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javacomparatormethod-reference
JiaY
JiaY

Reputation: 39

Does Java Integer class have a compare method which returns Comparator?

I was reading the Java tutorial and saw this line of code:

Comparator<Integer> normal = Integer::compare;

About the right hand side, I tried looking for documentation which explains how compare returns a Comparator for an Integer object. But I found none. The Java API docs show the following:

public static int compare(int x, int y)

What am I missing?

Upvotes: 1

Views: 1786

Answers (2)

rolling stone
rolling stone

Reputation: 645

Integer class implements only Comparable interface.

The compare method in Integer class is NOT Comparator's compare method Implementation. compare is a static helper method in Integer class itself.

Even before Java 8 we have compare method in Integer class. Integer class has two methods in this context one is

public static compare(Integer val1,Integer val2)

another

public int compareTo(Integer val). //Comparable Interface Implementation

Therefore, If you invoke compare method of Integer class it internally still invoke the impmentation of compareTo Method of the Comparable interface defined in the Integer class as below.

public static int compare(int x, int y) {
 Integer.valueOf(x).compareTo(Integer.valueOf(y));  
}

Upvotes: 0

Matt
Matt

Reputation: 13923

Comparator<T> is a functional interface with the signature int compare(T o1, T o2);. From the documentation of java.util.function: "Functional interfaces provide target types for lambda expressions and method references."

The method Integer#compare(int x, int y) matches this signature. Its method reference can therefore be assigned to a variable of type Comparator<Integer>.

To better understand this, I suggest that you read about lambda expressions, method references and functional interfaces.

Upvotes: 3

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