A tömb egy olyan adatszerkezet, amelyet azonos típusú adatok tárolására használnak. A tömbök elemeit összefüggő memóriahelyeken tárolják.

A Java-ban a tömbök objektumok. Az osztályobjektum összes metódusa meghívható egy tömbben. Rögzített számú elemet tárolhatunk egy tömbben.

Nyilatkozjunk egy egyszerű primitív tömbtípust:

int[] intArray = {2,5,46,12,34};

Most próbáljuk meg kinyomtatni a System.out.println()módszerrel:

System.out.println(intArray); // output: [I@74a14482

Miért nem nyomtatta ki Java a tömbünket? Mi történik a motorháztető alatt?

A System.out.println()metódus az általunk átadott objektumot karakterlánccá alakítja hívással String.valueOf(). Ha megnézzük a String.valueOf()módszer megvalósítását, akkor ezt látni fogjuk:

public static String valueOf(Object obj) { return (obj == null) ? "null" : obj.toString(); }

Ha az átadott objektum nullnull értéket ad vissza, különben hív obj.toString(). Végül System.out.println()felhívja toString()a kimenetet.

Ha az objektum osztálya nem írja felül a Object.toString()megvalósítást, akkor meghívja a Object.toString()metódust.

Object.toString()visszatér . Egyszerű megfogalmazással a következőket adja vissza: „class name @ object hash code”.getClass().getName()+‘@’+Integer.toHexString(hashCode())

Korábbi kimenet [I@74a14482, az [azt állítja, hogy ez egy tömb, és Ijelentése int (a típus a tömb). 74a14482a tömb hash-kódjának előjel nélküli hexadecimális ábrázolása.

Amikor saját egyéni osztályokat hozunk létre, a legjobb Object.toString()módszer a módszer felülbírálása .

Nem nyomtathatunk tömböket Java-ban egyszerű System.out.println()módszerrel. Ehelyett a következő módon nyomtathatunk tömböt:

1. Hurkok: hurokhoz és minden hurokhoz
2. Arrays.toString() módszer
3. Arrays.deepToString() módszer
4. Arrays.asList() módszer
5. Java Iterator felület
6. Java Stream API

Lássuk őket egyenként.

1. Hurkok: hurokhoz és minden hurokhoz

Íme egy példa a for ciklusra:

int[] intArray = {2,5,46,12,34}; for(int i=0; i
    

All wrapper classes override Object.toString() and return a string representation of their value.

And here's a for-each loop:

int[] intArray = {2,5,46,12,34}; for(int i: intArray){ System.out.print(i); // output: 25461234 }

2. Arrays.toString() method

Arrays.toString() is a static method of the array class which belongs to the java.util package. It returns a string representation of the contents of the specified array. We can print one-dimensional arrays using this method.

Array elements are converted to strings using the String.valueOf() method, like this:

int[] intArray = {2,5,46,12,34}; System.out.println(Arrays.toString(intArray)); // output: [2, 5, 46, 12, 34]

For a reference type of array, we have to make sure that the reference type class overrides the Object.toString() method.

For example:

public class Test { public static void main(String[] args) { Student[] students = {new Student("John"), new Student("Doe")}; System.out.println(Arrays.toString(students)); // output: [Student{name='John'}, Student{name='Doe'}] } } class Student { private String name; public Student(String name){ this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + '}'; } }

This method is not appropriate for multidimensional arrays. It converts multidimensional arrays to strings using Object.toString() which describes their identities rather than their contents.

For example:

// creating multidimensional array int[][] multiDimensionalArr = { {2,3}, {5,9} }; System.out.println(Arrays.toString(multiDimensionalArr)); // output: [[I@74a14482, [I@1540e19d]

With the help of Arrays.deepToString(), we can print multidimensional arrays.

3. Arrays.deepToString() method

Arrays.deepToString() returns a string representation of the “deep contents” of the specified array.

If an element is an array of primitive type, it is converted to a string by invoking the appropriate overloading of Arrays.toString() .

Here is an example of the primitive type of multidimensional array:

// creating multidimensional array int[][] multiDimensionalArr = { {2,3}, {5,9} }; System.out.println(Arrays.deepToString(multiDimensionalArr)); // output: [[2, 3], [5, 9]]

If an element is an array of reference type, it is converted to a string by invoking Arrays.deepToString() recursively.

Teacher[][] teachers = {{ new Teacher("John"), new Teacher("David") }, {new Teacher("Mary")} }; System.out.println(Arrays.deepToString(teachers)); // output: [[Teacher{name='John'}, Teacher{name='David'}],[Teacher{name='Mary'}]]

We have to override Object.toString() in our Teacher class.

If you are curious as to how it does recursion, here is the source code for the Arrays.deepToString() method.

NOTE: Reference type one-dimensional arrays can also be printed using this method. For example:

Integer[] oneDimensionalArr = {1,4,7}; System.out.println(Arrays.deepToString(oneDimensionalArr)); // output: [1, 4, 7]

4. Arrays.asList() method

This method returns a fixed-size list backed by the specified array.

Integer[] intArray = {2,5,46,12,34}; System.out.println(Arrays.asList(intArray)); // output: [2, 5, 46, 12, 34]

We have changed the type to Integer from int, because List is a collection that holds a list of objects. When we are converting an array to a list it should be an array of reference type.

Java calls Arrays.asList(intArray).toString() . This technique internally uses the toString() method of the type of the elements within the list.

Another example with our custom Teacher class:

Teacher[] teacher = { new Teacher("John"), new Teacher("Mary") }; System.out.println(Arrays.asList(teacher)); // output: [Teacher{name='John'}, Teacher{name='Mary'}]

NOTE: We can not print multi-dimensional arrays using this method. For example:

Teacher[][] teachers = {{ new Teacher("John"), new Teacher("David") }, { new Teacher("Mary") }}; System.out.println(Arrays.asList(teachers)); // output: [[Lcom.thano.article.printarray.Teacher;@1540e19d, [Lcom.thano.article.printarray.Teacher;@677327b6]

5. Java Iterator Interface

Similar to a for-each loop, we can use the Iterator interface to loop through array elements and print them.

Iterator object can be created by invoking the iterator() method on a Collection. That object will be used to iterate over that Collection’s elements.

Here is an example of how we can print an array using the Iterator interface:

Integer[] intArray = {2,5,46,12,34}; // creating a List of Integer List list = Arrays.asList(intArray); // creating an iterator of Integer List Iterator it = list.iterator(); // if List has elements to be iterated while(it.hasNext()) { System.out.print(it.next()); // output: 25461234 }

6. Java Stream API

The Stream API is used to process collections of objects. A stream is a sequence of objects. Streams don’t change the original data structure, they only provide the result as per the requested operations.

With the help of the forEach() terminal operation we can iterate through every element of the stream.

For example:

Integer[] intArray = {2,5,46,12,34}; Arrays.stream(intArray).forEach(System.out::print); // output: 25461234

Now we know how to print an array in Java.

Thank you for reading.

Cover image by Aziz Acharki on Unsplash.

You can read my other articles on Medium.

Happy Coding!