dictionary assignment java

in is a symbol that is used to perform operations. For example: +, -, *, / etc.

There are many types of operators in Java which are given below:

Java Unary Operator

The Java unary operators require only one operand. Unary operators are used to perform various operations i.e.:

• incrementing/decrementing a value by one
• negating an expression
• inverting the value of a boolean

Java Unary Operator Example: ++ and --

Java unary operator example 2: ++ and --, java unary operator example: ~ and , java arithmetic operators.

Java arithmetic operators are used to perform addition, subtraction, multiplication, and division. They act as basic mathematical operations.

Java Arithmetic Operator Example

Java arithmetic operator example: expression, java left shift operator.

The Java left shift operator << is used to shift all of the bits in a value to the left side of a specified number of times.

Java Left Shift Operator Example

Java right shift operator.

The Java right shift operator >> is used to move the value of the left operand to right by the number of bits specified by the right operand.

Java Right Shift Operator Example

Java shift operator example: >> vs >>>, java and operator example: logical && and bitwise &.

The logical && operator doesn't check the second condition if the first condition is false. It checks the second condition only if the first one is true.

The bitwise & operator always checks both conditions whether first condition is true or false.

Java AND Operator Example: Logical && vs Bitwise &

Java or operator example: logical || and bitwise |.

The logical || operator doesn't check the second condition if the first condition is true. It checks the second condition only if the first one is false.

The bitwise | operator always checks both conditions whether first condition is true or false.

Java Ternary Operator

Java Ternary operator is used as one line replacement for if-then-else statement and used a lot in Java programming. It is the only conditional operator which takes three operands.

Java Ternary Operator Example

Another Example:

Java Assignment Operator

Java assignment operator is one of the most common operators. It is used to assign the value on its right to the operand on its left.

Java Assignment Operator Example

Java assignment operator example: adding short.

After type cast:

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Java Tutorial

Java methods, java classes, java file handling, java how to's, java reference, java examples, java booleans.

Very often, in programming, you will need a data type that can only have one of two values, like:

• TRUE / FALSE

For this, Java has a boolean data type, which can store true or false values.

Boolean Values

A boolean type is declared with the boolean keyword and can only take the values true or false :

Try it Yourself »

However, it is more common to return boolean values from boolean expressions, for conditional testing (see below).

Boolean Expression

A Boolean expression returns a boolean value: true or false .

This is useful to build logic, and find answers.

For example, you can use a comparison operator , such as the greater than ( > ) operator, to find out if an expression (or a variable) is true or false:

Or even easier:

In the examples below, we use the equal to ( == ) operator to evaluate an expression:

Real Life Example

Let's think of a "real life example" where we need to find out if a person is old enough to vote.

In the example below, we use the >= comparison operator to find out if the age ( 25 ) is greater than OR equal to the voting age limit, which is set to 18 :

Cool, right? An even better approach (since we are on a roll now), would be to wrap the code above in an if...else statement, so we can perform different actions depending on the result:

Output "Old enough to vote!" if myAge is greater than or equal to 18 . Otherwise output "Not old enough to vote.":

Booleans are the basis for all Java comparisons and conditions.

You will learn more about conditions ( if...else ) in the next chapter.

Test Yourself With Exercises

Fill in the missing parts to print the values true and false :

Start the Exercise

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Assign variable value inside if-statement [duplicate]

I was wondering whether it is possible to assign a variable a value inside a conditional operator like so:

if((int v = someMethod()) != 0) return v;

Is there some way to do this in Java? Because I know it's possible in while conditions, but I'm not sure if I'm doing it wrong for the if-statement or if it's just not possible.

• That'd be nice. –  Josh M. Commented Feb 3, 2022 at 20:57
• You were so preoccupied with whether you could, you didn’t stop to think if you should... That's some real confusing code right there and it can get even worse. You can have this other abomination: if(v = doSomething()) { /* do stuff */ } . By the time you realise that's a = and not == , you already interpreted the code wrong for hours trying to understand why it doesn't work –  Guilherme Taffarel Bergamin Commented Mar 3, 2023 at 13:03

9 Answers 9

Variables can be assigned but not declared inside the conditional statement:

