Java function return list

Returning Collections From Methods in Java

Writing software often means fetching and returning data. In an object-oriented language like Java, we generally express data in terms of properties of a class. Unlike many newer OO languages, Java doesn’t offer properties as a first-class construct, so we instead use JavaBean getters and setters, in order to maintain encapsulation. As a result, most of us have grown accustomed to writing code like the following:

public class MyClass private String data; 
public String getData() < return this.data; > 
public void setData(String data) < this.data = data; >
>

In the example above, we see a simple class with a simple property: a String called data. Of course, data can either contain a value, or it can be null.

How do we deal with Collection -type properties? Let’s fabricate a simple class that encapsulates a List :

public class MyClass  private List myStrings;>

We have a field — a List of String s — called myStrings , which is encapsulated in MyClass . Now, we need to provide accessor methods:

public class MyClass  private List myStrings; public void setMyStrings(List s) this.myStrings = s; 
> public List getMyStrings() return this.myStrings; 
>>

Here we have a properly-encapsulated — if verbose — class. So we’ve done good, right? Hold that thought.

Optional Lessons

Consider the Optional class, introduced in Java 8. By now, you’ve probably heard the mantra that you should never return null from a method that returns an Optional . Why? Consider the following contrived example:

public class Foo  private String bar; public Optional getBar() // don't actually ever do this 
return (bar == null) ? null : Optional.of(bar); 
>>

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Return an ArrayList in Java

1. Return an ArrayList From a Non-Static Function in Java
2. Return an ArrayList From a Static Function in Java

An ArrayList is a resizable class of java.util package. It is a scalable array, which means that the size of an array can be modified whenever you want. However, it can be a little slow to use at times.

In this tutorial, we will return an ArrayList from a function inside a class in Java.

Return an ArrayList From a Non-Static Function in Java

We will work with a function that creates and returns an ArrayList of some size. We will try to invoke this function in another class. This function is non-static, so an object of the class will be needed to invoke it.

In the following code, we create such a function.

import java.util.ArrayList; public class ClassA   public static void main(String args[])    ClassB m1 = new ClassB();  List listInClassA = m1.myNumbers();  System.out.println("The List is "+listInClassA);  > > public class ClassB   public ArrayListInteger> myNumbers()   ArrayListInteger> numbers = new ArrayListInteger>();  numbers.add(10);  numbers.add(15);  numbers.add(30);  return(numbers);  > > 

The function myNumbers() is not static. So, we need to create an instance of ClassB in ClassA . Now we will have access to the ArrayList method myNumbers() of ClassB .

Return an ArrayList From a Static Function in Java

A static function can be accessed or invoked without creating an object of the class to which it belongs.

If the static method is to be called from outside its parent class, we have to specify the class where that static function was defined.

We can modify our code slightly while working with a static function.

import java.util.ArrayList; public class ClassA   public static void main(String args[])    List listInClassA = classB.myNumbers();  System.out.println("The List is "+listInClassA);  > > public class ClassB   public static ArrayListInteger> myNumbers()   ArrayListInteger> numbers = new ArrayListInteger>();  numbers.add(10);  numbers.add(15);  numbers.add(30);  return(numbers);  > > 

In the above example, we referred to the function from classB in classA without creating an object of classB .

Related Article — Java ArrayList

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Return ArrayList in Java

This article discusses cases of returning an ArrayList in Java from a method.

An ArrayList in Java is a collection of elements of the same data type under a single variable name. In different cases, you can return an ArrayList from a method in Java. These cases are as given below.

Return ArrayList in Java From a Static Method

As you might know, static methods in Java do not need any class instance to call them. You can call a static method using the class name.
Returning an ArrayList from a static method is simple.

• Create a static method with the return type as ArrayList .
• Create an ArrayList and instantiate it.
• Input the data into the list.
• Return the ArrayList.

Let us see an example code for the same.

Return ArrayList in Java From a Non-static Method

The methods that you normally define under a normal class name are the non-static methods.

You can not directly call the non-static methods using the class name. For calling a non-static method, you would need to create an instance of the class and then call the method using that instance.

This method of returning the ArrayList is similar except that the receiving method must use the class instance to call the returning method.

Let us see the example code.

Further reading:

Print ArrayList in Java

Initialize ArrayList with values in Java

Conclusion

This is all about returning the ArrayList from a method in Java.

Hope you enjoyed reading the article. Stay tuned for more such articles. Happy Learning!

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