Pass an object as a parameter and modify it within the method

Question

Let's say I got a Map<String, String> and I wanna remove all the entries that value contains foo. What is the best way to do it, in terms of optimization/memory/etc.? The four syso below are printing the same result, that is to say {n2=bar}.

public static void main(String[] args) {

    Map<String, String> in = new HashMap<String, String>();
    in.put("n1", "foo");
    in.put("n2", "bar");
    in.put("n3", "foobar");

    // 1- create a new object with the returned Map
    Map<String, String> in1 = new HashMap<String, String>(in);
    Map<String, String> out1 = methodThatReturns(in1);
    System.out.println(out1);

    // 2- overwrite the initial Map with the returned one 
    Map<String, String> in2 = new HashMap<String, String>(in);
    in2 = methodThatReturns(in2);
    System.out.println(in2);

    // 3- use the clear/putAll methods
    Map<String, String> in3 = new HashMap<String, String>(in);
    methodThatClearsAndReadds(in3);
    System.out.println(in3);

    // 4- use an iterator to remove elements
    Map<String, String> in4 = new HashMap<String, String>(in);
    methodThatRemoves(in4);
    System.out.println(in4);

}

public static Map<String, String> methodThatReturns(Map<String, String> in) {
    Map<String, String> out = new HashMap<String, String>();
    for(Entry<String, String> entry : in.entrySet()) {
        if(!entry.getValue().contains("foo")) {
            out.put(entry.getKey(), entry.getValue());
        }
    }
    return out;
}

public static void methodThatClearsAndReadds(Map<String, String> in) {
    Map<String, String> out = new HashMap<String, String>();
    for(Entry<String, String> entry : in.entrySet()) {
        if(!entry.getValue().contains("foo")) {
            out.put(entry.getKey(), entry.getValue());
        }
    }
    in.clear();
    in.putAll(out);
}

public static void methodThatRemoves(Map<String, String> in) {
    for(Iterator<Entry<String, String>> it = in.entrySet().iterator(); it.hasNext();) {
        if(it.next().getValue().contains("foo")) {
            it.remove();
        }
    }
}

Answer 1

The best way is methodThatRemoves because:

1. In terms of memory consumption: it doesn't create a new map so doesn't add memory overhead.
2. In terms of CPU use: iterator has O(1) complexity for calling next or removing the current element.

Answer 2

The most efficient way methodThatRemoves, because it

• Uses almost no memory
• Creates no objects except the (lightweight) iterator
• Is extremely fast (doesn't use any map lookups)

I would not make a copy first though, unless you have an unmodifiable map or you need to preserve the original.