiate from other methods

   valueOf returns an Integer object, not a primitive. toString converts int to string. intValue() is called on an Integer object to get primitive int.

3. Final Answer:

   Integer.parseInt(String s) -> Option C

4. Quick Check:

   parseInt returns int [OK]

Hint: parseInt returns primitive int, valueOf returns Integer object [OK]

Common Mistakes:

• Confusing parseInt and valueOf
• Using intValue() without an Integer object
• Thinking toString converts string to int

2. Which of the following is the correct syntax to convert an int variable num to an Integer object?

easy

A. Integer numObj = Integer.valueOf(num);

B. Integer numObj = Integer.parseInt(num);

C. Integer numObj = num.intValue();

D. Integer numObj = Integer.toString(num);

Solution

1. Step 1: Identify method to get Integer object

   valueOf converts primitive int to Integer object.

2. Step 2: Check other options

   parseInt returns primitive int, not Integer object. intValue() is a method of Integer, not int. toString returns String, not Integer.

3. Final Answer:

   Integer.valueOf(num) -> Option A

4. Quick Check:

   valueOf returns Integer object [OK]

Hint: Use valueOf to get Integer object from int [OK]

Common Mistakes:

• Using parseInt with int input
• Calling intValue() on primitive int
• Confusing toString with valueOf

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

String s = "123";
Integer obj = Integer.valueOf(s);
int n = obj.intValue();
System.out.println(n + 10);

medium

A. 133

B. Compilation error

C. NumberFormatException

D. 12310

Solution

1. Step 1: Convert String to Integer object

   Integer.valueOf("123") creates Integer object with value 123.

2. Step 2: Extract primitive int and add 10

   intValue() returns 123 as int. Adding 10 results in 133.

3. Final Answer:

   133 -> Option A

4. Quick Check:

   123 + 10 = 133 [OK]

Hint: valueOf + intValue() converts string to int for math [OK]

Common Mistakes:

• Thinking output is string concatenation
• Expecting NumberFormatException for valid number string
• Confusing intValue() usage

4. Identify the error in the following code snippet:

String s = "456";
int n = Integer.valueOf(s);
System.out.println(n);

medium

A. Missing semicolon after Integer.valueOf(s)

B. String s cannot be converted to Integer

C. Cannot assign Integer object to int directly

D. No error, code runs fine

Solution

1. Step 1: Check return type of Integer.valueOf

   valueOf returns an Integer object.

2. Step 2: Auto-unboxing allows assignment

   Java (5+) automatically unboxes Integer to int, so the code compiles and runs, printing 456.

3. Final Answer:

   No error, code runs fine -> Option D

4. Quick Check:

   valueOf(String) auto-unboxes to int [OK]

Hint: valueOf(String) auto-unboxes to int [OK]

Common Mistakes:

• Forgetting auto-unboxing feature
• Assuming valueOf returns primitive int
• Thinking String can't convert to Integer

5. You have a list of numeric strings: ["10", "20", "30", "40"]. Which code snippet correctly converts this list to a list of Integer objects using wrapper methods?

hard

A. List<Integer> nums = list.stream().map(String::parseInt).collect(Collectors.toList());

B. List<Integer> nums = list.stream().map(Integer::valueOf).collect(Collectors.toList());

C. List<Integer> nums = list.stream().map(s -> s.intValue()).collect(Collectors.toList());

D. List<Integer> nums = list.stream().map(s -> Integer.toString(s)).collect(Collectors.toList());

Solution

1. Step 1: Understand method return types

   Integer.valueOf(String) returns an Integer object, suitable for list of Integer.

2. Step 2: Check other options

   String::parseInt invalid - no parseInt on String. s.intValue() not a method on String. toString returns String, not Integer.

3. Final Answer:

   list.stream().map(Integer::valueOf).collect(Collectors.toList()) -> Option B

4. Quick Check:

   valueOf converts String to Integer object [OK]

Hint: Use valueOf to convert String to Integer object in streams [OK]

Common Mistakes:

• Thinking parseInt is a String method
• Calling intValue() on String
• Using toString instead of valueOf