how to create a generic class around Map in java

Question

I'm new to java and java generics, I want to create a generic doubleBuffer class using java's Map

the code is:

import java.util.Random;
import java.util.concurrent.locks.ReentrantLock;
import java.util.Map;
import java.util.HashMap;
class DoubleMap<K, V, M extends Map<K, V>> {
    private ReentrantLock lock = new ReentrantLock();
    private M frontend, backend;

    public DoubleMap() {
        frontend = new M<K, V>();
        backend = new M<K, V>();
    }   

    public void put(K k, V v) {
        lock.lock();
        frontend.put(k, v); 
        lock.unlock();
    }   

    public Map<K, V> getBackend() {
        lock.lock();
        Map<K, V> ret = backend;
        backend = frontend;
        frontend = new M<K, V>();
        lock.unlock();
        return ret;
    }   
}
public class DoubleMapTest implements Runnable {
    static DoubleMap<Integer, Integer, HashMap<Integer, Integer>> map = new DoubleMap<Integer, Integer, HashMap<Integer, Integer>>();
    public void run() {
        Random rand = new Random();
        while (true) {
            map.put(rand.nextInt(), rand.nextInt());
            try {
                Thread.sleep(200);
            } catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }   
        }   
    }   

    public static void main(String[] args) {
        DoubleMapTest test = new DoubleMapTest();
        Thread t = new Thread(test);
        t.start();
        while (true) {
            Map<Integer, Integer> toMerge = map.getBackend();
            for (Map.Entry<Integer, Integer> entry : toMerge.entrySet()) {
                System.out.println(entry.getKey() + " = " + entry.getValue());
            }   
            try {
                Thread.sleep(500);
            } catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }
        }
    }
}

when i compile it, i got this error message

DoubleMapTest.java:10: error: unexpected type frontend = new M(); ^ required: class found: type parameter M where M,K,V are type-variables: M extends Map declared in class DoubleMap K extends Object declared in class DoubleMap V extends Object declared in class DoubleMap DoubleMapTest.java:11: error: unexpected type backend = new M(); ^ required: class found: type parameter M where M,K,V are type-variables: M extends Map declared in class DoubleMap K extends Object declared in class DoubleMap V extends Object declared in class DoubleMap DoubleMapTest.java:24: error: unexpected type frontend = new M(); ^ required: class found: type parameter M where M,K,V are type-variables: M extends Map declared in class DoubleMap K extends Object declared in class DoubleMap V extends Object declared in class DoubleMap 3 errors

score 4 · Answer 1 · edited May 23 '17 at 11:43

4

You can not use new to create instance of generic type in java because of type erasure. That's why you have errors in your program.

Have a look at this SO post Create instance of generic type in Java?

edited May 23 '17 at 11:43

Community

1
1

answered May 26 '15 at 06:33

ring bearer

20,383
7
59
72

how to create a generic class around Map in java

1 Answers1