1.2: Interfaces in Java

A Java interface specifies a set of methods; any class that implements this interface has to provide these methods. For example, here is the source code for Comparable, which is an interface defined in the package java.lang:

public interface Comparable {
    public int compareTo(T o);
}

This interface definition uses a type parameter, T, which makes Comparable a generic type. In order to implement this interface, a class has to

• Specify the type T refers to, and
• Provide a method named compareTo that takes an object as a parameter and returns an int.

For example, here’s the source code for java.lang.Integer:

public final class Integer extends Number implements Comparable {
    public int compareTo(Integer anotherInteger) {
        int thisVal = this.value;
        int anotherVal = anotherInteger.value;
        return (thisVal<anotherVal ? -1 : (thisVal==anotherVal ? 0 : 1));
    }
    
    // other methods omitted
}

This class extends Number, so it inherits the methods and instance variables of Number; and it implements Comparable<Integer>, so it provides a method named compareTo that takes an Integer and returns an int.

When a class declares that it implements an interface, the compiler checks that it provides all methods defined by the interface.

As an aside, this implementation of compareTo uses the "ternary operator", sometimes written ?:. If you are not familiar with it, you can read about it at thinkdast.com/ternary.

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