Practice›Java Fundamentals›Module 9. Generics›9.1 Generic classes

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9.1 Generic classes — more than one type parameter

Lesson

Two placeholders at once

A class may declare several type parameters, separated by commas: class Pair<K, V>. Each is filled in independently, so a Pair<String, Integer> holds a String key and an Integer value. The names are conventional — T for type, E for element, K and V for key and value — but any identifier works.

Worked example

class Pair<K, V> {
    private K key;
    private V value;
    public K getKey() { return key; }
    public V getValue() { return value; }
}

Pair<String, Integer> p = new Pair<String, Integer>("age", 30);
System.out.println(p.getKey());     // age
System.out.println(p.getValue());   // 30

Two independent placeholders, two different actual types.

▶ Try it: run this — a Pair with a String key and an Integer value; toString() prints them joined.

class Pair<K, V> {
    private K key;
    private V value;
    public Pair(K key, V value) { this.key = key; this.value = value; }
    public K getKey() { return key; }
    public V getValue() { return value; }
    public String toString() { return key + "=" + value; }
}
public class Main {
    public static void main(String[] args) {
        Pair<String, Integer> p = new Pair<String, Integer>("age", 30);
        System.out.println(p.getKey());
        System.out.println(p.getValue());
        System.out.println(p);
    }
}

How do you declare two type parameters?

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In Pair<String, Integer>, getValue() returns...

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Type exactly what this program prints.

class Pair<K, V> {
    private K k; private V v;
    public Pair(K k, V v){ this.k=k; this.v=v; }
    public K getKey(){ return k; }
    public V getValue(){ return v; }
}
public class Main { public static void main(String[] a){
    Pair<String, Integer> p = new Pair<String, Integer>("x", 7);
    System.out.println(p.getKey());
    System.out.println(p.getValue());
}}

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Match each conventional type-parameter name to its usual meaning.

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True or false?

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