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Generic Collections

Similar to Collections (but, this is a different concept), you can store multiple datatypes in one collection.

danger

This is one of those "It works, but shouldn't do it without a good reason" concepts.

Java is a typed language, meaning that variables have types. Typed languages are meant to help you as a developer! You'll have more secure, predictable code.

But, you may see this in the real world, so it's worth covering the topic.

Generic Collections​

In Java, an ArrayList is a class that can store a collection of objects, and the type of the objects stored in the ArrayList can be specified using a generic type parameter.

For example, to create an ArrayList that can store objects of type String and Integer, you can use the following code:

ArrayList<Object> list = new ArrayList<Object>();
list.add("hello");
list.add(123);

In this example, the ArrayList is declared with a generic type parameter of Object, which means that it can store objects of any type. The add() method is used to add a String and an Integer to the ArrayList.

Typically, you will be creating an ArrayList that is specific to a certain type of object. For example, to create an ArrayList that can store only Strings, you can use the following code:

ArrayList<String> list = new ArrayList<String>();
list.add("hello");
list.add("world");

In this example, the ArrayList is declared with a generic type parameter of String, which means that it can store only objects of type String. The add() method is used to add two Strings to the ArrayList.

Should you ever use generic collections?​

Most developers will say "No, never. Typed languages are typed for a reason".

Untyped languages do exist. When you define variables in these languages, you do not need to specify the type.

For example, here's some Python code

# Lists can contain any data types
myList = ["Hello", 123, True, 9000, "male", False]

# variables can be declared with one type, and reassigned to another type
hello = "Hello!"
hello = 42
hello = True
# no errors

Python lists can contain any combination of data types by default. This flexibility can be nice, but also problematic! Take a look at this Java code:

ArrayList<Object> myBills = new ArrayList<Object>();
myBills.add(123.00);
myBills.add(55.12);
myBills.add("Rent was 800 dollars")

Consider the following:

Should this be allowed? Most likely not. If you're writing an app to keep track of your bills, this will likely result in one of two things:

  1. A compile error
  2. Incorrect values

You may be able to think of creative used for Generic Collections, but 99.9% of the time, using a Generic Collection shouldn't even cross your mind. Typed languages benefit you, the developer.