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I've been learning about HashMaps recently but I have one question that I can't seem to get a clear answer on. The main difference between -
HashMap hash1 = new HashMap();
vs
HashMap<,>hash1 = new HashMap <,> (); //Filled in with whatever Key and Value you want.
I thought when you define a HashMap it requires the Key and Value. Any help would be much appreciated. Thank You.
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Java has a lot of HashMap methods that allow us to work with hashmaps. In this reference page, you will find all the hashmap methods available in Java. For example, if you need to add an element to the hashmap, use the put () method.
You should take a look on Java generics, if you don't specify the types of the HashMap, both key and value will be Object type. So, if you want a HashMap with Integer keys and String values for instance: HashMap<Integer, String> hashMap= new HashMap<Integer, String> ();
Internally, for every element, a separate hash is generated and the elements are indexed based on this hash to make it more efficient. 2. Changing Elements: After adding the elements if we wish to change the element, it can be done by again adding the element with the put () method.
HashMap stores key value pairs (for example records of students as value and roll number as key) and HashSet stores only keys (for example a set if integers). Please refer HashSet in Java for details. Hashmap methods in Java with Examples | Set 2 (keySet (), values (), containsKey ())
Those are the options you have:
J2SE <5.0 style:
Map map = new HashMap();
J2SE 5.0+ style (use of generics):
Map<KeyType, ValueType> map = new HashMap<KeyType, ValueType>();
Google Guava style (shorter and more flexible):
Map<KeyType, ValueType> map = Maps.newHashMap();
You should take a look on Java generics, if you don't specify the types of the HashMap, both key and value will be Object type.
So, if you want a HashMap with Integer keys and String values for instance:
HashMap<Integer, String> hashMap= new HashMap<Integer, String>();
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