How to Compare Two ArrayList in Java

There are following ways to compare two ArrayList in Java:

• Java equals() method
• Java removeAll() method
• Java retainAll() method
• Java ArrayList.contains() method
• Java contentEquals() method
• Java Stream interface

Java equals() method

Java equals() method of List interface compares the specified object with the list for equality. It overrides the equals() method of Object class.

Syntax

This method accepts an object to be compared for equality with the list. It returns true if the specified object is equal to the list, else returns false.

Example

In the following example, we have create two ArrayList firstList and secondList. Comparing both list by using equals() method, it returns true. We have added another element in the secondList to create a difference between firstList and secondList. Now, if we perform comparison, it returns false.

Output

[Apple, Pears, Guava, Mango]
[Apple, Pears, Guava, Mango]
true
false

Java removeAll() method (finding the missing element)

Java removeAll() method of ArrayList class is used to remove all elements from the list that are contained in the specified collection. It overrides the removeAll() method of AbstractCollection<E> class.

Syntax

This method accepts Collection as a parameter containing elements to be removed from this list. It returns true if this list changed as a result of the call.

Example

In the following example, we have created two ArrayList firstList and secondList. The removeAll() method removes all the elements of the firstList because the same elements are also present in the secondList, except Papaya. So, Papaya is the missing element in the firstList. Hence, it returns Papaya. The method returns an empty list [] if both the list have same elements.

Output

[Papaya]

Let's see another example of removeAll() method that returns the elements from firstList which are not present is the secondList.

Example

Output

First array list: 
[12, 4, 67, 90, 34]
Second array list: 
[12, 4, 67, 0, 34]
Un-common element of the first list: 
[90]

Java retainAll() method (returns common elements in both lists)

Java retainAll() method of ArrayList class retains only the elements of the list that are contained in other list also. It overrides the retainAll() method of AbstarctCollection<E> class.

Syntax

This method accepts a Collection as a parameter that contains elements to be retained in the list. It returns true if this list changed as a result of the call.

Example

In this example, we have created two ArrayList firstList and secondList by using the asList() method of the Arrays class. The asList() method returns a list view of the specified array.

Output

First arraylist: 
[M, W, J, K, T]
Second arraylist: 
[M, W, E, K, T]
Common elements in both list:
[M, W, K, T]

Java ArrayList.contains() method

Java ArrayList.contains() method is used for comparing two elements of different ArrayList. Java ArrayList.contains() method overrides the contains() method of AbstrarctCollection<E> class.

Syntax

This method parses an element whose presence in the list is to be checked. It returns true if the element is matched, else returns false.

Example

In this example, we have created two ArrayList firstList and secondList of String type. We have compared these ArrayList using contains() method. If the elements of firstList match with the elements of the secondList, it return Yes and stores this value into thirdList. Similarly, if the element does not match, it return No.

Output

[Yes, No, No]

Java contentEquals() method

Java contentEquals() method compares the String with the StringBuffer and returns a boolean value. It belongs to String class.

Syntax

This method accepts StringBuffer as a parameter to compare against the String. It returns true if the String represents the same sequence of characters as the specified StringBuffer, else returns false.

Example

In this example, we have created two ArrayList firstList and secondList of String type. We have created a static method compareList() which parses two ArrayList ls1 and ls2 as an argument and returns a boolean value. The method converts a list into String. The contentEquals() method compares the String to the specified StringBuffer.

We have added another element in the secondList to create the difference between both lists and again call the compareList() method, which returns false.

Output

When Lists are same: true
When Lists are not same: false

Java Stream Interface

Java Stream is an interface. It belong to java.util package. It provides a filter() method to filter stream elements on the basis of given elements. It is an intermediate process.

Syntax

This method accepts Predicate (functional interface) as an argument and returns a new stream of resultant elements.

collect() method

Another method is used in the following example is the collect() method of Stream interface.

It performs a mutable reduction operation on the elements of this stream using a Collector. The Collector encapsulates the functions used as arguments to collect(Supplier, BiConsumer, BiConsumer). It allows to reusing of collection strategies and composition of collect operations. It is a terminal operation.

Syntax

Collectors.toList() method

The toList() is a static method of Collectors class. It belong to java.util.Stream.Collectors class.

Syntax

Where T is the type of elements. It returns a Collector which collects all the input elements into a List, in an encounter (unordered) order.

Example

In the following example, we have created two ArrayList firstList and secondList of String type. The filter() method returns a List of common elements which are present in both Lists.

Output

First List: [Java, Python, Ruby, Go]
Second List: [Java, Python, Ruby, Go, Perl]
Common elements: [Java, Python, Ruby, Go]