• 3 So if you need the declaration anyway just place the someMethod() assign in front of the declaration. int v = someMethod() –  wviana Commented Feb 17, 2016 at 19:53
• 3 Pitty, no oneliner for me :( –  Pieter De Bie Commented Dec 20, 2016 at 9:02
• 1 i pitty the oneliner >:D –  Jason K. Commented Apr 6, 2017 at 16:15
• 15 I pity the int foo . –  xdhmoore Commented May 9, 2018 at 22:44
• 1 @wviana In some situations you may not want to run someMethod() and spend run time until after other checks are made in the conditional. If someMethod() takes a long time, and you don't want to check the result of that until a later "else". With your way, that time would be spent on it whether it is used or not. –  Michael K Commented Oct 21, 2022 at 15:43

You can assign , but not declare , inside an if :

an assignment returns the left-hand side of the assignment. so: yes. it is possible. however, you need to declare the variable outside:

• 3 As per specification , assignment returns value of the left -hand side (the variable that was assigned to). –  StenSoft Commented Mar 12, 2015 at 15:45
• 1 @rmalchow Correction, you don't necessarily have to initialize variable, it depends on what if statement does. If it returns out, for instance, then there is no need for initialization. –  randomUser56789 Commented Jul 30, 2015 at 11:24
• @StenSoft - true. however ... i wonder if, other than an implit cast - as in long i = (int)2; - this would have any significance? –  rmalchow Commented Jul 23, 2016 at 6:50
• @randomUser56789 can you elaborate? "if((int v = call()) != 4)" just won't work. –  rmalchow Commented Jul 23, 2016 at 6:51
• @rmalchow it won't work indeed. I said that you don't have to initialize the variable outside, but you do have to declare it outside. –  randomUser56789 Commented Jul 25, 2016 at 7:38

Yes, you can assign the value of variable inside if.

I wouldn't recommend it. The problem is, it looks like a common error where you try to compare values, but use a single = instead of == or === .

It will be better if you do something like this:

• 1 This is easy to shoot yourself in the foot with because it compiles just fine... boolean b = false; if ( b = true ) { //oops } –  Edward J Beckett Commented Mar 16, 2018 at 12:52
• @EddieB This isn't like shooting yourself in the foot. More about not knowing the kind of gun you have in your hands. Gosling messed up Java a lot in the name of the former when most of the time it had to do with the latter. –  stillanoob Commented Jul 14, 2020 at 9:53

I believe that your problem is due to the fact that you are defining the variable v inside the test. As explained by @rmalchow, it will work you change it into

There is also another issue of variable scope. Even if what you tried were to work, what would be the point? Assuming you could define the variable scope inside the test, your variable v would not exist outside that scope. Hence, creating the variable and assigning the value would be pointless, for you would not be able to use it.

Variables exist only in the scope they were created. Since you are assigning the value to use it afterwards, consider the scope where you are creating the varible so that it may be used where needed.

Yes, it's possible to do. Consider the code below:

I hope this will satisfy your question.

You can assign a variable inside of if statement, but you must declare it first

Yes, it is possible to assign inside if conditional check. But, your variable should have already been declared to assign something.

Because I know it's possible in while conditions, but I'm not sure if I'm doing it wrong for the if-statement or if it's just not possible.

HINT: what type while and if condition should be ??

If it can be done with while, it can be done with if statement as weel, as both of them expect a boolean condition.

Not the answer you're looking for? Browse other questions tagged java or ask your own question .

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HashMap in Java

Let us start with a simple Java code snippet that demonstrates how to create and use a HashMap in Java.

In Java, HashMap is a part of Java’s collection since Java 1.2. This class is found in java.util package. It provides the basic implementation of the Map interface of Java. HashMap in Java stores the data in (Key, Value) pairs, and you can access them by an index of another type (e.g. an Integer). One object is used as a key (index) to another object (value). If you try to insert the duplicate key in HashMap, it will replace the element of the corresponding key. 

• What is HashMap?

Java HashMap is similar to HashTable , but it is unsynchronized. It allows to store the null keys as well, but there should be only one null key object and there can be any number of null values. This class makes no guarantees as to the order of the map. To use this class and its methods, you need to import java.util.HashMap package or its superclass.

Table of Content

Java HashMap Examples

• HashMap Declaration
• Hierarchy of Java HashMap

Creating HashMap in Java

Java hashmap constructors, performing various operations on hashmap, complexity of hashmap in java, internal structure of hashmap.

• Advantages and Disadvantages of Java HashMap

Below is the implementation of an example of Java HashMap:

 HashMap Declaration:

Parameters:.

