Common wrapper methods in Java - Cheat Sheet & Quick Revision

overview

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)

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)

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)

Which of these is NOT a wrapper class in Java?

A Integer

B Double

C String

D Character

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

A A Double object

B A primitive double

C A String

D An Integer object

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

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