How do I use Map.getOrDefault() default method in Java?

By Wayan in Util Package 0 Comment

The Map.getOrDefault(Object key, V defaultValue) method in Java 8 is a convenience default method to return the value for a given key. If the map does not contain a mapping for the key, then it returns the default value.

This method can be particularly useful in situations where you’re working with a map and need to fetch a value for a key, but aren’t sure if the key exists in the map. It helps you handle these scenarios without a need to write extra conditional code to check if the key is present (i.e., using containsKey(Object key)) before trying to get the value.

Here’s a common use case without getOrDefault():

Map<String, Integer> map = new HashMap<>();
// fill map...

Integer value;
if (map.containsKey("key")) {
    value = map.get("key");
} else {
    value = -1;
}

Here’s a basic example of how to use getOrDefault():

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapGetOrDefaultExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        // Get a value of the key "A". It will return value 1 as
        // "A" is present in the map.
        Integer value = map.getOrDefault("A", -1);
        System.out.println("Value: " + value);

        // Try to get a value of the key "Z". As "Z" is not present
        // in the map, it will return the default value -1.
        value = map.getOrDefault("Z", -1);
        System.out.println("Value: " + value);
    }
}

In this example, “Value: 1” and then “Value: -1” will be printed in the console. In the first case, the key “A” is in the map, so the associated value 1 is returned. In the second case, the key “Z” does not exist in the map, so the default value of -1 is returned.

How do I use the Map.forEach() default method?

By Wayan in Util Package 0 Comment

The forEach() method in the Map interface in Java 8, allows you to iterate over each entry in the map, allowing you to use each key-value pair in some way.

Here’s a basic usage of the forEach() method:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapForEachExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 10);
        map.put("Orange", 20);
        map.put("Banana", 30);

        // Use the forEach method. Here, each key-value pair is printed.
        map.forEach((key, value) -> System.out.println("Key: " + key + ", Value: " + value));
    }
}

Output:

Key: Apple, Value: 10
Key: Orange, Value: 20
Key: Banana, Value: 30

In this example, a HashMap is created and populated with some data. The forEach method is then called on this map, with a lambda expression that accepts a key and a value, then prints them. The key and value parameters represent the current key-value pair the forEach method is handling. In this lambda expression, they are printed to the console.

This operation is applied to each entry in the map, hence the name forEach.

Using the forEach method with lambda expressions has several benefits:

  1. Improved Readability: Traditional iteration requires creating an iterator, a while or for loop, and handling each element. With forEach and lambdas, you can express what you want to do with each element clearly and concisely, making the code easier to read and understand
  2. Concurrency Safety: The forEach method is inherently safer to use in concurrent environments. You don’t need to worry about ConcurrentModificationException errors which you might get while using an Iterator and modifying the collection concurrently.
  3. Less Boilerplate Code: The forEach function in combination with a lambda function provides a way to iterate over a collection with fewer lines of code compared to using iterators
  4. Functional Programming: Lambda expressions and functional interfaces pave the way towards functional programming in Java, which allows for more expressive ways to manipulate collections.

Remember, although forEach can make your code more concise, it does not necessarily make it faster.

How do I use List.replaceAll() method?

By Wayan in Util Package 0 Comment

The List.replaceAll() method was introduced in Java 8. This method replaces each element of the list with the result of applying the operator to that element. The operator or function you pass to replaceAll() should be a UnaryOperator.

Here is a simple example:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;
import java.util.function.UnaryOperator;

public class ListReplaceAllExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        // Define an UnaryOperator to square each number
        UnaryOperator<Integer> square = n -> n * n;

        // Use replaceAll() method to square each number in the list
        numbers.replaceAll(square);

        System.out.println(numbers);
    }
}

Outputs:

[1, 4, 9, 16, 25]

In this example, the UnaryOperator square squares each element. The List.replaceAll() method applies this operator to all elements in the list.

Note that replaceAll() modifies the original list and does not return a new list. Please also be aware that this operation is in-place and hence modifies the original List. If you want to keep the original List unchanged, create a new List and add elements to it after applying the function.

The primary purpose of the List.replaceAll() method in Java is to perform an in-place transformation of all elements within a list based on a given unary function or operation.

A Unary function or operation is one that takes a single input and produces a result. In the context of replaceAll(), the unary operation is typically provided as a lambda expression or method reference which is applied to each element in the list in turn.

If successful, replaceAll() modifies the list such that each original element has been replaced by the result of applying the provided unary operation to that element. This operation is performed on the original list, and no new list is created, making it an efficient option for transforming large lists.

