Java HashMap detailed tutorial

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This is a detailed description about how to use HashMap in Java. Lot of code examples makes it easy to understand it's behaviour.

Tutorial info:

Name:	Java HashMap detailed tutorial
Total steps:	1
Category:	Basics
Date:	2011-04-13
Level:	Beginner
Product:	See complete product
Viewed:	6855

Bookmark Java HashMap detailed tutorial

Step 1 - How to use Java HashMap

Java HashMap detailed tutorial

HashMap is an implementation of the Map interface. It means that it stores key-value pairs. Both the key and the value also must be an Object. You can not put primitive types into a HashMap, in this cases use their wrapper classes like Integer. Both the key and the value can be null.

Here is a simple example how to create and fill a HashMap.

Code:

```
HashMap hm = new HashMap();
```
```
 
```
```
hm.put("A","Test-A");
```
```
hm.put("B","Test-B");
```
```
 
```
```
System.out.println(hm.get("A"));
```

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If you use the same key object more than once, then the value field will be overwritten as you can see in this example:

Code:

```
hm.put("A","Test-A");
```
```
hm.put("B","Test-B");
```
```
System.out.println(hm.get("A"));
```
```
hm.put("A","This is an other value");
```
```
System.out.println(hm.get("A"));
```

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If you want to get out a key from the HashMap that doesn’t exist then it returns with null. You can also check it with the containsKey() method as presented here:

Code:

```
if (hm.containsKey("NonExistingKey")) {
```
```
	System.out.println("Key exists");
```
```
} else {
```

	System.out.println("Key doesn't exist");

```
}
```

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Iterating over a HashMap

In cases when you want to process every item from the HashMap you need to call the entrySet() method on the HashMap to obtain a Set of Map.Entry elements. As next step you can get the Iterator of this step and make a loop to visit every item. Here is a simple demonstration how you can iterate over a HashMap:

Code:

```
HashMap hm = new HashMap();
```
```
 
```
```
hm.put("A","Test-A");
```
```
hm.put("B","Test-B");
```
```
hm.put("C","Test-C");
```
```
 
```
```
Set hmSet = hm.entrySet();
```
```
 
```
```
Iterator iter = hmSet.iterator();
```
```
 
```
```
while(iter.hasNext()){
```

    Map.Entry me = (Map.Entry)iter.next();

    System.out.println(me.getKey() + " -> " + me.getValue() );

```
}
```

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Or if you simply need the values and not interested the keys you can use this form:

Code:

```
iter = hm.values().iterator();
```
```
while(iter.hasNext()){
```
```
    System.out.println(iter.next());
```
```
}
```

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Remove items from the HashMap

If you want to remove an item from the HashMap then you need to use the remove() method, which requires a key object that you want to remove. You can also remove all of the items using the clear() method.

Code:

```
HashMap hm = new HashMap();
```
```
 
```
```
hm.put("A","Test-A");
```
```
hm.put("B","Test-B");
```
```
hm.put("C","Test-C");
```
```
 
```

System.out.println("--- The original content ---");

```
 
```
```
Iterator iter = hm.values().iterator();
```
```
while(iter.hasNext()){
```
```
    System.out.println(iter.next());
```
```
}
```
```
 
```
```
hm.remove("A");
```

System.out.println("--- After one item was removed ---");

```
 
```
```
iter = hm.values().iterator();
```
```
while(iter.hasNext()){
```
```
    System.out.println(iter.next());
```
```
}
```
```
 
```
```
hm.clear();
```

System.out.println("--- After all items were removed ---");

```
 
```
```
iter = hm.values().iterator();
```
```
while(iter.hasNext()){
```
```
    System.out.println(iter.next());
```
```
}
```

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Tags: java hashmap, hashmap usage, hashmap example, hashmap code, hashmap iterator

Java HashMap detailed tutorial - Table of contents
Step 1 - How to use Java HashMap

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Java HashMap detailed tutorial

Tutorial info:

Bookmark Java HashMap detailed tutorial

Step 1 - How to use Java HashMap