It takes two parameters namely as follows:

• The type of keys maintained by this map
• The type of mapped values

Note: K eys and value can’t be primitive datatype. Key in Hashmap is valid if it implements hashCode() and equals() method , it should also be immutable (immutable custom object ) so that hashcode and equality remains constant. Value in hashmap can be any wrapper class, custom objects, arrays, any reference type or even null . For example : Hashmap can have array as value but not as key.

HashMap in Java implements Serializable , Cloneable , Map<K, V> interfaces.Java HashMap extends AbstractMap<K, V> class. The direct subclasses are LinkedHashMap and PrinterStateReasons .

Hierarchy of HashMap in Java

Characteristics of Java HashMap:

A HashMap is a data structure that is used to store and retrieve values based on keys. Some of the key characteristics of a hashmap include:

• Fast access time : HashMaps provide constant time access to elements, which means that retrieval and insertion of elements are very fast, usually O(1) time complexity.
• Uses hashing function : HashMaps uses a hash function to map keys to indices in an array. This allows for a quick lookup of values based on keys.
• Stores key-value pairs: Each element in a HashMap consists of a key-value pair. The key is used to look up the associated value.
• Supports null keys and values : HashMaps allow for null values and keys. This means that a null key can be used to store a value, and a null value can be associated with a key.
• Not ordered: HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved. However, LinkedHashMap is a variation of HashMap that preserves the insertion order.
• Allows duplicates : HashMaps allow for duplicate values, but not duplicate keys. If a duplicate key is added, the previous value associated with the key is overwritten.
• Thread-unsafe : HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies. If thread safety is required, ConcurrentHashMap can be used.
• Capacity and load factor : HashMaps have a capacity, which is the number of elements that it can hold, and a load factor, which is the measure of how full the hashmap can be before it is resized.

Let us understand how we can create and do some operations on HashMap in Java with an example mentioned below:

HashMap provides 4 constructors and the access modifier of each is public which are listed as follows:

• HashMap(int initialCapacity)
• HashMap(int initialCapacity, float loadFactor)
• HashMap(Map map)

Now discussing the above constructors one by one alongside implementing the same with help of clean Java programs.

1. HashMap()

It is the default constructor which creates an instance of HashMap with an initial capacity of 16 and a load factor of 0.75.

2. HashMap(int initialCapacity)

It creates a HashMap instance with a specified initial capacity and load factor of 0.75.

3. HashMap(int initialCapacity, float loadFactor)

It creates a HashMap instance with a specified initial capacity and specified load factor.

4. HashMap(Map map)

It creates an instance of HashMap with the same mappings as the specified map.

HashMap<K, V> hm = new HashMap<K, V>(Map map);

1. Adding Elements  in HashMap in Java

In order to add an element to the map, we can use the put() method. However, the insertion order is not retained in the Hashmap. 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 in HashMap in Java

After adding the elements if we wish to change the element, it can be done by again adding the element with the put() method. Since the elements in the map are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we wish to change.

3. Removing Element from Java HashMap

In order to remove an element from the Map, we can use the remove() method. This method takes the key value and removes the mapping for a key from this map if it is present in the map.

4. Traversal of Java HashMap

We can use the Iterator interface to traverse over any structure of the Collection Framework. Since Iterators work with one type of data we use Entry< ? , ? > to resolve the two separate types into a compatible format. Then using the next() method we print the entries of HashMap.

HashMap provides constant time complexity for basic operations, get and put if the hash function is properly written and it disperses the elements properly among the buckets. Iteration over HashMap depends on the capacity of HashMap and the number of key-value pairs. Basically, it is directly proportional to the capacity + size. Capacity is the number of buckets in HashMap. So it is not a good idea to keep a high number of buckets in HashMap initially.

Important Features of HashMap

To access a value one must know its key. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to a small String that represents the same String. A shorter value helps in indexing and faster searches. HashSet also uses HashMap internally. A few important features of HashMap are: 

• HashMap is a part of java.util package.
• HashMap extends an abstract class AbstractMap which also provides an incomplete implementation of the Map interface.
• It also implements a Cloneable and Serializable interfaces. K and V in the above definition represent Key and Value respectively.
• HashMap doesn’t allow duplicate keys but allows duplicate values. That means A single key can’t contain more than 1 value but more than 1 key can contain a single value.
• HashMap allows a null key also but only once and multiple null values.
• This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. It is roughly similar to HashTable but is unsynchronized.

