How do I convert Array to Collection?

By I Wayan Saryada in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.Arrays;
import java.util.List;

public class ArrayToListExample {
    public static void main(String[] args) {
        // Creates an array of object, in this case we create an
        // Integer array.
        Integer[] numbers = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55};

        // Convert the created array above to collection, in this
        // example we convert it to a List.
        List<Integer> list = Arrays.asList(numbers);

        // We've got a list of our array here and iterate it.
        for (Integer number : list) {
            System.out.print(number + ", ");
        }
    }
}

How do I convert collection to ArrayList?

By I Wayan Saryada in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.ArrayList;
import java.util.LinkedList;

public class CollectionToArrayList {
    public static void main(String[] args) {
        // We create LinkedList collection type at put some values
        // in it. Here we put A, B, C and D letter into it.
        LinkedList<String> linkedList = new LinkedList<>();
        linkedList.push("A");
        linkedList.push("B");
        linkedList.push("C");
        linkedList.push("D");

        // Let say you want to convert it to other type of collection,
        // for instance here we convert it into ArrayList. To do it
        // we can pass the collection created above as a parameter to
        // ArrayList constructor.
        ArrayList<String> arrayList = new ArrayList<>(linkedList);

        // Now we have converted the collection into ArrayList and
        // printed what is inside.
        for (String s : arrayList) {
            System.out.println("s = " + s);
        }
    }
}

How do I create an empty collection object?

By I Wayan Saryada in Core API, Util Package 3 Comments

Sometimes you need to return an empty collection from your Java methods. The java.util.Collections utility class have three different static constants for creating empty List, Set and Map.

  • Collections.EMPTY_LIST
  • Collections.EMPTY_SET
  • Collections.EMPTY_MAP

There are also methods when you want to create type-safe empty collections.

  • Collections.emptyList()
  • Collections.emptySet()
  • Collections.emptyMap()

Bellow it the code example.

package org.kodejava.util;

import java.util.*;

public class EmptyCollectionDemo {
    public static void main(String[] args) {
        List list = Collections.EMPTY_LIST;
        System.out.println("list.size()  = " + list.size());
        Set set = Collections.EMPTY_SET;
        System.out.println("set.size()   = " + set.size());
        Map map = Collections.EMPTY_MAP;
        System.out.println("map.size()   = " + map.size());

        // For the type-safe example use the following methods.
        List<String> strings = Collections.emptyList();
        System.out.println("strings.size = " + strings.size());

        Set<Long> longs = Collections.emptySet();
        System.out.println("longs.size() = " + longs.size());

        Map<String, Date> dates = Collections.emptyMap();
        System.out.println("dates.size() = " + dates.size());
    }
}

The output are:

list.size()  = 0
set.size()   = 0
map.size()   = 0
strings.size = 0
longs.size() = 0
dates.size() = 0

How do I convert a collection object into an array?

By I Wayan Saryada in Core API, Util Package 0 Comment

To convert collection-based object into an array we can use toArray() or toArray(T[] a) method provided by the implementation of Collection interface such as java.util.ArrayList.

package org.kodejava.util;

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

public class CollectionToArrayExample {
    public static void main(String[] args) {
        List<String> words = new ArrayList<>();
        words.add("Kode");
        words.add("Java");
        words.add("-");
        words.add("Learn");
        words.add("Java");
        words.add("by");
        words.add("Examples");

        String[] array = words.toArray(new String[0]);
        for (String word : array) {
            System.out.println(word);
        }
    }
}

Our code snippet result is shown below:

Kode
Java
-
Learn
Java
by
Examples

How do I convert an array into a collection object?

By I Wayan Saryada in Core API, Util Package 0 Comment

To convert array based data into List / Collection based we can use java.util.Arrays class. This class provides a static method asList(T... a) that converts array into List / Collection.

package org.kodejava.util;

import java.util.Arrays;
import java.util.List;

public class ArrayAsListExample {
    public static void main(String[] args) {
        String[] words = {"Happy", "New", "Year", "2021"};
        List<String> list = Arrays.asList(words);

        for (String word : list) {
            System.out.println(word);
        }
    }
}

The results of our code are:

Happy
New
Year
2021