Object get keys java

Java: How to Get Keys and Values from a Map

Key-value stores are essential and often used, especially in operations that require fast and frequent lookups. They allow an object — the key — to be mapped to another object, the value. This way, the values can easily be retrieved, by looking up the key.

In Java, the most popular Map implementation is the HashMap class. Aside from key-value mapping, it’s used in code that requires frequest insertions, updates and lookups. The insert and lookup time is a constant O(1).

In this tutorial, we’ll go over how to get the Keys and Values of a map in Java.

Get Keys and Values (Entries) from Java Map

Most of the time, you’re storing key-value pairs because both pieces of info are important. Thus, in most cases, you’ll want to get the key-value pair together.

The entrySet() method returns a set of Map.Entry objects that reside in the map. You can easily iterate over this set to get the keys and their associated values from a map.

Let’s populate a HashMap with some values:

Map map = new HashMap<>(); map.put("David", 24); map.put("John", 35); map.put("Jane", 19); map.put("Billy", 21); 

Now, let’s iterate over this map, by going over each Map.Entry in the entrySet() , and extracing the key and value from each of those entries:

for (Map.Entry pair : map.entrySet()) < System.out.println(String.format("Key (name) is: %s, Value (age) is : %s", pair.getKey(), pair.getValue())); > 

Key (name) is: Billy, Value (age) is: 21 Key (name) is: David, Value (age) is: 24 Key (name) is: John, Value (age) is: 35 Key (name) is: Jane, Value (age) is: 19 

Get Keys and Values (Entries) from Java Map with forEach()

Java 8 introduces us to the forEach() method for collections. It accepts a BiConsumer action . The forEach() method performs the given BiConsumer action on each entry of the HashMap .

Let’s use the same map from before, but this time, add a year to each of the entries’ ages:

map.forEach((key, value) -> System.out.println(key + " : " + value)); 

Billy : 21 David : 24 John : 35 Jane : 19 

Or, instead of consuming each key and value from the map, you can use a regular Consumer and just consume entire entries from the entrySet() :

map.entrySet() .forEach((entry) -> System.out.println(entry.getKey() + " : " + entry.getValue())); 

Billy : 21 David : 24 John : 35 Jane : 19 

Get Keys from a Java Map

To retrieve just keys, if you don’t really need any information from the values, instead of the entry set, you can get the key set:

Get Values from a Java Map

Similarly, one may desire to retrieve and iterate through the values only, without the keys. For this, we can use values() :

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for(String value: map.values())

Check if Map Contains a Key

The HashMap class has a containsKey() method, which checks if the passed key exists in the HashMap , and returns a boolean value signifying the presence of the element or lack thereof.

Let’s check if a key, 5 exists:

boolean result = map.containsKey(5); System.out.println(result); 

boolean result = map.containsKey("John"); System.out.println(result); 

Conclusion

In this article, we’ve gone over a few ways to get keys and values (entries) of a Map in Java. We’ve covered using an iterator and going through each Map.Entry , as well as using a forEach() method both on the map itself, as well as its entry set.

Finally, we’ve gone over how to get the key set and values separately, and check if a map contains a given key.

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How to get keys and value from object in Java

I’m guessing you’ve come to Java from a language in which objects have loose types and are accessed approximately as key, value pairs which are often defined at run time. An example is json converting directly to JS objects with minimal translation.

That does not describe Java at all. The type and structure of Java objects are explicitly defined at compile time and expected to be accessed through methods available at compile time. For an experienced Java coder, your question is confusing because accessing object variables as keys and values is just not how things are generally done in Java.

If your object is going to store a list of maps from string keys to date values (for example), then that would normally be expressed in java as a variable of type List> .

Solution – 2

I know this is kind of an old question, but for those who actually want an answer to guide them in the (more or less) right direction, you can do something along the lines of below to get an object’s fields/values:

import java.lang.reflect.Field; public String getObjectFields(obj) < // See: // - https://stackoverflow.com/questions/13400075/reflection-generic-get-field-value // - https://www.geeksforgeeks.org/reflection-in-java // - https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#getDeclaredField-java.lang.String- ClassobjClass = obj.getClass(); Field[] objFields = objClass.getDeclaredFields(); Map entriesMap = new HashMap<>(); for (Field field : objFields) < String fieldSuffix = field.toString().replaceAll("(^.*)(\.)([^\.]+)$", "$3"); field.setAccessible(true); entriesMap.put(fieldSuffix, field.get(obj)); >return entriesMap; > 

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Interface Map

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.

The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map’s collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

All general-purpose map implementation classes should provide two «standard» constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map , which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.

The «destructive» methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be «superimposed» is empty.

Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the containsKey(Object key) method says: «returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)) .» This specification should not be construed to imply that invoking Map.containsKey with a non-null argument key will cause key.equals(k) to be invoked for any key k . Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate.

Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the clone() , equals() , hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.

Unmodifiable Maps

• They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
• They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException .
• They are serializable if all keys and values are serializable.
• They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in IllegalArgumentException .
• The iteration order of mappings is unspecified and is subject to change.
• They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
• They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

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Get Keys From HashMap in Java

1. Use keySet() to Get a Set of Keys From a HashMap in Java
2. Use keySet() to Get an Array of Keys From a HashMap in Java

This tutorial discusses methods to get the keys from a HashMap in Java.

Use keySet() to Get a Set of Keys From a HashMap in Java

The simplest way to get the keys from a HashMap in Java is to invoke the keySet() method on your HashMap object. It returns a set containing all the keys from the HashMap .

In the example below, we will first create a HashMap object, insert some values in it, and then use keySet() to get the keys.

import java.util.*;  public class MyClass   public static void main(String args[])   // Create a HashMap with some values  HashMapString, Integer> map = new HashMapString, Integer>();  map.put("Monday", 5);  map.put("Tuesday", 6);  map.put("Wednesday", 10);   // Invoke keySet() on the HashMap object to get the keys as a set  SetString> keys = map.keySet();  for ( String key : keys )   System.out.println( key );  >  > > 

Use keySet() to Get an Array of Keys From a HashMap in Java

Often we prefer to work with an array instead of a set . The below example illustrates how to use keySet() to get an array of keys from a HashMap in Java.

import java.util.*;  public class MyClass   public static void main(String args[])   // Create a HashMap with some values  HashMapString, Integer> map = new HashMapString, Integer>();  map.put("Monday", 5);  map.put("Tuesday", 6);  map.put("Wednesday", 10);   // Invoke keySet() and use toArray() to get an array of keys  Object[] keys = map.keySet().toArray();  for (int i=0; ikeys.length; i++)   System.out.println( keys[i] );  >  > > 

Related Article — Java HashMap

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