Internally HashMap contains an array of Node and a node is represented as a class that contains 4 fields: 

It can be seen that the node is containing a reference to its own object. So it’s a linked list. 

Performance of HashMap

The performance of HashMap depends on 2 parameters which are named as follows:

• Initial Capacity
• Load Factor

1. Initial Capacity – It is the capacity of HashMap at the time of its creation (It is the number of buckets a HashMap can hold when the HashMap is instantiated). In java, it is 2^4=16 initially, meaning it can hold 16 key-value pairs. 2. Load Factor – It is the percent value of the capacity after which the capacity of Hashmap is to be increased (It is the percentage fill of buckets after which Rehashing takes place). In java, it is 0.75f by default, meaning the rehashing takes place after filling 75% of the capacity. 3. Threshold – It is the product of Load Factor and Initial Capacity. In java, by default, it is (16 * 0.75 = 12). That is, Rehashing takes place after inserting 12 key-value pairs into the HashMap. 4. Rehashing – It is the process of doubling the capacity of the HashMap after it reaches its Threshold. In java, HashMap continues to rehash(by default) in the following sequence – 2^4, 2^5, 2^6, 2^7, …. so on. 

If the initial capacity is kept higher then rehashing will never be done. But by keeping it higher increases the time complexity of iteration. So it should be chosen very cleverly to increase performance. The expected number of values should be taken into account to set the initial capacity. The most generally preferred load factor value is 0.75 which provides a good deal between time and space costs. The load factor’s value varies between 0 and 1. 

Note: From Java 8 onward, Java has started using Self Balancing BST instead of a linked list for chaining. The advantage of self-balancing bst is, we get the worst case (when every key maps to the same slot) search time is O(Log n). 

Synchronized HashMap

As it is told that HashMap is unsynchronized i.e. multiple threads can access it simultaneously. If multiple threads access this class simultaneously and at least one thread manipulates it structurally then it is necessary to make it synchronized externally. It is done by synchronizing some object which encapsulates the map. If No such object exists then it can be wrapped around Collections.synchronizedMap() to make HashMap synchronized and avoid accidental unsynchronized access. As in the following example: 

Now the Map m is synchronized.  Iterators of this class are fail-fast if any structure modification is done after the creation of the iterator, in any way except through the iterator’s remove method. In a failure of an iterator, it will throw ConcurrentModificationException.   Applications of HashMap: 

HashMap is mainly the implementation of hashing. It is useful when we need efficient implementation of search, insert and delete operations. Please refer to the applications of hashing for details.

Methods in HashMapassociate

• K – The type of the keys in the map.
• V – The type of values mapped in the map.

 Methods inherited from class java.util.AbstractMap

Methods inherited from interface java.util.Map

Advantages of Java HashMap

• Fast retrieval: HashMaps provide constant time access to elements, which means that retrieval and insertion of elements is very fast.
• Efficient storage : HashMaps use a hashing function to map keys to indices in an array. This allows for quick lookup of values based on keys, and efficient storage of data.
• Flexibility : HashMaps allow for null keys and values, and can store key-value pairs of any data type.
• Easy to use : HashMaps have a simple interface and can be easily implemented in Java.
• Suitable for large data sets : HashMaps can handle large data sets without slowing down.

Disadvantages of Java HashMap

• Unordered : HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved.
• Not thread-safe : HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies.
• Performance can degrade : In some cases, if the hashing function is not properly implemented or if the load factor is too high, the performance of a HashMap can degrade.
• More complex than arrays or lists : HashMaps can be more complex to understand and use than simple arrays or lists, especially for beginners.
• Higher memory usage : Since HashMaps use an underlying array, they can use more memory than other data structures like arrays or lists. This can be a disadvantage if memory usage is a concern.
• Hashmap vs Treemap
• Hashmap vs HashTable
• Recent Articles on Java HashMap

FAQs on Java HashMap

Q. what is a hashmap in java.

HashMap in Java is the class from the collection framework that can store key-value pairs inside it.

Q. Why use HashMap in Java?

HashMap in Java is used for storing key-value pairs where each key is unique.

Q. What is the benefit of HashMap?

HashMap is used because of it provides features like: Fast retrieval Efficient storage Flexible to use Easy to use Suitable for large data sets

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