Here is an example which doubles each integer in a list:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class ListReplaceAllSecondExample {
    public static void main(String[] args) {
        List<Integer> ints = new ArrayList<>();

        ints.add(1);
        ints.add(2);
        ints.add(3);

        // Double every integer in the List
        ints.replaceAll(n -> n * 2);

        System.out.println(ints); 
    }
}

Outputs:

[2, 4, 6]

In conclusion, List.replaceAll() provides a convenient and efficient way to modify all elements in a list according to a specified operation or function. It’s especially useful when using the Streams API and functional programming techniques introduced in Java 8.

How do I use Collection.removeIf() method?

By Wayan in Util Package 0 Comment

The Collection.removeIf() method was introduced in Java 8, and it allows for the removal of items from a collection using a condition defined in a lambda expression.

The primary purpose of the Collection.removeIf() method in Java is to filter out elements from a collection based on a certain condition or predicate. It’s a more efficient and concise way of performing this type of operation than traditional for or iterator-based loops.

The method iterates over each element in the collection and checks whether it satisfies the condition described by the given Predicate. If the Predicate returns true for a particular element, removeIf() removes that element from the collection.

Here’s a simple example:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class CollectionRemoveIfExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        // Use removeIf method to remove all numbers greater than 2
        numbers.removeIf(n -> n > 2);

        System.out.println(numbers); // Outputs: [1, 2]
    }
}

In this example, n -> n > 2 is a lambda expression that defines a Predicate, which returns true for all numbers greater than 2. The removeIf() method uses this Predicate to determine which elements to remove.

Please be aware that not all Collection implementations support the removeIf() method. For example, if you try to use it with an unmodifiable collection (like the ones returned by Collections.unmodifiableList()), it will throw an UnsupportedOperationException.

As removeIf() is a default method, it’s provided with a default implementation, and it’s available for use with any classes that implement the Collection interface (like ArrayList, HashSet, etc.) without requiring those classes to provide their own implementation.

However, classes can still override this method with their own optimized version if necessary. Here’s another example of removeIf() method:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class CollectionRemoveIfSecond {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        names.add("David");
        names.add("Rosa");

        // Remove names that start with 'B'
        names.removeIf(name -> name.startsWith("B"));

        System.out.println(names); // Outputs: [Alice, Charlie, David, Rosa]
    }
}

Remember, it’s a bulk operation that can lead to a ConcurrentModificationException if the collection is modified while the operation is running (for example, removing an element from a collection while iterating over it with removeIf()), except if the collection is a Concurrent Collection.

In conclusion, the Collection.removeIf() default method provides a unified, efficient, and convenient way to remove items from a collection based on certain conditions.

What is Default Methods in Java?

By Wayan in Basic 0 Comment

Default methods are a feature introduced in Java 8, allowing the declaration of methods in interfaces, apart from abstract methods. They are also known as defender methods or virtual extension methods.

With the use of default keyword, these methods are defined within the interface and provide a default implementation. This means they can be directly used by any class implementing this interface without needing to provide an implementation for these methods.

The main advantage of default methods is that they allow the interfaces to be evolved over time without breaking the existing code.

Here’s an example of a default method in an interface:

interface MyInterface {
    void abstractMethod();

    default void defaultMethod() {
        System.out.println("This is a default method in the interface");
    }
}

In the above example, any class implementing MyInterface needs to provide an implementation for abstractMethod(), but not for defaultMethod() unless it needs to override the default implementation.

Before Java 8, we could declare only abstract methods in interfaces. It means that classes which implement the interface were obliged to provide an implementation of all methods declared in an interface. However, this was not flexible for developers, especially when they wanted to add new methods to the interfaces.

For instance, here is an interface used by multiple classes:

interface Animal {
    void eat();
}

Now, if we wanted to add a new method called run(), all classes that implement Animal would need to define this method, which could potentially introduce bugs and is quite cumbersome if we have many classes that implement the interface.

To mitigate such issues, Java 8 introduced default methods in interfaces. With default methods, we can now add new methods in the interface with a default implementation, thereby having the least impact on the classes that implement the interface.

interface Animal {
    void eat();

    default void run() {
        System.out.println("Running");
    }
}

So in the updated Animal interface, the run() method is a default method. Classes implementing Animal can choose to override this method, but they are not obliged to do so. If a class does not provide an implementation for this method, the default implementation from the interface will be used.

Here’s an example implementation of the Animal interface:

class Dog implements Animal {
    @Override
    public void eat() {
        System.out.println("Dog is eating");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.eat();  // Output: Dog is eating
        dog.run();  // Output: Running
    }
}

As you can see, the Dog class didn’t implement the run method, but we’re still able to call dog.run() because of the default implementation in the Animal interface.

Note: In case a class implements multiple interfaces and these interfaces have default methods with identical signatures, the compiler will throw an error. The class must override the method to resolve the conflict.