Why method invoke return null for method put from proxied Map

Question

There is a problem when method invoke is called for the method- 'V put(K key, V val)' from proxied Map it returns null. But for the method - V get(K key) it is all right.

See code example

package com.dynamic.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class LogHandler implements InvocationHandler {

    private final Object target;

    public LogHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Start");
        Object o = method.invoke(target, args);
        System.out.println("Finish");
        return o;

    }
}

-----
package com.dynamic.proxy;

import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.Map;

public class Main {

    public static void main(String[] args) {

        LogHandler logHandler = new LogHandler(new HashMap<>());
        Map<Integer, String> map = 
        (Map) Proxy.newProxyInstance(logHandler.getClass().getClassLoader(),
                new Class[]{Map.class},
                logHandler);

        System.out.println(map.put(0, "value-0")); //null
        System.out.println(map.get(0));            // value-0
    }
}

That is a standard return value for a Map - if the key you are putting did not exist then return null. What are you expecting to get returned and why? — takendarkk, May 17 '19 at 18:43

score 2 · Answer 1 · answered May 17 '19 at 18:44

To quote the documentaion, Map#put retruns "the previous value associated with key, or null if there was no mapping for key". Since this is the first time you introduce the key 0 to the map, the return value of put(0, "value-0") is indeed null. You would get the same result if the map wasn't proxied.

score 1 · Answer 2 · answered May 17 '19 at 18:44

1

This is expected - put returns the previous value on the map, or null if it doesn't exist

See from javadoc:
Returns: the previous value associated with key, or null if there was no mapping for key.

answered May 17 '19 at 18:44

Nir Levy

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Thank you, it is really expected – May 17 '19 at 18:45

Why method invoke return null for method put from proxied Map

2 Answers2