Comparing Objects in Java – ==, equals(), compareTo() and compare()

TLDR;

When to use == in Java

When comparing two operands. It is used to check whether two operands are equal or not.
It’s best to only use it when working with primitive data types, such as int, float, char or sometimes even Booleans

When to use equals() in Java

This method is best used when comparing String types.

When to use compareTo() in Java

It is good to use compareTo when you need to compare two String literals.

When to use compare() in Java

When using the compare method, it is ideal to use this on Integer types.


Java has a few different ways of comparing variables and objects with one another.

==, equals(), compareTo() and compare().

Java provides various operators such as == and != which check for equality and inequality respectively. They are used in a programming flow to check if certain criteria match or are similar, such as:

On line 8 we check if `someString` is the same (==, known as a reference equal) as the String test, which it is, so it returns `true` (in this case).

On line 16 we check if `someString2` is equal to the String test by using the Strings built in `.equals` method. This is for example, the recommended method of comparing Strings.

The function checks the actual contents of the string, the `==` operator checks whether the references to the objects are equal. (Alnitak, 2009)

Our reference equals usage above compares the objects references to see if they point to the same object. This means that they dont actually check to see if the end values are equal, potentially returning a false positive in some instances.

A reference equals is very useful for primitive data types such as `int`s but soon becomes a problem when comparing more complex objects. Even in our example above we had to use `.equals` to compare the actual value of the variables as opposed to the reference types.

Another useful comparison method is `compareTo`, which performs a lexicographical comparison of two objects. It is specifically useful for sorting comparisons passed to it as the comparison is done by comparing a range of String characters and or Double values.

For example, (String) abc is `-1` when compared to (String) abd because the c is lexicographically one less than d. Therefore two strings with the same value would return a `0` as they are the same and have `0` differences. The order of this check is therefore extremely important.

`compareTo` is part of the `Comparable Interface`.

The `compare()` is another method that is useful for comparing objects, it is different to `compareTo()` in that it can sort multiple elements and returns the value instead of modifying the original class.


`compare` is part of the `Comparator Interface`.

Comparable and Comparator are both Interfaces and can both be used to sort collections of elements.

We will highlight some key differences between the two.

ComparableComparator
Sorting on a single elementSorting on multiple elements
Modifies the original classResult is passed back and original class is not modified
Invoked by means of compareTo() methodInvoked by means of compare() method
Part of the java.lang packagePart of the java.util package

References:

Java String.equals versus == (2009) – Available from: http://stackoverflow.com/questions/767372/java-string-equals-versus (Accessed on 9th April 2017)

String Class – intern (n.d.) – Available from: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#intern%28%29 (Accessed on 9th April 2017)

Interface Comparable<T> (n.d.) – Available from: https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html (Accessed on 9th April 2017)

Generics in the Java Programming Language (2004) – Available from: http://www.cs.unibo.it/martini/PP/generics-tutorial.pdf (Accessed on 9th April 2017)

==, .equals(), compareTo(), and compare() (2007) – Available from: http://perso.ensta-paristech.fr/~diam/java/online/notes-java/data/expressions/22compareobjects.html (Accessed on 9th April 2017)

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