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This chapter is from the book

Generic Methods

Generic methods are methods that introduce their own type parameters. This is similar to declaring a generic type, but the type parameter’s scope is limited to the method where it is declared. Static and nonstatic generic methods are allowed, as are generic class constructors.

The syntax for a generic method includes a type parameter, inside angle brackets, that appears before the method’s return type. For static generic methods, the type parameter section must appear before the method’s return type.

The Util class includes a generic method, compare, which compares two Pair objects:

public class Util {
    // Generic static method
    public static <K, V> boolean compare(Pair<K, V> p1, Pair<K, V> p2) {
        return p1.getKey().equals(p2.getKey()) &&
               p1.getValue().equals(p2.getValue());
    }
}

public class Pair<K, V> {

    private K key;
    private V value;

    // Generic constructor
    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    // Generic methods
    public void setKey(K key) { this.key = key; }
    public void setValue(V value) { this.value = value; }
    public K getKey()   { return key; }
    public V getValue() { return value; }
}

The complete syntax for invoking this method is as follows:

Pair<Integer, String> p1 = new Pair<>(1, "apple");
Pair<Integer, String> p2 = new Pair<>(2, "pear");
boolean same = Util.<Integer, String>compare(p1, p2);

The type has been explicitly provided, as shown in bold. Generally, this can be left out and the compiler will infer the type that is needed:

Pair<Integer, String> p1 = new Pair<>(1, "apple");
Pair<Integer, String> p2 = new Pair<>(2, "pear");
boolean same = Util.compare(p1, p2);

This feature, known as type inference, allows you to invoke a generic method as an ordinary method, without specifying a type between angle brackets. This topic is further discussed later on, in “Type Inference.”

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