Reference data types in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are reference data types in Java?

Reference data types store the address (reference) of objects in memory, not the actual data value itself. Examples include classes, arrays, and interfaces.

beginner

How do reference types differ from primitive types in Java?

Primitive types store actual values (like int, double), while reference types store memory addresses pointing to objects. Reference types can be null; primitives cannot.

intermediate

What happens when you assign one reference variable to another?

Both variables point to the same object in memory. Changes through one reference affect the same object seen by the other.

beginner

Can reference variables be null? What does that mean?

Yes, reference variables can be null, meaning they do not point to any object in memory. Accessing methods or fields on a null reference causes a NullPointerException.

beginner

Give an example of a reference data type in Java.

Example:

String name = "Alice";

Here, name is a reference variable pointing to a String object containing "Alice".

Which of the following is a reference data type in Java?

A int

B double

C String

D boolean

String is a class in Java, so it is a reference data type. int, double, and boolean are primitive types.

What does a reference variable store?

A The actual data value

B The size of the object

C The data type of the object

D The memory address of an object

Reference variables store the memory address (reference) of the object they point to.

What happens if you try to use a method on a null reference variable?

A The program crashes with NullPointerException

B The method returns null

C The variable automatically gets initialized

D The method runs normally

Using a method on a null reference causes a NullPointerException at runtime.

If you assign one reference variable to another, what happens?

A Both variables point to the same object

B A new object is created

C The original object is deleted

D The variables become primitive types

Both reference variables will point to the same object in memory.

Which of these is NOT a reference data type?

A Interface

B char

C Array

D Class

char is a primitive data type, not a reference type.

Explain what reference data types are in Java and how they differ from primitive types.

Think about how variables hold data versus how they hold addresses.

Describe what happens when you assign one reference variable to another in Java.

Consider what happens to the memory address stored.