Comparing maps with lists in java

Question

Ive come across a nice problem recently, I have a Map

 LinkedHashMap<String, List<MyCustomObject>>

I need to compare this to another Map, which will be populated in the exact same method is the data in the DB is the same.

What really is happening is , that I am populating map1 at time 0, and populating map2 at time t, using the same java method(when I say this, i mean using the same DB/tables etc) So if the data on the DB side is same, then these two maps should have the same content, in the same order(hopefully!!)

Now I want to compare these two maps and if they are not equal (i.e some new data was fetched from the DB), then I want to do some action.If they are the same, I just want to leave it like that.

Can someone point me out to tutes that will help me understand how to do that??

Is serialization an answer to this??

Thanks, Neeraj

Do you want to know the differences or do you simply want to know if the two differ? — Adam Paynter, Jun 16 '11 at 12:15
Java Serialisation is not the answer. What's wrong with `equals`? — Tom Hawtin - tackline, Jun 16 '11 at 12:18
would invoking equals take care of iterating through the list(value to each key) and also iterating through all the keys , and comparison all at the same time?? — Neeraj, Jun 16 '11 at 12:22
You've first got to insure that you are pulling the records from the database to make sure the map will be in order. If that's the case, your LinkedHashMap will be in order according to the javadocs which are referenced in this question: http://stackoverflow.com/questions/1190083/does-entryset-in-a-linkedhashmap-also-guarantee-order — cgp, Jun 16 '11 at 12:24

score 4 · Accepted Answer · answered Jun 16 '11 at 12:18

just use equals:

if (!map1.equals(map2)) {
    //they differ
}

you will have to implement equals (and hashCode!) on MyCustomObject, since (if I understand correctly) the objects will have the same content but different identity (the default equals just compares the pointers - i.e., the memory address in which the object live in).

score 2 · Answer 2 · answered Jun 16 '11 at 12:19

Make sure that your MyCustomObject class implements an appropriate equals and hashcode methods and then it's as easy as getting the result of map1.equals(map2).

This is what the javadocs have to say about it:

Compares the specified object with this map for equality. Returns true if the given object is also a map and the two Maps represent the same mappings. More formally, two maps t1 and t2 represent the same mappings if t1.entrySet().equals(t2.entrySet()). This ensures that the equals method works properly across different implementations of the Map interface.

score 1 · Answer 3 · answered Jun 16 '11 at 13:05

The other answers are good and here's an illustration that indeed, the map functions iterate through the keys and the values (and compare items from the lists) to do equality comparison. As they say, you're going to need to override hashCode and equals.

static public List<String> getStringList(String... arg) {
    ArrayList<String> arrayList = new ArrayList<String>();
    for(int x=0;x<arg.length;x++) arrayList.add(arg[x]);
    return arrayList;
}

static public void main(String[] arg) {
    LinkedHashMap<String, List<String>> mapA = new LinkedHashMap<String, List<String>>();
    LinkedHashMap<String, List<String>> mapB = new LinkedHashMap<String, List<String>>();
    LinkedHashMap<String, List<String>> mapC = new LinkedHashMap<String, List<String>>();
    mapA.put("same", getStringList("a", "b", "c"));
    mapB.put("same", getStringList("a", "b", "c"));
    mapC.put("same", getStringList("a", "b", "d"));
    System.out.println(mapA.equals(mapB) ? "true" : "false");
    System.out.println(mapA.equals(mapC) ? "true" : "false");
}

Comparing maps with lists in java

3 Answers3